synthesis and characterization of nanostructures
TRANSCRIPT
Synthesis and Characterization of
Nanostructures
By
Rafaqat Hussain
CIIT/SP04-PPH-006/ISB
Ph.D Thesis
In
Physics
COMSATS Institute of Information Technology
Islamabad- Pakistan
Spring, 2011
ii
COMSATS Institute of Information Technology
Synthesis and Characterization of Nanostructures
A Thesis Presented to
COMSATS Institute of Information Technology, Islamabad
In partial fulfillment
of the requirement for the degree of
Ph.D (Physics)
By
Rafaqat Hussain
CIIT/SP04-PPH-006/ISB
Spring, 2011
iii
Synthesis and Characterization of Nanostructures
A Post Graduate Thesis submitted to the Department of Physics as partial
fulfillment of the requirement for the award of Degree of M.S/Ph.D (Physics).
Name Registration Number
Rafaqat Hussain CIIT/SP04-PPH-006/ISB
Supervisor
Dr. Ehsan Ullah Khan, (T.I.)
Professor in Physics
Co-Supervisor
Dr. Syed Tajammul Hussain
Director, Nanoscience and Catalysis Division
National Centre for Physics, Islamabad, Pakistan.
June, 2011
iv
Final Approval
This thesis titled
Synthesis and Characterization of Nanostructures By
Rafaqat Hussain
CIIT/SP04-PPH-006/ISB Has been approved
For the COMSATS Institute of Information Technology, Islamabad
External Examiner 1: ___________________________________________________
Prof. Dr. Asghari Maqsood, S.I
Meritorious Professor, Department of Physics, CESET, Islamabad
External Examiner 2: ___________________________________________________
Prof. Dr. Younis Nadeem
Department of Physics, Bahauddin Zakariya University, Multan
Supervisor: ___________________________________________________________
Prof. Dr. Ehsan Ullah Khan, T.I
Department of Physics, CESET, Islamabad
Co-Supervisor: ________________________________________________________
Prof. Dr. Syed Tajammul Hussain, PoP
Director NS & CD, National Centre for Physics, Islamabad
HoD: ________________________________________________________________
Prof. Dr. Mahnaz Haseeb
Department of Physics, Islamabad Campus, CIIT
Chairman:_____________________________________________________________
Prof. Dr. Sajid Qamar
Department of Physics, CIIT, Islamabad
Dean, Faculty of Sciences: ___________________________________________
Prof. Dr.Arshad Saleem Bhatti, T.I
v
Declaration
I Rafaqat Hussain, registration number CIIT/SP04-PPH-006/ISB hereby declare that I
have produced the work presented in this thesis, during the scheduled period of study. I
also declare that I have not taken any material from any source except referred to
wherever due that amount of plagiarism is within acceptable range. If a violation of HEC
rules on research has occurred in this thesis, I shall be liable to punishable action under
the plagiarism rules of the HEC.
Date: ________________ Signature of the student:
_____________________________
Rafaqat Hussain
CIIT/SP04-PPH-006/ISB
vi
Certificate
It is certified that Mr. Rafaqat Hussain, registration number CIIT/SP04-PPH-006/ISB has
carried out all the work related to this thesis under our supervision at the Department of
Physics, COMSATS Institute of Information Technology, Islamabad and the work fulfills
the requirement for an award of PhD degree.
Date: ____________________
Supervisor:
___________________________
Prof. Dr. Ehsan Ullah Khan, TI
Professor in Physics
Co-Supervisor:
___________________________
Prof. Dr. Syed Tajammul Hussain
Director NS & CD, NCP, Islamabad
Head of Department:
_______________________
Department of Physics
vii
Dedicated
To
Holy Prophet Muhammad (Peace be upon him) and his
companions who laid the foundations of modern
civilization and paved the way for social, moral,
political, economical, cultural and physical revolution
xi
Table of Contents
1.1 Motivations................................................................................................................1
1.2 Current Status of the Field .........................................................................................5
1.3 Objectives ..................................................................................................................7
1.4 Summary of the thesis ................................................................................................7
2.1. The Magnetic Moment and Magnetisation ................................................................8
2.1.1 Origins of Magnetism in Bulk Materials ..............................................................8
2.1.2. Thin Film Magnetism ....................................................................................... 10
2.2 Demagnetizing Field ................................................................................................ 11
2.3 Magnetic Anisotropies ............................................................................................. 11
2.3.1 Magnetocrystalline Anisotropy .......................................................................... 12
2.3.2 Exchange Anisotropy ........................................................................................ 12
2.3.3 Shape Anisotropy .............................................................................................. 13
2.3.4 Other Contributions to Anisotropy ..................................................................... 15
2.4 Magnetic Domain .................................................................................................... 15
2.5 Reversal Mechanisms .............................................................................................. 16
2.5.1 (Coherent reversal) Stoner Wohlfarth model ...................................................... 17
2.5.2 Incoherent Reversal ........................................................................................... 18
2.5.3 Domain wall pinning mechanism or domain nucleation ..................................... 21
2.5.4 Reversal process in EB thin films ...................................................................... 23
2.6 Exchange Bias ......................................................................................................... 26
2.6.1 Earlier Theories ................................................................................................. 26
2.6.2 Technological Importance ................................................................................. 27
2.6.3 Recent development in the field of EB............................................................... 28
2.7. York Protocol ......................................................................................................... 29
2.7.1. Theory .............................................................................................................. 29
xii
2.7.1.1. Grain Size Distribution .............................................................................. 29
2.7.1.2 Blocking Temperature ................................................................................. 31
2.7.1.3 Measurement of EB from York Protocol ..................................................... 33
2.8 Training Effect in Exchange Bias ............................................................................. 34
2.9 Diluted Magnetic Semiconductors ........................................................................... 36
2.9.1 Types of interactions in DMS ............................................................................ 37
3.1. Synthesis Technologies ........................................................................................... 42
3.1.1. High Target Utilization Sputtering (HITUS) ..................................................... 42
3.1.2. Aerosol Assisted Chemical Vapor Deposition (AACVD) ................................. 44
3.1.2.1. Advantages of AACVD over conventional CVD ........................................ 45
3.2. Characterization Techniques ................................................................................... 46
3.2.1. X-Ray Diffractometer ....................................................................................... 46
3.2.2 Field Emission Electron Microscope (FESEM).................................................. 47
3.2.3. Zeiss Particle Size Analyzer ............................................................................. 48
3.2.4. Transmission electron microscopy (TEM) ........................................................ 49
3.2.5 Magnetometers .................................................................................................. 51
3.2.5.1. Vibrating Sample Measurement (VSM) ..................................................... 51
3.2.5.2 Alternating Field Gradient Magnetometer (AGFM)..................................... 53
3.2.5.3. Superconducting Quantum Interference Device (SQUID) .......................... 54
3.2.6. Rutherford Back scattering (RBS) .................................................................... 55
4.1 Introduction ............................................................................................................. 58
4.2 Experimental ........................................................................................................... 59
4.3 Results and Discussion ............................................................................................ 61
4.3.1 Effect of substrate cutting .................................................................................. 61
4.3.2 Effects of sample shape ..................................................................................... 63
Summary ....................................................................................................................... 64
5.1 Experimental ........................................................................................................... 66
5.1.1 Fabrication process and conditions .................................................................... 66
5.1.2 Setting process .................................................................................................. 67
xiii
5.1.2.1 York Protocol ............................................................................................. 67
5.2 Results and Discussion ............................................................................................ 69
5.2.1 Grain Size Analysis ........................................................................................... 69
5.2.2 Exchange Bias ................................................................................................... 72
5.2.3 Blocking Temperature ....................................................................................... 74
5.3 Training Effect......................................................................................................... 77
5.3.1 Sample Preparation............................................................................................ 78
5.4. Results and Discussion............................................................................................ 79
Summary ....................................................................................................................... 82
6.1 Introduction ............................................................................................................. 84
6.2 Nickel Doped TiO2 Thin Films ................................................................................ 85
6.2.1 Experimental ..................................................................................................... 86
6.2.2 Results and Discussion ...................................................................................... 87
6.2.2.1 XRD analysis .............................................................................................. 87
6.2.2.2 Rutherford Back Scattering ...................................................................... 92
6.2.2.3 Scanning electron microscopy ..................................................................... 96
6.2.2.4 Magnetic properties .................................................................................... 98
6.3 Cobalt Doped TiO2 Thin Films .............................................................................. 101
6.3.1 Experimental ................................................................................................... 101
6.3.2 Results and Discussion .................................................................................... 102
6.3.2.1 XRD Analysis ........................................................................................... 102
6.3.2.2 Rutherford Back Scattering ....................................................................... 105
6.3.2.3 Scanning electron microscopy ................................................................... 106
Summary ..................................................................................................................... 112
7.1 Exchange Bias ....................................................................................................... 113
7.2 Diluted magnetic semiconductors .......................................................................... 115
7.3 Future Work .......................................................................................................... 117
References ................................................................................................................... 131
xiv
List of Publications ...................................................................................................... 131
LIST OF FIGURES ....................................................................................................... xv
LIST OF TABLES .................................................................................................... xviii
xv
LIST OF FIGURES
_____________________________________________________________
Fig 1. 1 a) magnetic semiconductor b) a nonmagnetic semiconductor c) a dilute magnetic
semiconductor .................................................................................................................4
Fig 2.1: Schematic representation of a magnetisation curve for a typical ferromagnet,
showing the hysteretic behavior and the main characterizing parameters….……………10
Fig 2.2: Hysteresis loop of CoO/Co particles after sample was cooled through TN of CoO
in a saturating magnetic field ......................................................................................... 13
Fig 2. 3: Domain formation and minimization of energy ................................................ 15
Fig 2.4: Coherent rotation of magnetisation by rotation – the Stoner-Wohlfarth model. . 17
Fig 2.5: Variation of coercivity with axial ratio using the Stoner-Wohlfarth model of
reversal. ......................................................................................................................... 19
Fig 2.6: Fanning (a) and coherent (b) reversal modes for an N = 2 chain of spheres. ...... 20
Fig 2.7: Magnetization Reversal in Exchange Bias Thin Films....................................... 25
Fig 2. 8: Magnetization Reversal in soft magnetic film .................................................. 25
Fig 2.9: Schematic of the energy barrier reversal, showing the proportion of AFM grains
set parallel or anti-parallel to the original set direction. .................................................. 32
Fig 2.10: Comparison of Blocking Temperature (TB) measured from a) Conventional
method b) York Protocol ............................................................................................... 32
Fig 2.11: Schematic of the grain size distribution after the setting of the AFM and cooling
to a temperature at which the AFM is thermally unstable. .............................................. 33
Fig 2. 12: Consecutive hysteresis loops of a Co − CoO system measured with torque
balance. The observed overshoot is an instrumental effect. ............................................ 34
Fig 2. 13: Schematic showing the FM and AFM sublattice magnetizations in an exchange
bias system where the AFM anisotropy has biaxial symmetry during the 1st and 2nd
hysteresis loop measurements. ...................................................................................... 36
xvi
Fig 2. 14: Interaction of two bound magnetic polarons. The polarons are shown with gray
circles. Small and large arrows show impurity and hole spins, respectively. Shaded region
shows the effect of two BPMs on impurity spins. ........................................................... 40
Fig 3. 1: Schematic representation of HiTUS sputtering technology……………………42
Fig 3. 2: Schematic representation of AACVD .............................................................. 46
Fig 3. 3: Schematic Diagram of a Transmission Electron Microscope ............................ 50
Fig 3. 4: Schematic of a standard VSM. ......................................................................... 52
Fig 3. 5: Schematic of a standard AGFM. ...................................................................... 53
Fig 3. 6: Schematic diagram Josephson junction ............................................................ 55
Fig 3. 7: Schematic diagram of RBS basic function ....................................................... 56
Fig 4. 1: Sample structure………………………………………………………………..59
Fig 4. 2: SEM images of the edges of the three sample types ......................................... 60
Fig 4. 3: Hysteresis loops for three samples produced by (a) cutting with a diamond
scribe, (b) depositing through a mask and (c) cutting with an ultrasonic cutter ............... 62
Fig 4. 4: Hysteresis curve a) and b) shows the effect of sample shape ............................ 63
Fig 5. 1: Sample structure with Mn doping……………………………………………...67
Fig 5. 2: a) Schematic diagram and b) measurement steps of the York protocol. 14
......... 68
Fig 5. 3: TEM Image ..................................................................................................... 70
Fig 5. 4: Graph showing the log-normal distribution of grain sizes................................. 71
Fig 5. 5: Typical hysteresis loop obtained using the York protocol. ................................ 73
Fig 5. 6: Typical loop shift for different temperatures under a constant reverse field. ..... 75
Fig 5. 7: Measurement of Blocking Temperature (TB) by York Protocol ........................ 76
Fig 5. 8: Schematic diagram of sample structure ............................................................ 78
Fig 5. 9: Training effect with a) NiCr seed layer b) Cu seed layer .................................. 79
Fig 5. 10: a) comparison of bias voltage vs. training effect for NiCr and Cu under layer b)
grain size vs. training effect for NiCr under layer ........................................................... 80
Fig 6. 1: a)Topography image and corresponding b) MFM images of Ni doped TiO2 thin
films ……………………………………………………………………………………85
Fig 6. 2. XRD pattern of Ni (2%) doped TiO2 ................................................................ 88
Fig 6. 3. XRD pattern of Ni (4%) doped TiO2 ................................................................ 89
Fig 6. 5. XRD pattern of Ni (6%) doped TiO2 ................................................................ 89
xvii
Fig 6. 6. XRD pattern of Ni (8%) doped TiO2 ................................................................ 90
Fig 6. 7: XRD pattern of Ni (15%) doped TiO2 thin films .............................................. 90
Fig 6. 8: Crystal structure of [Ni2Ti2(OEt)2(l-OEt)6(acac)4] ............................................ 91
Fig 6. 9: Comparison of experimental and simulated RBS spectra on Ni doped TiO2 thin
film................................................................................................................................ 94
Fig 6. 10: RBS spectra of Ni doped TiO2 thin films with various concentrations ............ 95
Fig 6. 11: SEM images of Ni doped TiO2 thin films with a) 2wt.% Ni b) 4wt.% Ni c)
6wt.% Ni d) 8wt.% Ni and e) 15wt.% Ni doping ............................................................ 97
Fig 6. 12: Magnetic moment of Ni doped TiO2 thin films at 100K ............................... 100
Fig 6. 13: Magnetic moment of Ni doped TiO2 thin films AT 300K ............................. 100
Fig 6. 14: XRD pattern of Ni doped TiO2 thin films with a) 2wt.% Ni b) 4wt.% Ni c)
6wt.% Ni d) 8wt.% Ni and e) 15wt.% Ni doping .......................................................... 104
Fig 6. 15: RBS spectra of Co doped TiO2 thin films with various concentrations ......... 105
Fig 6. 16: SEM images of Co doped TiO2 thin films with a) 2wt.% Co b) 4wt.% Co c)
6wt.% Co d) 8wt.% Co and e) 15wt.% Co doping ........................................................ 108
Fig 6. 17: Hysteresis loop of Co doped TiO2 thin films with various Co doping
concentrations ............................................................................................................. 109
xviii
LIST OF TABLES
_____________________________________________________________
Table 2. 1: calculated values of c for elongated iron particles26
................................... 19
Table 5. 1: The results obtained for the average grain size at different bias voltages…...72
Table 5. 2: The results obtained for and c at different bias voltages....................... 73
Table 5. 3: The results obtained for TB at different reverse fields. ................................... 77
Table 5. 4: Amount of training for Cu and NiCr underlayer ........................................... 81
Table 6. 1. Lattice parameters, Cell volume and crystallite size calculated from XRD
data……………………………………………………………… ………….92
Table 6. 2: Ni concentration and film thickness as calculated from RBS spectra ............ 95
Table 6. 3: Magnetic moment of Ni doped TiO2 thin films at 100K and 300K ............. 101
Table 6. 4: Co concentration and film thickness as calculated from RBS spectra .......... 106
Table 6. 5: Magnetic moment of Co doped TiO2 at 300K ............................................. 111
viii
ACKNOWLEDGEMENTS
First of all I thank Almighty ALLAH, the most Merciful, and the most
Beneficent, Who blessed me with sound health and opportunity to complete this research
work successfully. I pay gratitude to my supervisors Prof. Dr. Syed Tajammul Hussain
and Prof. Dr. Ehsan Ullah Khan (T.I.) for their kind supervision, guidance and
cooperation during this research work. I also appreciate continuous support from NCP
and Department of Chemistry, QAU, Islamabad during the course of my PhD work. I pay
my gratitude to all the lab fellows and staff members in NCP and QAU for their good
wishes and support to my research. My whole heartedly appreciation to my foreign
supervisor Prof. Kevin O’ Grady, Department of Physics, the University of York, UK for
his kind guidance, encouragement and cooperation during my six months stay at the
Department of Physics, the University of York, UK to perform experimental work.
Without his careful consideration and encouragement this research work could have
never been completed. I am obliged to Dr. Barbara Kaeswurm for her guidance and keen
interest to help me to work in the project. I am also thankful to Mr. Nick Cramp and all
the other lab fellows in the University of York for their help in the project and enjoyable
company.
I am thankful to Dr. Tadachika Nakayama, the University of Nagaoka, Japan and
Ms. Naila Jabeen Berkeley labs, USA for their help to characterize my samples.
I commend the cooperation and the supports extended to me by Prof. Arshad
Saleem Bhatti, Dean Faculty of Sciences and appreciate his arduous efforts to prop up
post graduate research program. I am gratified to Dr. Ishaq Ahmad, Head of the
Department of Physics, for creating a lively scientific environment.
Extraordinary credit to the authorities of HEC, Government of Pakistan, for
bestowing upon me the scholarship under Indigenous 5000 Scheme and reverend
authorities in the University of York for giving me opportunity to work in Magnetism
Lab in the University of York.
I, from the core of my heart, thank all of my fellows at CIIT, NCP and QAU for
their moral and manual help throughout this research work. I am enormously grateful to
my loving parents, brothers and sisters who always pray for my success in every walk of
life.
ix
ABSTRACT
Synthesis and Characterization of Nanostructures
This thesis is mainly focused on synthesis and characterization of (magnetic)
nanostructures in the form of multilayers and magnetic oxides thin films for spintronics
applications. Exchange bias phenomenon which has a critical role in
ferromagnetic/antiferromagnetic multilayer system was studied experimentally with a
theoretical understanding of very recent model of exchange bias namely York Model.
Standard IrMn and CoFe multilayer system (Si/Cu/IrMn/CoFe/Ta) was fabricated using
High Target Utilization Sputtering (HiTUS) to study various aspects of exchange bias.
Effect of Mn doping showed a decrease in the blocking temperature. Chemical reaction
of Mn at the interface and diffusion of Cu from the under layer in IrMn layer were
considered to be cause of this decrease. Training effect in exchange coupled IrMn and
CoFe multilayer thin films was investigated for varying grain size that was controlled
during the fabrication process through bias voltage. It was observed that smallest grains
gave rise to a larger training effect as larger anti-ferromagnetic grain volumes give rise to
thermally stable bias fields and consequently smaller training effects. The result is found
reproducible and in agreement with the literature. The effects of nucleation were also
studied. It was determined that nucleation arises from both sample shape effects and the
process used to cut the sample. The obtained results showed that sample edge roughness
leads to a distribution of nucleation fields and hence changes the shape of the hysteresis
loop. It was concluded that the best way to cut samples of nucleation controlled materials
is by cracking for the application in spintronics devices.
Second part of the study was about Ni and Co doped TiO2 diluted magnetic
semiconductors thin films grown by Aerosol Assisted Chemical Vapor Deposition
(AACVD). AACVD method was adopted for synthesizing these films due to certain
advantages over other chemical routes. Further, synthesis routes may vary various
properties and there are only a handful reports in the literature in which AACVD method
x
was employed to synthesize diluted magnetic oxides. Ni and Co doped TiO2 films were
prepared at 450 C and 650 C respectively with Argon as a carries gas. XRD, FESEM
and RBS were carried out to see phase, morphology, and stoichiometry and film
thickness. Magnetic properties of the films were investigated using SQUID. Ni and Co
doping resulted in ferromagnetism in TiO2 at room temperature attributed to the
formation of Bound Magnetic Polaron (BMP).
Chapter 1
1
Chapter 1
Introduction
1.1 Motivations
The study of the materials at nano-scale has been a focus of recent investigations.
But to initiate, what is nano? Here we do not go into definitive details. Simply,
nanoscience is an emerging area of science and technology. It has gotten attention from
researchers all over the world. The word nano describes physical length scales that are
equivalent of billionth of a meter in length.
The increasing interest in the nanomaterials is because of the numerous
possibilities that they can induce when they are on atomic scales. Nanomaterial can
induce significant change in the mechanical, optical, chemical and magnetic properties,
which are very dissimilar in comparison with bulk materials of the same composition.1 In
this respect different fields like physics, chemistry, biology and engineering, strived to
explain various phenomena that these materials exhibit.2
In the past, materials science focused basically upon utilization of the natural
elements (iron, silicon, etc.) for developing new compounds. As a result scientists have
attained knowledge of making devices using artificial structures in which the atoms are
deposited layer by layer and later on they managed to redesign the device structures to
control the properties at molecular level. This allowed fabricating the system with novel
properties. With these technological advances it has become possible now to make
artificial nanometer systems in which the effects of the quantum confinement is
pronounced.
Chapter 1
2
How can nanoscale materials be used to improve our lives whether in health,
environment, energy needs of the day and improved performance of existing electronic
devices etc.? The quest for answer compels us to make research in this field.
Investigation on nanoparticles has also made its contribution in many other fields as well.
The size of the devices is continuously reducing, while the speed and efficiency is
improving. The advancements are being made and new concepts for reducing size,
power utilization and exploring multi functions of material are being investigated
regularly. The use of the spin electrons, hole, nuclei, or ions to explore and enhance new
functionalities analog and digital electronics is one of the favored topics today.3-4
The
charge, mass and spin of the electron lays the foundation for the information technology,
data storage and many other areas being in use at present time. Semiconducting materials
are in used for construction of Integrate circuit and high-frequency devices.4-5
On the
other hand, mass storage of information is carried out by magnetic recording such as
magnetic tapes, hard disks and optical discs. All this can be achieved by using spin of the
electron in a ferromagnetic metal. 6-7
So far the charge and spin is used separately. However the combination of this
two degree of freedom may bring an enhancement in the performance in current
technology. It may also bring new functionalities which are not possible by using one out
of them. If we can combine these two properties we can actually store and process the
information together. On the other hand, it is possible to inject spin-polarized current to
control the spin state of carriers into semiconductors, which may allow us to work on
qubit (quantum bit) operation which is essential for quantum computing. This branch of
the electronic in which we play with both charge and spin of electron at the same time is
called ―spintronics‖.8
Due to the difficulties to integrate such ferromagnetic materials with conventional
semiconductors being used, such as silicon (Si) and gallium arsenide (GaAs), these
devices could not be made yet. It is because of huge difference in physical and chemical
properties, crystal structure and lattice parameters between these two classes of materials.
Chapter 1
3
Further, Si and GaAs do not have magnetic ions and have nonmagnetic properties. To
achieve a meaningful variance in the energy between the two possibilities of orientation
of spin (up and down) the required applied magnetic field would be too high for common
use.9
In some ferromagnetic semiconducting materials (Fig 1.1a) magnetism and
semiconducting properties are known to exist simultaneously, such as ferromagnetic and
ferrimagnetic semiconducting spinels and europium chalcogenides 10-11
. These materials
have a periodic array of magnetic element. In practice, these classes of semiconductors
are difficult to fabricate and are incompatible with the current industrial semiconductors
like Silicon and Gallium arsenide. The reason is mismatch in the crystalline structure of
these materials and have low Curie Temperatures (Tc) which is about 100 K.9
Diluted magnetic semiconductors (Fig 1.1c) can be one possible solution of this
problem. When small concentrations of magnetic ions are doped into the non-magnetic
host semiconductors it can give sufficiently high Curie temperature as far as the
theoretical predictions are concerned (Fig 1.1b). This category of semiconductor is then
known as dilute magnetic semiconductor or (DMSs). In recent time many non-magnetic
impurities have also shown ferromagnetism at room temperature.
It provides the possibility of using and studying a variety of magnetic and
magneto-optical phenomena which are seldom present in conventional non-magnetic
semiconductor. Recently a large number of investigations have been focused in the
elaboration of new DMS material in different semiconductor host.12
These efforts have
been made to develop ferromagnetism on room temperature to find a new class of
spintronics devices such as transistors, spin valves, magnetic sensors, light emitting
diodes, non-volatile memory, optical isolators, logic devices and ultra-fast optical
switches. The predicted advantages of these spintronics devices will be of greater
efficiency, higher speed, and better stability, in addition to the low energy requirement
for flipping a spin.4
Chapter 1
4
Fig 1. 1 a) magnetic semiconductor b) a nonmagnetic semiconductor c) a dilute magnetic
semiconductor9
Diluted magnetic semiconductors are the class of materials which have the spin
polarized electrons retaining the semiconducting properties, along with the magnetism.
However, it is essential to enhance Curie temperature well above the room temperature to
make them useful for their application in spintronics devices.9
Besides DMS, multilayered structures of ferromagnetic antiferromagnetic
materials coupled together have a range of application in the spintronics devices such as
magnetic random access memory (MRAM), spin valve etc. The working principle of
these structures is mainly based on Exchange Bias (EB). Exchange bias is the
phenomenon in which the characteristic hysteresis loop which determines the properties
of the magnetic materials is shifted on one side of the origin.
Exchange bias played a vital role in the development of the spin valve.13
Without
this the giant magneto resistant (GMR) read head would not have been possible, and
consequently the storage densities of the modern hard disk drive (HDD) would not have
been technologically feasible. The development of MRAM has also seen considerable
focus. This technology would compete with current SRAM and DRAM technologies and
the development of which would bring the GMR sensor into another large market.
Element 1: Nonmagnetic Element 2: Nonmagnetic Element 3: Magnetic
Chapter 1
5
To understand the operation of the spin-valve a basic understanding of GMR is
required. Thompson13
has recently written an in depth review on this topic. In
Ferromagnetic materials (FMs) spin up and spin down electrons experience different
probabilities of scattering. This is due to a difference in the number of each electron in
the d band, known as a spin-split structure. As such it can be considered that the current
of spin up electrons is separate to that of the spin down electrons, as one current is
favored and the other is not, they are called the majority and minority electrons. In a
simple FMs bi-layer, two spin channels can be considered. In the first case, when the
FMs magnetizations are parallel, one spin channel carries only the majority electrons
whilst the other carries only minority electrons. In such a case the majority electron spin
channel is greatly favored for carrying charge and as such the overall resistance is low. In
the second case, when the FMs magnetizations are anti-parallel, both spin channels must
carry both majority and minority electrons. As such neither channel is favored, as they
have equal resistance, and so the overall resistance is higher.
For a spin-valve device to be possible a pinned and free ferromagnetic (FM) layer
is required. Due to exchange bias this is possible. An antiferromagnetic (AFM) layer is
used to pin one of the FM layers. The exchanged biased bi-layer is then separated with a
spacer that destroys any exchange bias that could arise between the two FM films. This
allows for one of the FM layers to maintain a constant magnetisation, whilst the other is
free to rotate within an applied field.
1.2 Current Status of the Field
The current state of exchange bias was reviewed and challenged by O‘Grady et.
al14
with the proposal of new definitions and explanations for a number of phenomena
associated with the magnetic measurement and characterization of sputtered
polycrystalline thin films of exchange coupled ferromagnetic/antiferromagnetic bilayer
systems in general.
Chapter 1
6
The largest of contributions by O′grady et al was that of the York Protocol14
, a
series of steps in which the magnetic history of samples could be controlled and therefore
reproducible measurements made. This allowed for comparable measurements of effects
that could not previously be compared. This also gave rise to a new definition of the
blocking temperature (TB) as well as a new explanation for the main contributing factors
to the value of Exchange field ( ), both of which will be elaborated on in later sections.
This knowledge has allowed the design of AFM/FM materials for specific applications
and setting conditions.
It is also very important to investigate the reproducibility of the measurements.
Also, the errors and the losses in the magnetic energy to make the devices more efficient
are essential to study. Very small parameters may bring a big impact on the functionality
of the devices e.g. the sample fabrication techniques currently used in the industry is
ultrasonic cutting. This thesis will provide a discussion on the best fabrication techniques
to be applied.
The present thesis is a discussion on the determination of the structural and
magnetic properties, of the synthesis of transition metal doped TiO2 thin films by Aerosol
Assisted Chemical Vapour Deposition (AACVD). There are many advantages of using
this technique such as fast evaporation of precursor, relatively shorter delivery time and
higher deposition rate, low cost and precise stoichiometry.15-17
Due to its versatility to use
a variety of precursors and the reaction environment, this method allows us to control the
size, shape, and size distribution of grains, which are hard to achieve through other
fabrication techniques. Until recently a great attention is given to synthesize diluted
magnetic semiconductor nanostructures from various chemical methods and physical
techniques.18-22
However direct chemical approaches are generally compatible with large-
scale production. Despite the fact that there is huge uncertainty in understanding the
origin of room temperature ferromagnetism in doped semiconductors with different
dopant as for as the small curie temperature is concerned than the theoretically predicted
value.9 This has opened a need to investigate the new synthesis routes to achieve the
Chapter 1
7
theoretical values of the magnetization to make a better understanding of factors affecting
on it.23
1.3 Objectives
The objective of the present research can be summarized as follows.
Investigate the effect of Mn doping on the Blocking temperature (TB).
To study the Sample Shape and fabrication process effect on Exchange Bias (EB)
Grain size effect on Training Effect
Synthesis and characterization of Ni and Co doping in TiO2 by AACVD
1.4 Summary of the thesis
A brief outline of the content in the thesis is listed below.
Chapter 2 is devoted to introduction and literature review on history of
magnetism, the related articles on exchange bias and the description of basic ideas to
establish a background necessary to understand the discussion chapters.
Chapter 3 describes the experimental setup/characterization facilities, working
principals and how their use in current research work was carried out.
Chapter 4 is devoted to results and discussion on the effect of shape and the
fabrication process on exchange bias (EB)
Chapter 5 describes effect of Mn doping in the antiferromagnetic (AFM) layer on
the Blocking Temperature (TB). Effect of grain size on the training effect is also included
in this chapter.
Chapter 6 is devoted to the synthesis of Ni and Co doping in TiO2 by AACVD.
The structure and magnetic properties are also discussed.
Chapter 7 is devoted to conclude the output of the research and the suggestions
for the future work.
Chapter 2
8
Chapter 2
Theory of Magnetism in Exchange Bias and Diluted
Magnetic Semiconductors
2.1. The Magnetic Moment and Magnetisation
Magnetic moment μ=|μ| is the most fundamental property in magnetism. It can be
expressed in terms of an analogy to a simple current loop in classical picture 24
. However,
in reality, the magnetic response of materials is quantum mechanical phenomenon which
arises from spin interaction of unpaired electron. In ferromagnetic materials, the unpaired
spins interact via indirect exchange mechanism and hence give rise to the magnetic
moment.
In bulk materials another most important characterizing parameters is the
magnetisation, M= |M|. Magnetisation is defined as the magnetic moment per unit
volume, μ/V. However, this statement is valid for homogeneous systems only. Actual
systems, such as magnetic thin films used in data storage are rarely homogeneous. Single
domain Co-alloy thin films are used for storage material. The system of Co-alloy thin
films are highly non-uniform as the grains of the thin film are decoupled from one
another which is a need to increase the storage capability.
2.1.1 Origins of Magnetism in Bulk Materials
The magnetic materials can be divided into three distinct classes owing to their
response in an external field. The weakest form of response is known as diamagnetism
and arises due to change in orbital velocity of electron in external field HA; hence all
materials with paired core electrons possess a diamagnetic component. Hence all
materials are diamagnetic. The change in velocity opposes the field that causes it and
Chapter 2
9
therefore susceptibility χ, the ratio of magnetisation to field, is negative. The diamagnetic
response is linear, independent of temperature and decreases with the magnitude of the
applied field. In general, the effect is very weak but in small particles and thin metallic
films it can be significant.
Another class of magnetic materials is known as paramagnetic materials. Like
diamagnetic behavior, the response to an applied field is positive although it is relatively
weak. Paramagnetism is temperature dependent in that the susceptibility χ, the ratio of
magnetisation to field, varies with the inverse of temperature. The origin of
paramagnetism is the fixed dipole moments atom or molecule in a material from there
being no complete cancellation of the spin and orbital components of angular
momentum25
. In the absence of any external field, all the moments are randomly oriented
and so the net magnetisation is zero. Eventually the moments will all align, typically at
high field ~ 10T and low temperatures T=4.2K. However, the degree of alignment is
limited owing to thermal effects that will attempt to randomize the orientation of the
moments and hence the paramagnetic effect is weak but increases with the magnitude of
the applied field.
Ferromagnetic materials (Fe, Co and Ni) are the most striking manifestation of the
magnetism due to their spontaneous magnetization that does not require an applied field
in order to be induced. The magnetic moment μ arises from the ordering of the atomic
spins via quantum mechanical exchange coupling between them. A ferromagnet is
characterized by their most prominent phenomenon known as hysteresis (Fig 2.1).
Hysteresis, from the Greek hysteresis, describes the situation where an effect lags behind
its cause. In the field of magnetism, this is manifested in the lagging of the magnetisation
M behind a swept applied field HA and it is this behavior that is central to the use of
ferromagnetic materials as data storage media.
Chapter 2
10
Fig 2.1: Schematic representation of a magnetisation curve for a typical ferromagnet,
showing the hysteretic behavior and the main characterizing parameters26
For a typical hysteresis loop plot, the applied magnetic field A and normalized
magnetisation M/Ms are used as the abscissa and the ordinate respectively. The
normalization is important to eliminate the large error associated with measuring the
magnetically-active volume of a thin-film sample. A complete loop contains two
important parameters as indicated in Fig 2.1: the remanent magnetisation (remanence) Mr
and the coercive field (intrinsic coercivity) c. The properties of both can be used to
assess the suitability of a material for use as a potential storage medium. The remanence
constitutes an effective ‗memory‘ for the material and the coercivity is the field required
to reduce the magnetisation to zero.
2.1.2. Thin Film Magnetism
In the initial applications of thin-film recording media, the reversal mechanism
was primarily dictated by an irreversible process induced by domain wall motion and the
associated pinning of the domain walls by strains27
or inclusions28
. With the rapid
advancement of recording media technology, the reduction in grain size has seen a move
towards single domain-type grains which has also resulted in an enhanced coercivity. In a
medium comprising single domains with uniaxial anisotropy, Stoner-Wohlfarth type
Chapter 2
11
behavior results and therefore there is a clear switch between two magnetisation states.
In the ideal Stoner-Wohlfarth case, a square hysteresis loop will come as a consequence
of having a system of moments with near-perfect orientation. Such a requirement is hard
to achieve in longitudinal media where all the moments lay in-plane and a circumferential
texture is necessary. It is not possible to obtain such a texture that will give the optimum
alignment.
2.2 Demagnetizing Field
According to Maxwell‘s equations for magnetism the magnetic field H and flux
density (|B|) must be continuous in both materials and free space. This is also known as
law of continuity of flux. When a field is applied on a piece of magnetic material it is
magnetised along a certain direction, north and south ‗poles‘ will form at each end. Thus
a magnetic field opposite to the applied field will arise. Due to its direction, this field is
known as the demagnetizing field HD. The magnetizing field depends both on the
magnetisation of the material and its shape. The demagnetising field can be calculated
from
(2.1)
Where M is the magnetisation of the sample and ND is demagnetising factor
which depends upon the geometry.
2.3 Magnetic Anisotropies
Due to the bonding of the ions in the crystals the spins on the atoms cannot take
up all orientations. Electron orbits in the solids are fixed by the crystal field and spins are
coupled to the orbits via L-S coupling. Hence to orient the spins requires that either the
L-S coupling is overcome or the hard movement. This means that the properties vary
along crystal axes due to varying atomic separation.
Chapter 2
12
2.3.1 Magnetocrystalline Anisotropy
Magnetocrystalline anisotropy is where a magnetic material is more easily
magnetised along a particular crystallographic direction. The origin of this effect is in the
spin-orbit coupling and the coupling of orbital magnetic moments to the crystal lattice.
The orbital magnetic moments are quenched so there is no orbital contribution to the
atomic magnetic moment. Consequently large magnetic fields will have no effect on the
orientation of the electron orbit. The orientation is therefore very strongly fixed to the
crystal lattice. When a field is applied, the electron spins will try to align. But as the spins
are coupled to the orbital angular momentum which is in turn fixed to the lattice, the
material will resist the reorientation of the spins. Therefore an anisotropic material will be
easier to magnetise along a certain crystallographic direction known as the easy axis, and
the axis where magnetisation is hardest defined as the hard axis.
2.3.2 Exchange Anisotropy
Exchange anisotropy was first discovered by Meiklejohn and Bean (1956).29
Ferromagnetic Co particles with diameter of ~20nm were oxidized forming a shell of
antiferromagnetic CoO around a ferromagnetic Co core. When the Co/CoO particles were
heated above the Néel temperature, TN, of the AFM CoO their magnetic properties were
the same as Co particles. However when the Co/CoO particles were cooled from 300K to
77K though the TN of the CoO, TN = 293K, with a saturating magnetic field applied the
hysteresis loop of the particles was shifted as shown in Fig 2.2.
Chapter 2
13
Fig 2.2: Hysteresis loop of CoO/Co particles after sample was cooled through TN
of CoO in a saturating magnetic field29
The shift in the hysteresis loop is called exchange bias. This type of anisotropy
exists due to the exchange coupling of ferromagnetic and antiferromagnetic materials
whether in the form of particles or thin films. The moment existing on the interface
opposes the reversal direction. Hence the coercivity is not the same in each direction and
the loop is no longer symmetrical. In extreme cases loop can shift entirely towards the
field cooling side.
2.3.3 Shape Anisotropy
In some materials, especially in polycrystalline thin films, lack of preferred grain
orientation gives rise to absence of crystal anisotropy. In such a case, shape of the
particles or grains gives rise to demagnetising field. A finite sample exhibiting poles at its
surfaces leads to a stray field outside the sample which results in a demagnetizing field
inside the sample. In the presence of its own stray field, energy of a sample is given be
the eq,26
Chapter 2
14
(2.2)
Where, is the demagnetizing field inside the sample. The solution of this eq is
much complicated for general shape. For symmetric shapes, it follows this way,
An ellipsoid contain demagnetizing field given in equation (2.1)
The stray field energy is thus given as
(2.3)
(2.4)
Where, is the volume of the sample.
Using these equations and taking the dimensions into account, the stray field energy
density for long cylinder can be calculated by using eq,
(2.5)
For infinitely extended thin sheet with a = b = , the stray field energy density amounts
to,
(2.6)
For thin magnetic films and layers, the above eq. can be rewritten as
(2.7)
With
At θ=90 , the stray field energy gets the minimum value of energy. This means that the
shape anisotropy dominates the magneto crystalline anisotropy in thin films which result
in, in-plane magnetization of the thin films.30
Chapter 2
15
2.3.4 Other Contributions to Anisotropy
Anisotropy can also be induced by stress in some specific direction, annealing in a
magnetic field or due to plastic deformation.
2.4 Magnetic Domain
Different regions of a macroscopic system break symmetry in different ways.
Weiss26
proposed that a ferromagnet contains a number of small regions called domains,
with each of which the local magnetization reaches the saturation value. Domains are
separated by domain walls. Domain structure is a natural consequence of various
contributions to the energy, exchange energy ( ), anisotropy energy ( ),
demagnetizing energy ( ) and Zeeman energy ( ). So the total energy without an
external field will be
(2.8)
If there is no external field the = 0 and = constant
(2.9)
If no dipole setup at the surface of a ferromagnetic material, there would be no
domains. Domain form solely to minimize the magnetostatic energy, that results when
m.n≠0 at an interface. Fig 2.3. shows how the domains are formed and how the energy of
the system minimizes.
Fig 2. 3: Domain formation and minimization of energy26
In Fig 2.3. a) Single domain is formed as a consequence of magnetic dipole
formed on the surface of the crystal. This configuration will have the energy as
a) b) c) d)
Chapter 2
16
(2.10)
i.e. if as is opposite to
So is positive
In case of Fig 2.3 b) the magnetic energy is reduced to ½ of a) and in case of c) if
there are N number of grain, the energy will reduce 1/N of a). In fig d) and e) the domain
arrangement is in such a way that the magnetic energy is zero. So domain always has its
origin in the possibility of lowering the energy of the system by going from a saturated
configuration with high energy to a domain configuration with lower energy.
Because of the domain walls formation, the demagnetizing energy decreases
but domain wall energy decrease which is equal to the sum of and .
The domain walls can be classified according to the angle between them. The
magnetization in the two domains lies as under. A domain 180 domain wall separates
domains of opposite magnetization. A 90 domain wall separates domain of
perpendicular magnetization. The most common of the domain walls is Bloch wall in
which the magnetization rotates in a plane parallel to the plane of the wall. Another
possible configuration is the Neel wall in which the magnetization rotates in a plane
perpendicular to the plane of the wall.
2.5 Reversal Mechanisms
The process of magnetisation reversal in ferromagnet generally follows one of
three possible mechanisms. The first of these is the mechanism of coherent rotation that
occurs in single domain particles either in powder form or in the form of a thin film
where the individual particles are not exchange coupled. Here the mechanism of
reversal is by coherent or incoherent rotation of the atomic moments over an energy
barrier determined by the anisotropy energy density and the grain volume. Generally
the energy barrier in zero field is of the form
Chapter 2
17
ΔE = KV (2.11)
However in small elements or at the edges of even large samples the energy barrier is
modified by shape demagnetising effects so that even in zero field
ΔE = KV′ (1-Hd /HK) 2
(2.12)
Where, V′ is now the element volume or the volume of an asperity at the sample
surface.
2.5.1 (Coherent reversal) Stoner Wohlfarth model
In 1948 Stoner and Wohlfarth predicted that reversal mechanism of magnetisation
in uniaxial single domain particles is by rotation. The model was based on the assumption
that the spins of the atoms in these single domain particles remain aligned during the
reversal process. Stoner-Wohlfarth model of magnetization reversal gives the idea of
coherent rotation during the reversal process in single domain particles.
Fig 2.4: Coherent rotation of magnetisation by rotation – the Stoner-Wohlfarth model.31
Chapter 2
18
For the case of an ellipsoid, with minor and major axes a and c respectively, an
applied field A will attempt to rotate the magnetisation vector Ms away from the
preferred easy axis direction (along c). This is resisted by some kinds of anisotropy of the
particle usually the shape, stress, or crystal anisotropy, or some combination of these. The
form of rotation of Ms is known as coherent reversal and is ultimately dependent upon
the precise alignment between the applied field and easy axis direction. In addition,
thermal activation will affect the nature of reversal for any system that is not at absolute
zero. The non-stationary nature of the grains at a finite temperature will entail that perfect
alignment is never achieved and so the grains in the system will possess a distribution of
energy barriers.
Stoner-Wohlfarth particles can provide insight into the reversal behavior of both
continuous and patterned perpendicular media in an applied field. This is because the
grains in such a medium are single-domain in nature and are exchange decoupled from
each other.
2.5.2 Incoherent Reversal
The Stoner-Wohlfarth model of coherent reversal is remarkable; however, it is not
successful in describing the reversal properties of some real materials, especially in the
case of multilayer thin films, where the domains have a very little or almost no
elongations. Fig 2.5 shows the coercivity data of system of non-interaction single domain
particles given by Stoner-Wohlfarth model.
Chapter 2
19
Fig 2.5: Variation of coercivity with axial ratio using the Stoner-Wohlfarth model of
reversal.31
Coercivity data due to shape anisotropy for aligned elongated particle for Stoner
Wohlfarth type of coherent reversal is given as
c = (Na-Nc)Ms (2.13)
Where Na and Nc are the demagnetising factors for short and long axes.
The calculated values of c for elongated iron particles by above equation is given by
B.D. Cullity26
in reference book reproduced in table 2.1
Table 2. 1: calculated values of c for elongated iron particles26
Shape anisotropy
c/a
Na-Nc
(SI)
Coercive Field
c (Oe)
1.0 9 0
1.1 0.075 810
1.5 0.301 3,240
5 0.833 8,950
10 0.939 10,100
20 0.980 10,500
1 10,800
Chapter 2
20
As can be seen in above in table 2.1, the observed value of c is 10,100Oe for c/a ratio of
10 and 10,800Oe for infinite elongation of iron particles. However, later on, Luborsky32
showed that c cannot exceed more than 1800Oe for c/a above 10, which is less than
20% than the theoretical value. This contradiction imposed a need to study other
incoherent rotation modes that would give a better match of experimental coercivities.
The reassessment of the coherent reversal by rotation process was initiated by
experimental coercivity data on systems of fine particles being considerably lower than
the predicted values33
. In an attempt to refine the description of magnetisation reversal in
a fine particle system, Jacobs and Bean proposed the 'chain of spheres' model. In this
model, the reversal occurs via a so-called incoherent mechanism known as fanning.
Fig 2.6: Fanning (a) and coherent (b) reversal modes for an N = 2 chain of spheres.33
Jacobs and Bean proposed a ―chain of spheres‖ model for ―peanut shaped‖ particles
which were seen on electron microscopy in their experiment on iron particles as shown in
the Fig 2.6.33
They modified expression for the intrinsic coercivity brings the predicted
values more in line with that observed in experiments.
In their model they suggested two possible mechanisms of reversal
Chapter 2
21
1. Symmetric fanning in which Ms vectors in the adjacent spheres have the
alternate directions. (Shown in Fig 2.6 a)
2. Coherent rotation in which Ms Vector lies parallel.
The coercivities calculated are compared to as predicted by Stoner-Wohlfarth model in
prolate spheroid. In this way they modified expression for the intrinsic coercivity brings
the predicted values more in line with that observed in experiments.
In 1957 Frei et al.,34
proposed another model of incoherent rotation based on the
micromagnetics calculation of shape anisotropy. The model is based on the assumption
that the particle is a single domain with all spins initially parallel to the +z direction in
zero fields. Crystal anisotropy and thermal effects are ignored in this type of reversal
mode. The main observation of this mode of incoherent reversal mechanism is that the
coercivity strongly depends upon the shape and size of the particle. They found that there
is a critical size of the particles below which, coherent rotation is favored. The particles
above that critical size will reverse by curling.
A number of models were put forward, such as the row-reversal model35
(Andrä
et al., 1984), various mean-field theories (Wuori and Judy, 198436
) and theories involving
domain wall motion (Wielinga et al., 198237
; Andrä and Danan, 198738
). However, many
of these interpretations failed to capture all of the experimentally observed phenomena,
be it incorrect values for the coercivity or the unlikely existence of a comparatively large
domain wall contained within very small grains.
2.5.3 Domain wall pinning mechanism or domain nucleation
The alternative mechanisms that initiate reversal are either a domain wall
pinning mechanism or domain nucleation. In the case of domain wall pinning a reverse
domain may already exist within the sample after it has been saturated and the field
reduced to zero. The reverse domain will have been injected due to the magnetostatic
energy of the sample or simply by the effects of thermal energy acting on the sample. A
Chapter 2
22
dispersion of easy axis orientations will aid this process as the moment returns to the
easy axis after having been saturated in the direction of the applied field. As larger
reverse fields are applied generally reversal proceeds by domain wall motion
throughout the sample but where strong pinning sites exist within the material, for
example at grain boundaries or at defects or inclusions, further reverse domain
nucleation may occur where it is energetically favorable. For an overview of reversal
processes see 26
.
Certain materials and particularly those having high anisotropy such as NdFeB
or SmCo5 exhibit a different form of hysteresis due to the very high anisotropy and the
consequent difficulty of nucleating the reverse domain when a strong easy axis
alignment is present. Under these circumstances a saturated material remains at
saturation even when the field is reversed to zero and a reverse domain cannot form
until a nucleation field Hn is reached. The value of Hn is then greater than any domain
wall pinning fields within the material and hence the reverse domain, once nucleated,
sweeps out across the sample resulting in an almost perfectly square hysteresis loop.
Such materials are known as nucleation controlled magnets 39
.
Nucleation controlled hysteresis is not confined to permanent magnet materials
or other hard materials. Similar effects are observed in very soft materials such as
Permalloy in thin film form. Here the fact that the magneto static energy in the plane of
the film is very low means that a significant reverse applied field is required to nucleate
a reverse domain. Due to the low anisotropy of the alloy and the strong exchange
coupling between the grains, the domain wall is again able to sweep through the sample
resulting in a square hysteresis loop 40
.
The energy barrier to domain wall nucleation depends upon a number of factors.
However in a granular material the nucleation process is similar to that involving a
reversal of a region of volume V over an energy barrier in a similar manner to that
which occurs in a single domain particle and to a first approximation can also be
Chapter 2
23
described by equations 1 or 2. Hence for materials where the reversal process is
dominated by nucleation or even in the case where ongoing reverse domain nucleation
occurs as domain walls sweep through the sample, the effects of local demagnetising
fields can be of critical importance.
Other than for perfect ellipsoids of revolution, the demagnetising field in a
sample is generally non uniform. This non uniformity can be manifest, for example at
the corners of square samples, or where any sample edge roughness occurs due to the
process by which the sample was fabricated. Such effects are well known when samples
of NdFeB are produced for measurement. Here dramatic reductions in the coercivity
can be observed unless samples are produced with very smooth surfaces to prevent
nucleation on surface asperities and other defects41
.
2.5.4 Reversal process in EB thin films
One of the most prevalent is so-called domain wall-assisted reversal. The reversal
process is as follows: an applied field induces the nucleation of a domain in the soft layer,
followed by its propagation towards the interface with the hard layer where it is pinned
and compressed. This compression continues until the field is sufficient to de-pin the
domain wall, allowing its propagation through the hard layer. For example, this type of
behavior has been modeled for a single FePt (hard)/Fe (soft) grain 42
. Of course, such a
mechanism is only possible if a domain wall can fit inside the layers. It has been
suggested that such structures can offer advances over conventional perpendicular media
in terms of data density 43
. Experimental measurements of the coercivity and remanent
coercivity have been undertaken to support this reversal hypothesis 44
.
Goodman et al.,45
proposed a comprehensive detail of reversal process in
exchange bias thin films (Fig 2.7)
At point 1 in the hysteresis loop the ferromagnet (FM) is fully saturated and
contains a single domain.
Chapter 2
24
At point 2 the reversal of the pinned layer is initiated. At this stage in the
hysteresis curve the nucleation of reverse domains in the ferromagnet is indicated
by degree of saturation at point 1.
At point 3 the domains nucleated at point 2 are growing. This growth is promoted
by applied field, the coupling field within layer and characteristic thermal
activation process. The coercivity at point3 exhibits normal magnetic viscosity.
At point 4 the ferromagnet is now saturated in negative bias and thermal
activation process in antiferromagnetic (AFM) are driven by exchange field from
FM layer. Goodman et al.45
proposed that at this point there is a growth of
domains within the AFM.
At point 5 it would be expected that FM layer would reverse. However, this is not
the case, to explain this, additional energy barriers have to be applied in the
system. Goodman et al.(2001)45
proposed that those energy barriers arose from
the pinning of domain walls in AFM. This results in the introduction of an
additional effective anisotropy in the FM via the exchange between the two
layers.
At point 6 the reversal of the FM layer starts. There is a difference between the
nature of the reversal at point 2 and 6. Goodman et al.45
proposed some initial
domain nucleation between 5 and 6. These domains could develop either via
domain wall movement over pinning sites or domain rotation. The viscosity at
point 4 suggests that pinning had to be significant.
At point 7 anomalous behavior of the second coercivity was observed. Goodman
et al.(2001)45
proposed that c 2 was intermediate and critically depends on the
amount of the time spent at.
Chapter 2
25
Fig 2.7: Magnetization Reversal in Exchange Bias Thin Films45
Craig et al.,46
studied the domain wall nucleation and its propagation with the help of
Lorentz microscopy (Fig 2.8)
Fig 2. 8: Magnetization Reversal in soft magnetic film46
Chapter 2
26
2.6 Exchange Bias
In 1956 Meiklejohn and Bean29
observed a new type of magnetic anisotropy in
compacts of oxidized Co particles. It was described by them as exchange anisotropy, or
exchange bias, as the effect arose due to the interaction of the ferromagnetic (FM) Co
particles with their anti-ferromagnetic (AFM) CoO shells when field cooled through their
Néel temperature ( ). In over 50 years since the discovery, numerous materials have
been made, and measured, that displayed this effect to various degrees. However there is
still lack of a comprehensive theory that can explain all the pragmatic affects such as the
hysteresis loop shift ( ) and the enhancement of coercivity ( c), which is described as
half the loop width.
2.6.1 Earlier Theories
A number of models attempted to explain the exchange bias phenomena. The first
attempt was made by Meiklejohn and Bean 29
. Their model was based on their studies of
single domain oxidized Co particles having uniaxial anisotropy and an easy axis in the
direction of the applied field. This model was based on the assumption that AFM spin
structure is perfectly uncompensated at the interface. Other assumption is that these
uncompensated spins remains aligned along the easy axis due to a large anisotropy in the
AFM. Though this model is successful in other exchange bias systems47-49
, however, it
predicts two orders of magnitude larger than observed values of in smaller grain size
polycrystalline films. The second model, chronologically, was that of Néel 50
in which he
proposed an uncompensated AFM spin structure at the interface. However he pointed out
that during the reversal of the FM layer the AFM spin structure at the interface would be
subject to irreversible changes, therefore the values of and c would change by the
changes in the AFM spin structure during reversal of the FM layer. This model too
predicts unreasonable values for . A more successful, model was that of Fulcomer and
Charap 51
in which it was predicted that changes in the AFM due to thermal activation
would occur due to the exchange field from the FM layer. This model predicts a
Chapter 2
27
distribution of particle sizes and shapes in AFM. The model gave good agreement of
temperature dependence of and c with experimental observations.52
Some other
granular models were also proposed on the basis of assumptions proposed by Fulcomer
and Charap.53-54
More recently Xi55
extended Fulcomer and Charap model to describe
thermal effects. However this model is restricted to single grain volume and do not fits
well for real systems.
However, models discussed above and many other attempted models failed to
exchange correct values of EB in real systems and to provide a road map develop new
materials for practical applications. Major shortcomings in all previous models are that
none of them could explain effect of film thickness and grain size, role interface and
grain size on EB.14
The reasons for failure of models are that none of them considered possible
thermal instability and initial degree of order in AFM layer. Study of wide variety of
systems is another reason for the failure of models.
In 1999 Berkowitz and Takano 56
gave a comprehensive review of the field. They
also proposed that a successful model of the exchange bias phenomena would have to
answer a number of problems. In answer to this O‘Grady et al. 14
proposed new model for
the exchange bias in polycrystalline thin films which can answer the majority of the
problems. As such it is this theoretical model that this study will utilize.
2.6.2 Technological Importance
Exchange bias played a vital role in the development of the spin valve 13
. Without
this the giant magneto resistant read head would not have been possible, and
consequently the storage densities of the modern hard disk drive (HDD) would not have
been technologically feasible. The development of magnetic random access memory
(MRAM) has also seen considerable focus. This technology would compete with current
Chapter 2
28
SRAM and DRAM technologies and the development of which would bring the GMR
sensor into another large market.
To understand the operation of the spin-valve a basic understanding of GMR is
required. In FMs spin up and spin down electrons experience different probabilities of
scattering. This is due to a difference in the number of each electron in the d band, known
as a spin-split structure. As such it can be considered that the current of spin up electrons
is separate to that of the spin down electrons, as one current is favored and the other is
not they are called the majority and minority electrons. In a simple FM bilayer two spin
channels can be considered. In the first case, when the FM‘s magnetizations are parallel,
one spin channel carries only the majority electrons whilst the other carries only minority
electrons. In such a case the majority electron spin channel is greatly favored for carrying
charge and as such the overall resistance is low. In the second case, when the FM‘s
magnetizations are anti-parallel, both spin channels must carry both majority and
minority electrons. As such neither channel is favored, as they have equal resistance, and
so the overall resistance is higher. Thompson 13
has recently written an in depth review of
this topic.
For a spin valve device to be possible a pinned and free FM layer is required. Due
to exchange bias this is possible. AFM is used to pin one of the FM layers. The
exchanged biased bi-layer is then separated with a spacer that destroys any exchange bias
that could arise between the two FM films. This allows for one of the FM layers to
maintain a constant magnetization, whilst the other is free to rotate within an applied
field.
2.6.3 Recent development in the field of EB
The current state of exchange bias was reviewed and challenged by O‘Grady et
al.14
with the proposal of new definitions and explanations for a number of phenomena
Chapter 2
29
associated with the magnetic measurement and characterization of sputtered
polycrystalline thin films.
The largest of contributions by O'Grady et al 14
was that of the York Protocol, a
series of steps in which the magnetic history of samples could be controlled and therefore
reproducible measurements made. This allowed for comparable measurements of effects
that could not previously be compared. This also gave rise to a new definition of the
blocking temperature ( B) as well as a new explanation for the main contributing factors
to the value of , both of which will be elaborated on in later sections. This knowledge
has allowed the design of AFM/FM materials for specific applications and setting
conditions.
2.7. York Protocol
2.7.1. Theory
2.7.1.1. Grain Size Distribution
There are a number of probability densities used to describe experimental grain
growth and grain size distributions in thin films. Of these it has been show that the grain
size distribution in polycrystalline thin films follows a log-normal distribution 57
.
Furthermore, the log-normal distribution only returns positive values, which is necessary
for the description of grain sizes. Another advantage is that if a variable D follows the
log-normal distribution then so must both 2 and
3 which is especially significant for
magnetic materials as their properties are volume dependant. For a linear interval, dD, the
log-normal distribution is written as:
(2.14)
Where D is the grain diameter, σ is the standard deviation of ln and μ the mean of ln .
The standard deviation of the log-normal distribution is, however, given by:
(2.15)
Chapter 2
30
While the mean grain diameter is:
(2.16)
Both of which will be required to give a full physical description for comparisons.
In order to make a quantitative analysis of the experimentally found grain sizes,
the experimental histograms must be fitted using the log-normal distribution function.
The most direct method of achieving this would be to use standard data analysis software,
calculate σ and μ then recompose equation (2.14) and directly fit the experimental data.
However this requires a large number of grains, ≥1000 grains, to make a reproducible
distribution 57
. Vopsaroiu et al. 57
proposed the usage of the cumulative percentage
method (CPM). The advantage to this is that a much smaller number of grains are
required to form a reproducible distribution. Through this method σ and μ can be
calculated and then used to recompose equation (2.14).
The cumulative percentage data for a specific grain size is calculated using the
percentage undersize sum , where is the percentage of the total number of
particles with a given grain size diameter . This is then plotted against ln . As ln is
normally distributed it is possible to graphically calculate μ as it corresponds to 50% of
the cumulative percentage. It is similarly simple to calculate σ using:
(2.17)
Where, 84 and 16 correspond to the 84% and 16% points on the cumulative
percentage curve. Armed with both σ and μ the log-normal function can be generated and
fitted to the experimental data. 57
Chapter 2
31
2.7.1.2 Blocking Temperature
Conventionally the blocking temperature con is described as the temperature at
which the exchange bias reduces to zero. To determine con the procedure was based
on the idea of taking hysteresis loops whilst increasing the temperature until such a point
that loop shift became zero. At temperatures above con the exchange bias remains
equal to zero. Fulcomer and Charap 51
stated that the value of con would correspond to
the individual blocking temperature of the AFM grain with the largest anisotropy energy.
Following from this it can be said that each AFM grain in a polycrystalline system has its
own blocking temperature and so the bulk AFM is characterized by a distribution of
blocking temperatures. While using the conventional method of measurement, the AFM
would be prone to thermal activation during measurement at a logarithmic rate 58
. This
lead to a lack of reproducibility due to changes in the state of order of the AFM from
measurement to measurement.
O‘Grady et al.,14
demonstrated that due to the thermal activation of the AFM layer
it was possible to shift the hysteresis loop of an exchange biased system to the opposite of
that from when it was set. They further showed that using a specific set of procedures,
known as the York Protocol, this could be specifically controlled to measure the mean
blocking temperature . Thus by heating the FM layer whilst reversed the AFM
would experience changes in order from the original state to the opposite orientation, as
shown in Fig 2.9. As such they concluded that the amount of the AFM that undergoes
reversal will rely solely on the temperature and the exchange field from the FM layer.
Chapter 2
32
Fig 2.9: Schematic of the energy barrier reversal, showing the proportion of AFM grains
set parallel or anti-parallel to the original set direction.14
The York Protocol thus gives a different definition of < > and O‘Grady et al.
define it as ―The point where equal fractions of the volume of the AFM grains are
orientated in opposite senses‖. As such the value of can be described as proportional
to the difference between the fractions of AFM grains that are orientated in opposite
directions:
(2.18)
Fig 2.10: Comparison of Blocking Temperature (TB) measured from a) Conventional
method b) York Protocol14
Chapter 2
33
2.7.1.3 Measurement of EB from York Protocol
O‘Grady et al.,14
demonstrated that in an exchanged biased system the value of
is dependent on the amount of AFM grains that are thermally set. However as for
IrMn, the AFM used in most technological applications, is far higher than room
temperature it is not possible to fully set the AFM without damaging the structure of the
sample. As such the sample has to be set below , in which the setting process is via
thermal activation. Fig 2.11 demonstrates a situation where a sample has been set at a
temperature set for a time in which set was not sufficient to set the entire AFM
distribution. As such a fraction of the AFM grains with V>Vset are not aligned with the
FM layer, whilst a fraction of AFM grains with V<V were thermally unstable at the
temperature of measurement Tms. Consequently only the grains of volumes between VC
and Vset will contribute to . O‘Grady et al. assumed a constant value of KAF, the AFM
anisotropy constant, and wrote the proportion of:
Fig 2.11: Schematic of the grain size distribution after the setting of the AFM and cooling
to a temperature at which the AFM is thermally unstable.14
(2.19)
Chapter 2
34
It has been shown in practice that the application of a higher field during the
setting process increases the value of by orders of up to 20% and more. This implies
that the applied field somehow affects the interfaces while not affecting the bulk material.
O‘Grady et al. 14
showed there to be a correlation between the texture of the AFM and the
composition of the FM layers at the interface. Although they had insufficient data to
define the origin of this effect they were able to write an equation linking with the
degree of order and stability of the AFM and the behavior of interfacial coupling:
(2.20)
2.8 Training Effect in Exchange Bias
Training effect is one of the direct consequences of the exchange bias. It is found
that that with repeated cycling of the applied field there is a reduction in the exchange
bias in FM and AFM coupled system. The decrease of the shift of hysteresis loop with
consecutive cycles of the applied field is called training effect. Training effect may also
come from the change in the shape of the loop and change in the coercive field.
The training effect was discovered by Paccard et al59
in 1966 in the consecutive
loop of Co-CoO system. He found that the shift in the first and second loop is more
significant than the others.
Fig 2. 12: Consecutive hysteresis loops of a Co − CoO system measured with torque
balance. The observed overshoot is an instrumental effect.59
Chapter 2
35
He also found change in the shape of the loop and decrease in coercivity as shown
in the Fig 2.12. Thus training effect can be categorized in two classes. One is due to the
shift of 1st two measurements, which is more pronounced. The reason is the non-
equilibrium state of AFM caused during the cooling process. Second type of the training
is due to the further shift of the loop other than first two loops, which decreases with
increasing number of cycles. Paccard et al 59
shown that 2nd
type of training obeys inverse
power law. Fluctuations of the FM-AFM coupling, due to spins reconfiguration or the
domain state of the AFM, during consecutive cycles, is the cause for the 2nd
type of
training.
This occurs in both single crystal and polycrystalline thin film samples although
the effect is more important in polycrystalline systems60
. Training has been observed in
polycrystalline IrMn thin films such as those studied here 61.Training effect is important
due to the fact that it is the measure of the stability of exchange bias bilayer which is
inside many devices.
Hoffman (2004)62
, for the first time, put forward a theoretical model describing
the training effect successfully. He showed that training on the first loop may be a result
of multiple easy axes for the AFM layer62
. This model described that for AFM with
biaxial anisotropy there will be training effects between the first and second measured
hysteresis loops, however he ruled out any possibility of training effect in AFM
anisotropy that has uniaxial symmetry. This is in accordance with experimental data
where systems with an AFM having multiple easy axes (eg. cubic anisotropy), such as
IrMn, training is observed. However no training observed in FeF2 having AFs with
uniaxial anisotropy 63 .
Chapter 2
36
Fig 2. 13: Schematic showing the FM and AFM sublattice magnetizations in an exchange
bias system where the AFM anisotropy has biaxial symmetry during the 1st and 2nd
hysteresis loop measurements. 62
In Hoffman‘s model the training effect is caused by the AFM spins which are
non-collinear initially, relaxes into a collinear arrangement after the reversal of the field.
This is shown schematically in Fig 2.13
Another study of training in FeMn/CoFe systems by Fernandez-Outon et al64
showed that there were two contributions to the training effect. By measuring the training
at a temperature where the AFM grains that make up the AFM layer were thermally
stable, TNA, an athermal training effect where only the first branch of the hysteresis loop
shifts between the first and second hysteresis loops is observed. When the sample was
measured above TNA, both branches of the hysteresis loops shifted due to thermal
activation effect.
2.9 Diluted Magnetic Semiconductors
Diluted magnetic semiconductors are the class of materials which have the spin
polarized electrons retaining the semiconducting properties, along with the magnetism.
However it is essential to enhance Curie temperature well above the room temperature to
make them useful for their application in spintronics devices. Also the origin of
magnetism in such materials is still controversial.
Chapter 2
37
The DMS got much attention after the earliest observation of RT ferromagnetism
in the Co doped TiO2 system by Matsumoto et al.65
. Matsumoto and his coworkers
synthesized Ti1-xCoxO2 anatase films (0 ≤ x ≤ 0.08) on LaAlO3 (0001) and SrTiO3 (001)
substrates by laser molecular beam epitaxy. The same research group reported RT
ferromagnetism in rutile phase Ti1-xCoxO2 (0 ≤ x ≤ 0.05) thin films by same experimental
technique onto α-Al2O3 substrates. Since then, several physical and chemical deposition
techniques were employed to fabricate Co and many other transition metal doped TiO2
systems. Many techniques such as pulsed laser deposition (PLD)19-22, 66
, plasma-assisted
molecular beam epitaxy67-69
reactive co-sputtering70
, metal-organic chemical vapor
deposition (MOCVD)71
, molecular beam epitaxy (MBE)72-74
, and solgel method75
have
been used.
Below are the few theoretical models which are currently available to describe
this phenomenon.
2.9.1 Types of interactions in DMS
2.9.1.1 sp-d exchange interactions
The model of band structure of DMS can be used to start to interpret the source of
their properties. In this band structure two electronic subsystems are compared: one
consisting of the magnetic impurity electrons with magnetic moments localized in the
ionic open 3d (or 4f) shell, and the other containing delocalized, band electrons built
primarily of outer s and p orbital of constituting atoms. The magnetic properties produce
in the DMS system due to the strong spin dependent sp-d(f) exchange interactions and the
localized magnetic moments (d-d) interaction between these two systems.
The localized impurity magnetic moments in DMS and the spin dependent
interactions between band carriers are due to the two autonomous mechanisms, called the
hybridization mediated kinetic exchange and the direct Coulomb exchange 76
.
Chapter 2
38
2.9.1.2 d-d interactions between magnetic ions
DMS with magnetic properties are due to the interactions that couple the spins of
magnetic ions. There are a number of microscopic mechanisms that bring about the spin-
spin (d-d) interactions between two magnetic ions. The interaction can be considered as a
virtual transition between the ions and neighboring anions, in two main mechanisms,
namely the double exchange and superexchange. The spins of two ions are interrelated
due to the spin-dependent kinetic exchange interaction between each of the two ions and
the s, p band in the super-exchange mechanism. The magnetic behavior comes from the
superexchange is due to the dominant spin-spin interaction for group II-VI DMS and
have shown by Larson et al.77
. When the magnetic ions in DMS have the same chemical
but different charge state then the double exchange interaction occurs78
. The virtual
hopping of an ‗additional‘ electron from one ion to the other through interactions with the
p-orbitals shows the coupling of magnetic ions in different charge state related by double
exchange. Zener proposed this mechanism 79
in the 1950s and was applied to understand
ferromagnetism caused by the coexistence of the Mn2+
and Mn3+
in ZnO80
The
suprexchange interactions for many magnetic dopant should actually be
antiferromagnetic, and thus no contribution to the ferromagnetism.
2.9.1.3 RKKY interaction
The ion-ion interaction in DMS only when a high concentration of free carriers is
present could be described by using RKKY interaction. It can be understood as the
interaction between the surrounding electron gas and a local magnetic impurity. The
polarizable medium is the carriers that transmit spin polarization from one atomic site to
another. We can prove the different spin electrons scattered differently, as spin-up and
spin-down electrons feel a different potential in the neighborhood of a spin-polarized
impurity ion. The oscillation of the spin-up electron density is shifted comparative to the
oscillation of the spin down density due to the different phase shift. The superposition of
these two charge densities yields an oscillatory magnetization which decays according to
Chapter 2
39
the dimensionality of impurity considered. Atoms at a given distance from the impurity
experience either a negative or a positive polarization and consequently their orientation
describes their magnetic moment. The carrier induced RKKY interaction, adequately
long range to account for the magnetic interaction in dilute systems, and has been
propose to explain the carrier (hole) induced ferromagnetism observed in IV-VI thin
films and Mn-based III-V81-83
.
2.9.1.4 Zener model
Dietl et al 84-85
shown that , the Zener model has to be invoked to explain the
observed properties of Mn-based II–VI and III–V thin films and heterostructures when
the mean ion-ion distance is small with respect to 1/kF. This is due to the significance of
the kp, carrier-carrier interactions and spin-orbit coupling which are hard to take into
account within the RKKY model. The mean field values of the ordering temperature
deduced from the Zener equals to the RKKY model, when these interactions are
neglected. In Zener model79
, the spin polarization of the localized spins results in a spin
splitting of the bands and in this situation the exchange coupling between the carriers and
the localized spins leads to ferromagnetism. The redeployment of the carriers between the
spin sub-bands lowers the energy of the holes carriers, which at suitably low temperatures
overcompensates an increase of the free energy related with a decrease in Mn entropy.
Dietl et a84
suggested that the holes in the extended or weakly localized states mediate
the long range interactions between the localized spins on both sides of the Anderson-
Mott metal-insulator transition (MIT) in the Mn doped II-VI and III-V DMS. They also
showed that the holes transmit magnetic information efficiently between the Mn spins
due to the large density of states in the valence band and strong spin-dependent p-d
hybridization.
The p-d Zener model has been successful in explaining a number of properties
observed in ferromagnetic DMS, particular (Ga,Mn)As and (In, Mn)As, including the
magnetocrystalline anisotropy 86
, ferromagnetic transition temperature87
etc.
Chapter 2
40
2.9.1.5 Polaron Percolation model
The polaron percolation theory has been developed to understand the
ferromagnetic ordering of the DMS with strongly localized carriers. The formation of
bound magnetic polaron (BMP) results from the exchange interaction of those strongly
localized carriers with magnetic impurities. Since the carrier concentration is much less
than the magnetic impurities density, a localized hole is surrounded by the impurity spins
in a BMP as shown in Fig.2.14 88
. Even though direct exchange interaction of the
localized carriers may be antiferromagnetic, the interaction between bound magnetic
polaron can be ferromagnetic89
if the concentration of the magnetic impurities is large
enough. The localized holes (large arrows) produce an effective field for the impurity
spins (small arrows). Shaded area shows overlap effect of two BMPs on impurity spins.
The maximum of this effective magnetic field is achieved when the spins of the localized
holes are parallel. When the direction of impurity spins is parallel to the effective field,
minimum energy and maximum field are also achieved. Therefore at low temperatures
the system should eventually reach the state where the spins of all holes point in the same
direction, and all impurity spins point in the same or in the opposite direction, depending
on the sign of the impurity-hole exchange interaction.
Fig 2. 14: Interaction of two bound magnetic polarons. The polarons are shown with gray
circles. Small and large arrows show impurity and hole spins, respectively.88
Shaded
region shows the effect of two BPMs on impurity spins.
Chapter 2
41
Taking into account the high defect concentration in a typical magnetic
semiconductor material, the localized charge carrier density in the systems is highly
inhomogeneous too. Since the exchange interaction between magnetic impurities is
transmitted through the charge carriers, this interaction must also be highly
inhomogeneous 90
. When the temperature is lowered, in the regions with higher charge-
carrier density, the ferromagnetic transition will first occur locally (align a spin in parallel
or antiparallel with all the impurity spins in the vicinity), leading to the formation of a
BMP. As temperature falls further, the polaron grows in size until its radius overlaps that
of neighboring polaron, enabling long-range interactions between TM ions and
ferromagnetic ordering in low carrier density systems. BMP begins to form at a certain
temperature and their diameter will increase with decreasing temperature and eventually
spreads over the whole system at the Curie temperature to produce ferromagnetism.
Chapter 3
42
Chapter 3
Experimental
3.1. Synthesis Technologies
3.1.1. High Target Utilization Sputtering (HITUS)
HITUS was used for study of exchange bias effect in multilayer thin film at
Department of Physics, University of York, UK. Details of its working and conditions for
current studies are discussed below:
Exchange bias effects are seen in multiple types of samples each with distinct
morphologies controlled by their formation processes. The most common sample types
are: nanoparticles, which have non-flat interfaces51
, epitaxial thin films, which have near-
flat interfaces91
and sputtered polycrystalline thin films, which have significantly rough
interfaces.92
It is, interestingly, the later that produces the greatest exchange bias at room
temperature.14
Fig 3. 1: Schematic representation of HiTUS sputtering technology.57
Chapter 3
43
Vopsaroiu et al,57, 93
describes a novel sputtering technology that allows direct control
over the grain size of the sputtered sample through close and accurate control of the
growth rate. Magnetron sputtering, an older sputtering method, does not allow sputtering
from thick ferromagnetic targets and has non-uniform wear of the target making it
inefficient for magnetic thin films. Whilst triode and diode sputtering systems may have
the advantage of uniform target wear, they are not suitable for reactive sputtering and
have low deposition rates. As such the sputtering technology as described by Vopsaroiu
et al.57
is given the acronym HiTUS (high target utilization sputtering >90% of the
target). A further benefit to the technology is its ability to both sputter from large
ferromagnetic targets as well as carry out reactive sputtering.57, 93
Samples were prepared using the sputtering system introduced above and shown
schematically in Fig 3.1. The system is based on high intensity plasma which is produced
in a separate arm attached to the main deposition chamber. The plasma is created using a
radio frequency (RF) electric field (max. 2.5kW) and is launched into the main chamber
through the interaction between the RF field and the launch electromagnet. A second
electromagnet is then used to steer the plasma onto the target. Sputtering cannot occur,
however, without applying a negative dc bias (-1V to -1000V) to the target. At voltages
over -100V the target current becomes saturated and independent of the voltage. This
allows for control over the energy of the Ar ions incident upon the target. As the number
of Ar ions is controlled by the energy of the RF field the sputter rate can be controlled by
keeping the RF field constant and varying the bias voltage. This in turn controls the grain
size.57
All samples were sputtered under the same conditions after pumping down to a
base pressure of 2.75x10-3
mbar. The average process pressure was 2.75x10-3
mbar and the
RF power was held at 1.5kW. A rotating substrate table allowed up to six samples to be
grown separately without breaking the vacuum. The design of the substrate table meant a
Si substrate and Transmission Electron Microscope (TEM) grid could be sputtered on
simultaneously. The distance between substrate and target was ~25cm which is quite
Chapter 3
44
large than normal, thus minimizing interaction between them and the film on both
materials was identical.93
Metallic targets (Cu, NiCr, IrMn, CoFe) used for this study
were pure up to 99.9999%. Prior to the sputtering of each layer a 60sec substrate/target
plasma cleaning process was employed to eliminate contaminants and any oxide layers.
Substrate heating was not employed and a temperature below 100°C was maintained.
Additionally, a permanent magnet was placed above the substrate table to attempt to
induce order in the AFM layer during growth.
Sample growth was predominately automated; however substrate exposure had to
be controlled manually. Details of growth conditions were recorded during growth for the
sputtering of each layer. In each series five samples were grown with variable bias
voltage (200,400,600,800 and 1000V) whilst keeping the composition constant.
3.1.2. Aerosol Assisted Chemical Vapor Deposition (AACVD)
Several recent studies suggest that any magnetic ordering in diluted magnetic
semiconductors depends on the synthesis routes and is sensitive to chemical ordering of
the TM ions and defects which may be vacancies or interstitials 94-96
.Many reported
properties of DMS have shown lot of variation in the experimental data depending
strongly on preparation technique and growth conditions. This has created a lot of
complications in interpretation of experimental data. Thus on one hand more studies are
required for the optimized results, on the other hand new and more sophisticated
synthesis techniques are required which are economical.
AACVD is a novel technique in which precursor solution is atomized into fine,
submicron size aerosol droplets that are transferred to heating zone through evaporation.
In heating zone it go through decomposition and homogeneous and/or heterogeneous
chemical reactions to form the desired product. AACVD has an advantage over the
conventional CVD for it overcomes the availability and delivery problems of the
precursors found in the conventional CVD.
Chapter 3
45
Samples were prepared on Si (100) and commercially available soda glass
substrates using the house-build AACVD assembly introduced above and shown
schematically in Fig 3.2. The system is based on a PIFCO ultrasonic air humidifier with a
piezoelectric modulator. Other parts of the assembly are the cylinder of carrier gas which
may be air or any other gas according to the requirements of the experiments. A micro
controller is used to control the gas flow and hence the deposition rate which alternatively
controls the films morphology, porosity and thickness over a certain period of time. The
aerosols are created using PIFCO ultrasonic air humidifier and launched into the heating
zone through with the help of carrier gas where precursor decomposes to form thin films
of the required metal oxide.
3.1.2.1. Advantages of AACVD over conventional CVD
AACVD has following distinctive advantages over the conventional CVD process15-17
i. AACVD enables a fast evaporation of the precursor. This give an advantage
relatively short delivery time to the heating zone and high deposition rate.
ii. Fabrication of multi component films with highly precise stoichiometry
iii. A number of different precursors may be used for AACVD which need not to be
volatile, but just to be merely soluble. Thus eliminating the need of volatile
precursor, where essential for conventional CVD.
iv. Condition of reaction environment during AACVD is flexible. It can be
performed both at low pressure and in open atmosphere.
v. AACVD is a low cost process, it do not need complicated and costly
instrumentation.
vi. Variety of products may be achieved through AACVD such as thin films, nano-
powder and nanotubes etc.
However, AACVD also have certain limitations in comparison with other techniques
e.g.
i. AACVD is not suitable technique to grow thin films/coatings on thermally
unstable substrates 97
Chapter 3
46
ii. Gas phase reaction produces defects like pin holes in the films.
iii. Adhesion of the film may become weak with time.
iv. Difficult to control film thickness.
Fig 3. 2: Schematic representation of AACVD
3.2. Characterization Techniques
3.2.1. X-Ray Diffractometer
The diffractometer determines the identity of crystalline solid based on the atomic
structure of the material. The XRD pattern gives the direct information of two things
i. Relative positions of the XRD peaks give size and shape of the unite cell
ii. Relative intensities of the peaks determine the atomic positions in the unit cell.
In diffractometer when a sample is exposed to the monochromatic x-rays
diffraction occurs when atoms in a periodic array scatters radiation coherently, producing
intensive constructive interference at specific angles. The electrons in an atom interact
with the oscillating electric field of the light wave and produce coherent scattering. In this
way Atoms in a crystal form a periodic array of coherent scatterers. Diffraction from
different planes of atoms produces a diffraction pattern, which contains information about
the atomic arrangement within the crystal. The simplistic model to understand the
conditions required for the interference is Bragg‘s law98
Flow Controller
Car
rier
Gas
Piezoelectric Modulator
Precursor
Solution
Aerosol
s
Glass Jar
Substrate
Hot
Plate
Chapter 3
47
(3.1)
The space between diffracting planes of atoms determines peak positions. The
peak intensity is determined by what atoms are in the diffracting plane.
The XRD used in this study was the PAN analytical, X‘pert PRO operated on
40kV and 22.5mA. The high intensity monochromatic Cu-K radiations
(λ=0.154184nm) source and PSD detector are fitted. The majority of the control of the
system was through an attached computer where scanning parameters could be controlled
using the proprietary software. To increase efficiency and accuracy of the measurements
the intervals between recordings were carefully chosen. A very slow scanning rate of 1
step/sec was applied where each step was 0.02 .
3.2.2 Field Emission Electron Microscope (FESEM)
The SEM takes advantage of the concepts put forward by Ernest Ruska (1906-
1987) and applies them to the potential applications of electrons in microscopy. This
allows high resolution images to be taken at high magnifications with relative ease.
FESEM is a high resolution SEM and lithography system. A cold cathode field
emitter or simply field emission source at the top of the apparatus accelerates the
electrons through a potential difference 20kV. The tube is kept at a constant high pressure
<1x10-7
to decrease the discharge through electron-particle interactions. Upon reaching
the condenser the electrons are condensed into the beam, which is the focused on the
sample by using the objective aperture and the magnetic lenses. This is an important step
as the magnification of the sample is proportional to the distances between the sample
and the plane of the lenses. As the beam is incident upon the sample the electrons are
diffracted by the atoms in the sample. These diverged beams then pass through the next
set of lenses and are focused on the phosphorus screen or charge-coupled device (CCD)
to produce an image.
Chapter 3
48
The FESEM used in this study was the FEI Sirion S-FEG FESEM. Control of the
system was done mostly through the integrated electronics and computers where the
electron gun voltage, position of the image and capturing process could be controlled
using the in-built software and electronics. To help improve the image quality an
additional aperture was added to reduce interference on the image. In addition to this
spectrometry data could be collected from emitted x-rays so as to confirm the
composition of the samples.
The Sirion S-FEG allows a very short working distance of 5mm to help to
increase the image resolution. Higher image resolution can be obtained at a very high
voltage of 20kV using conventional SEI detector or through a combination of the lenses
system. It has a facility to work on low voltage (1-5kV) images for bulk and non-
conducting samples. However, the standard working conditions are 5kV. Energy
Dispersive X-ray Spectroscopy (EDX) is fitted with the FESEM for chemical analysis of
the samples having Z > 4. EDX is operated using Oxford INCA analysis system which is
further upgraded by applying 30mm 2 light element capable ATW detector
3.2.3. Zeiss Particle Size Analyzer
The Zeiss Particle Size Analyzer works by using an equivalent circle method. It
has 48 units of measurements, or bins, which have been calibrated so that each bin
corresponds to a specific diameter. For a measurement to be made a TEM image of the
grains is placed on a light box in which an iris focuses a spot of light. It is then possible
to match the spot of light with an equivalently sized grain. Registration is then made in a
bin for this grain size using an especially written lab view program. The program then
keeps track of the grain sizes within a specified range and creates a frequency distribution
accordingly.
Chapter 3
49
3.2.4. Transmission electron microscopy (TEM)
The TEM takes advantage of the concepts put forward by M. Planck and L. De
Broglie and applies them to the classical light microscope by replacing photons with
electrons. This allows high resolution images to be taken at high magnifications with
relative ease.
An electron gun at the top of the apparatus accelerates the electrons through a
potential difference, 200kV in this instance, down a thin tube in the centre of the device.
This tube is kept at a constant high vacuum so as to decrease the chance of an electron-
particle interaction. Upon reaching the condenser aperture the electrons are condensed
into a beam, which is then focused onto the sample by the objective aperture and lens.
This is an important step as the magnification of the sample is proportional to the
ratio of distances between the sample and plane of the lens. As the beam is incident upon
the sample the electrons are diffracted by the atoms in the sample. These diverged beams
then pass through the next set of lenses and are focused on the phosphorus screen99
or
charge-coupled device (CCD) to produce an image. This is demonstrated schematically in
Fig 3.3.
Chapter 3
50
Fig 3. 3: Schematic Diagram of a Transmission Electron Microscope100
The TEM used in this study was the JEOL JEM-2100 TEM. Control of the system was
done mostly through the integrated electronics and computers where the electron gun
voltage, position of the image and capturing process could be controlled using the in-built
software and electronics. To help improve the image quality an additional aperture was
added to reduce interference on the image. In addition to this spectrometry data could be
collected from emitted x-rays so as to confirm the composition of the samples.
To reduce error ten images at 80,000x multiplication were taken per sample. The
criteria for an image were for there to be 50 or more grains clearly visible. Equivalent
circle method was then implied to study grain size. Two images would be taken per area
on the film so as to obtain an even distribution of grain sizes.
Chapter 3
51
3.2.5 Magnetometers
There are multiple methods for making measurements of the magnetic properties
of materials, each with its own advantages and disadvantages. The first method is the
Susceptometer Magnetometer which is based on Faraday‘s law of induction. By moving
a magnetised sample between two ‗sensing‘ coils a signal is produced which is
proportional to the magnetic moment of the sample. A major benefit to this is that the
susceptometer measures in absolute units of both magnetic susceptibility and moment
and therefore no calibration is required101
. The second method is the Superconducting
Quantum Interference Device (SQUID) Magnetometer. The SQUID magnetometer
operates by measuring the flux change, due to the magnetised sample, on a
superconducting detection coil attached to a SQUID device. The signal that results is
proportional to the magnetic moment of the sample. This method results in exceedingly
high sensitivity; however the system must be operated at superconducting temperatures
101. The third method is that of the Vibrating Sample Magnetometer (VSM). This method
is the most commonly used and versatile method of magnetic characterization and is the
method used solely in this study and so will be covered in more detail in the following
sections.
3.2.5.1. Vibrating Sample Measurement (VSM)
The basic principle of the VSM is if any sample is placed inside a uniform
magnetic field, created between the poles of two electromagnets, a magnetic dipole will
be induced. If the sample is then vibrated with sinusoidal motion then a sinusoidal
electric signal will be induced in the pickup coils. This induced signal will then have the
same frequency of vibration; however the amplitude will be proportional to the magnetic
moment, the amplitude of vibration and the relative position of the sample to the coils 101
.
The setup of this system is shown schematically in Fig 3.4.
Chapter 3
52
Fig 3. 4: Schematic of a standard VSM.
The VSM used in this study was the ADE Vector Model 10 VSM. The majority of the
control of the system was through an attached computer where temperature, field angle
and applied field could be controlled using the proprietary software. To increase
efficiency and accuracy of the measurements the intervals between recordings were
carefully chosen. During magnetic saturation of the FM large step sizes were used, 200
Oe, while during the loop shift small step sizes, 25 Oe, were used. This allowed for an
increase in resolution around the points of interest.
Due to the automation of the system the main sources of error were in how the
sample was placed in the machine. As the sample would be vibrating it was important to
ensure that it was rigidly attached to the sample holder to reduce excess modes of
vibration. This was achieved using both vacuum grease and a non-magnetic tape.
Secondly the ‗saddle point‘ of the field had to be found. Using knobs that varied the
Pickup
Coil
Vibration
direction
Probe body
Hall probe
Sample
Electromagne
t
Electromagne
t
Chapter 3
53
position of the sample it was possible to find the point at which a minimum signal was
detected in both the x and y planes92
.
3.2.5.2 Alternating Field Gradient Magnetometer (AGFM)
The Alternating Gradient Field Magnetometer (AGFM) operates in almost an
opposite fashion to that of the VSM. The sample is attached to a sample holder which is
placed in a magnetic field gradient and, due to Faraday‘s law of induction, a force felt. If
the field gradient is then varied with a frequency equal to that of the natural frequency of
the sample then, the sample will vibrate sinosodially with the amplitude at its maximum.
This vibration will then be detected by the piezoelectric bimorph which is attached, via
quartz legs, to the sample holder. The sinusoidal motion of the sample then induces a
proportional current in the bimorph. Like the VSM the frequency of this signal will be
equal to that of the alternating field; however the amplitude will be proportional to the
magnetic moment, the amplitude of the alternating field and the mass of the sample101
.
The setup of the system is shown schematically in Fig 3.5.
Fig 3. 5: Schematic of a standard AGFM.
Gradient
Coil
Piezo-electric
biomorph
Probe body
Hall probe
Sample
Quartz legs
Electromagn
et
Chapter 3
54
The AGFM used in this study was the MicroMag 2900 AGFM. The majority of
the control of the system was through an attached computer where the frequency and
magnitude of the magnetic field and sensitivity could be controlled using the proprietary
software. The natural frequency of the sample was found when the difference between
two auto-tunes was in the order of 0.25Hz in difference. Due to the speed of
measurement in the AGFM step size was not a concern and so step sizes of 30 Oe were
used.
Due to the fragility of the probe and operation of the system the main sources of
error were dependant on the mass of the sample, the condition of the probe and the
position of the sample between the magnets. The probe had to be handled with extreme
care so as not to damage or break the legs as this would drastically affect the signal.
However the mass of the sample was not an issue due to its small size whereas the
position of the sample was vital. This was controlled in identical fashion to that of the
VSM.
3.2.5.3. Superconducting Quantum Interference Device (SQUID)
Superconducting Quantum Interference Device (SQUID) magnetometer is an
extremely sensitive magnetometer (Typically 10-7
emu) based on the Josephson Effect. It
can give measurements on a range of temperature ranging from 2K (liquid He
temperature) to 350K. The Helium-cooled magnetometer could operate in the range of -
70 to +70 kOe.
RF SQUID design is very much similar to that Josephson junction shown in the
schematic diagram. Josephson junction is based on a superconducting coil sandwiching
an insulator. There is phase coherence in the electron pairs which are responsible for the
zero resistance in the superconductor. Therefore these electrons carrying current travel in
phase throughout the coil. Any minute change in the flux may change the phase of the
electrons. Due to quantum mechanical boundary conditions this phase change must be
Chapter 3
55
integral multiple of 2 i. The change in phase is an estimate in the allowed quantized
value of change in flux throughout the coil. Any small change in the flux can be
compensated by an additional small current. Voltage across the Josephson junction
arising due to the compensating current enables to measure any small change in the
magnetic flux.
Fig 3. 6: Schematic diagram Josephson junction
RF SQUID is based on the Josephson junction coupled with an inductor in an LC-tank
circuit also known as transformer. LC- tank is excited at its resonant frequency by
applying RF current. The sample is mounted at the centre of this LC tank. Mutual
induction between vibrating sample and the flux transformer induced a change in flux.
The corresponding voltage measured across the Josephson junction is used to measure the
magnetic induction of the sample.
3.2.6. Rutherford Back scattering (RBS)
RBS is a multi-elemental analytical technique based on the electrostatic repulsion
between high energy incident ions and target nuclei. Besides elemental analysis of the
sample it can measure properties such as the thickness, chemical composition at the
Chapter 3
56
surface, crystalline quality and contamination. Its depth resolution ability is from 0-1mm.
It is a fast (typically 10 min or less) and non-destructive technique with high precession
of ± 3% with a great sensitivity.
RBS is based on the elastic scattering of light nuclei H, He, Li having energy in
the range of few MeV. These accelerated ions penetrate into the samples where they lose
energy by two different scattering processes which are explained in next paragraph.
Backscattered ions again lose energy while coming out and finally enter into a detector
which can measure number of backscattered ions as well as their energies. Experimental
setup is shown in the schematic diagram
Fig 3. 7: Schematic diagram of RBS basic function
In RBS back scattered ions lose energy by two different scattering processes
i. Scattering with sample nuclei
ii. Scattering through sample electrons
The first process depends upon the scattering cross-section of sample nuclei and
therefore on atomic number and mass of the sample nuclei. The incident ions thus scatter
on different angles due to the discreet lose in energy, producing a separate peak for
Chapter 3
57
different nuclei. Position of the peaks is finger prints for each nuclei and the height of the
peaks gives the relative intensities of the nuclei present in the sample.
Backscattering of incident ions through results in continuous lose in energy as the
ions pass through certain depth occupied by some specific element as. The energy lose
depends upon the electron density and the distance travelled within the sample.
Continuous lose in energy results in shift of the peak on energy spectrum of the specific
element. The peak fades toward the lower energies. The lose in energy ∆E is directly
proportional to the thickness of the film ∆t as follows
(3.3)
Where = lab scattering angle, k is kinematic factor. Subscripts ‗in‘ and ‗out‘ are
for the rate of energy lose of incident and the backscattered ions.
In short compositional depth profile can be determined from measurement of the
energy spectra. The positions of peaks in the energy spectrum give information of the
elements contained. The width and shifted position of the peaks give the depth profile;
peak heights contain the information of the relative concentration.
By examining the crystal structure through RBS can give an idea about the
chemical structure, however, energy spectra cannot give any information of chemical
structure directly.
Chapter 4
58
Chapter 4
Sample Shape and Fabrication Effects in Exchange Bias
Systems
The magnetic properties of materials are determined by the reversal process. In
single domain granular systems the reversal process is by coherent or incoherent rotation.
In thin films this mechanism can apply but for exchange coupled granular films the
reversal process is usually that of reverse domain nucleation and domain wall
propagation. Hence local effects such as demagnetising fields have a significant influence
since these can affect the nucleation process significantly. In this work the effects of
nucleation is described arising from both sample shape effects and the process used to cut
the sample. It is found that cutting techniques such as the use of ultrasonic cutters leads to
a large increase in nucleation which distorts the hysteresis loop. Deposition through
masks causes shadowing effects at the edges that also distort the loops. Cutting with a
diamond scribe appears to give the best outcome. Implications for devices based on nano-
elements are discussed.
4.1 Introduction
In this work an exchange bias system consisting of a CoFe ferromagnetic layer
deposited on top of an IrMn antiferromagnetic layer is studied which generally exhibits
nucleation controlled behavior. Effects of sample shape and also the effect fabrication
method for the production of the sample on exchange bias reversal mechanism is
studied. Additionally details of the edges of samples produced by different techniques
in an attempt to establish the best method for sample fabrication for nucleation
controlled materials are also studied. Multilayered IrMn/CoFe system is studied
because the exchange bias effect will be critical in the development of GMR, TMR or
spin-torque switched MRAM devices which will consist of lithographically defined
elements39, 102
.
Chapter 4
59
4.2 Experimental
`
All samples studied in this work were produced by sputtering using a HiTUS
sputtering system described by O′grady et al. 93
.
Fig 4. 1: Sample structure
Details of growth conditions are discussed in section 3.1.1. Five samples were
grown with variable bias voltage (200,400,600,800 and 1000V) whilst keeping the
composition. Amorphous Ta was used as seed layer in order to remove substrates
effects due to the availability of similar substrates. Only sample grown at 800V is used
to carry out further studies.
Exchange bias system with the structure shown schematically in Fig 4.1 which
is used in the current studies. This is a standard exchange bias system which is already
reported103
and which has a square hysteresis loop with the reversal controlled on the
first part of the loop by a nucleation process. In exchange bias systems with this type of
structure it is generally the case that the forward going part of the loop is nucleation
controlled.
Samples were prepared by three methods:
1. Samples were cut from a continuous film using a diamond scribe which was
used to score the uncoated side of the substrate and subsequently snapping the
Chapter 4
60
sample.
2. The sample was sputtered through a thin stainless steel mask with a circular hole
cut by either a simple mechanical drill or by laser cutting. In practice It is found
that, for the samples produced by sputtering through a mask, the mask cutting
technique resulted in no discernible change in properties.
3. An ultrasonic cutting device was used that produces circular samples but where
the sample is cut through from the coated side.
In addition to examining the effect of sample cutting; effect of sample shape has
also studied. This was achieved via the cutting of square and round samples as
described above but also by direct growth of identical stacks onto preformed round and
square substrates. In this way any intrinsic effect of sample shape can be distinguished
from sample edge roughness caused by a cutting technique.
Fig 4. 2: SEM images of the edges of the three sample types
Fig 4.2.a) shows the image for the sample cut with a diamond scribe as can be seen in
this image a very crisp and smooth edge results. Fig 4.2.b) shows the edge of a sample
Chapter 4
61
deposited through a circular mask. Here a marked gradation in the film thickness is
clear due to the shadowing effect at the edge of the mask. Fig 2.4 c) shows the sample
cut with an ultrasonic cutter. Here an extremely jagged and fragmented edge results due
to the nature of the cutting process with these devices.
Samples grown onto preformed substrates were a 5mm square and a 5mm
diameter circle. Unfortunately substrates of exactly the same type were not available in
different shapes. However this effect was negated by growing on a Ta underlayer with
an amorphous structure. Hence there are no substrate related texture effects in the films.
4.3 Results and Discussion
4.3.1 Effect of substrate cutting
Fig 4.3. shows hysteresis loops for three samples produced by (a) cutting with a
diamond scribe, (b) depositing through a mask and (c) cutting with an ultrasonic cutter.
Only data for the circular mask produced with a laser cutter is shown because the data
for a similar mask produced with a simple mechanical drill was identical.
Clearly the curves from the three samples show dramatically different effects. The
hysteresis loop for the sample produced by cutting with a diamond scribe (a) shows
clear nucleation controlled behavior with a single reversal occurring just above 500 Oe
and then a rapid reversal to negative saturation. The sample produced by sputtering
through a mask (b) appears to show multiple nucleation events occurring even above
zero field but interestingly shows a similar measured coercivity to that of the sample
cut with a diamond scribe. The sample produced using the ultrasonic cutter (c) shows
an even greater degree of nucleation at low fields and shows a slightly reduced
coercivity compared to the other two samples.
Chapter 4
62
Fig 4. 3: Hysteresis loops for three samples produced by (a) cutting with a diamond
scribe, (b) depositing through a mask and (c) cutting with an ultrasonic cutter
The reason for these effects can be seen in the images in Fig. 4.2. The smooth
edge from the diamond scribe cut sample means that there appears to be very little, if
any, nucleation prior to the major nucleation event at just over 500 Oe. The sample
deposited through a mask shows a clear shadowing effect at its edge which will of
course change the nucleation field as the thickness of the deposited film changes the
demagnetising field at the edges. The effect of using an ultrasonic cutter is catastrophic
with many chips along the film edge leading to areas with a strong demagnetising field
that will readily nucleate multiple reversal events.
However, the interesting feature is that all three samples have a similar
coercivities even if the loop shape as the sample is demagnetised varies dramatically.
This is not a true nucleation field in the conventional sense but rather a field at which
-1.0 -0.5 0.0 0.5
-1.0
-0.5
0.0
0.5
1.0
H [kOe]
a) diamont cut
b) deposted through mask
c) ultrasonic cutter
M/Ms
Chapter 4
63
the domain wall or walls nucleated in the ferromagnet can overcome the exchange
coupling between the AFM layer and FM layer allowing the domain wall to sweep
through the sample.
A further unexpected result occurs on the recoil loop where the sample
deposited through a mask has a significantly different coercivity than the other two
materials. Understanding of the origin of the coercivity in exchange bias systems is not
clear at this stage of research in the related field but this variation in the return
coercivity was observed for both samples sputtered through masks but to differing
degrees. Although this variation is not explainable at this stage yet it is worth to note
that the results are reproducible.
4.3.2 Effects of sample shape
Fig 4.4. a) and b) shows the effect of sample shape. A comparison of samples
deposited directly onto pre-prepared substrates without the use of masks or cutting is
presented. For both square and circular substrates the curves reproduce and both give
the same shape of hysteresis loop and the same value of coercivity.
Fig 4. 4: Hysteresis curve a) and b) shows the effect of sample shape
-1.0 -0.5 0.0
-1.0
-0.5
0.0
0.5
1.0
a) square
b) round
M/Ms
H [kOe]
Chapter 4
64
This result implies that sample shape effects in nucleation materials are
relatively small and that the strong demagnetising fields that would occur for example
in the corner of the square sample, have a very limited effect. This indicates that the
coupling to the AFM layer is dominating over demagnetising effects from samples with
relatively smooth edges. Hence the exact shape of the sample appears to be much less
significant than the sample edge roughness. However it should be noted that when
samples such as these are measured using the vibrating sample magnetometer the
geometric response of the coils or other sensor in the magnetometer means that great
care should be exercised to ensure that the magnetometer is calibrated with a sample of
similar size, moment and particularly shape to that of the sample which is to be
measured.
Nucleation and domain reversal processes in small elements have been
examined in detail using Lorentz TEM by Craig et al46
. These workers found that sharp
corners in the elements played a major role in determining the onset and progress of
reversal. However the samples studied were Permalloy which is magnetically soft ( c
<100 Oe) and hence demagnetising fields due to a sample shape may easily nucleate
reverse domains. In a real MRAM device element a significantly higher reversal field
will be required. From our study it would appear that in these circumstances overall
element shape will be less critical than edge roughness. This will pose a significant
challenge for the lithography since edge roughness will cause an overall change in the
reversal process of an element. This will also contribute significantly to a switching
field distribution across the sample due to non-uniform roughness in different elements.
Summary
From this work it is concluded that great care must be exercised when making
magnetic measurements on magnetic thin films where the reversal process is controlled
by domain nucleation. Surprisingly the effect of sample shape is relatively minor in
those materials is studied here but this may not always be the case for samples having a
Chapter 4
65
lower anisotropy than the exchange bias materials studied here. The critical parameter
appears to be the edge roughness of the samples where it is essential to obtain sharp
edges that are smooth using tools such as a diamond scribe but presumably not a
diamond saw where chipping at the edge of the samples would also occur. The use of
ultrasonic cutters, which are popular for taking samples from films deposited on wafers
or thin film discs, are completely inappropriate as they will give rise to a completely
false shape to the hysteresis loop. The use of masks is only appropriate with great care
due to the shadowing effect which occurs at their edge. It is found that the samples for
VSM measurements and other open circuit techniques should always be prepared on
pre-cut substrates or by using a diamond scribe or similar cracking tool to cut the
sample. Even when using diamond scribes the samples should always be cut from the
reverse side to avoid chipping the magnetic thin film thereby leading to an excess of
nucleation sites. The significance of edge roughness will make demands on lithography
for MRAM technology.
Chapter 5
66
Chapter 5
Interface Modification and Training Effect in Exchange
Bias Mutilayer System
The effects of inserting mono-atomic layers of Mn at the interface of IrMn/CoFe
bi-layers were investigated. Samples were grown using HiTUS sputtering technology at
200, 400, 600, 800 and 1000V bias voltages. Magnetic characterization was carried out
using a MicroMag 2900 AGFM and ADE Vector Model 10 VSM whilst grain size
analysis was carried out using a JEOL JEM-2100 TEM and a Zeiss particle analyser.
Exchange bias of the bi-layer grown at 800V was increased from (-411±10) Oe to (-
479±10) Oe with the insertion of 2Å thick Mn layer whilst coercivity was increased from
(212±10) Oe to (252±10) Oe and blocking temperature was decreased from 408.3 K to
380.0 K. In addition to this it was found that blocking temperature decreased by 7.5 K
when set at 20 kOe as opposed to 5 kOe. The cause of the changes to the coercivity,
exchange bias and blocking temperature could be due to modification of the anti-
ferromagnetic spin structure at the interface.
5.1 Experimental
5.1.1 Fabrication process and conditions
All samples were sputtered under the same conditions as described in section
3.1.1. A few samples were grown without the 0.2nm Mn layer as a control at 400, 600
and 800V in order to measure the effect of the Mn . The sample structure is shown in Fig.
5.1
Chapter 5
67
Fig 5. 1: Sample structure with Mn doping
5.1.2 Setting process
5.1.2.1 York Protocol
The measurement of the exchange bias phenomenon presents a number of
challenges. Firstly, for materials which have technological applications it is difficult to
achieve TN of the AFM layer without causing damage to the sample as a whole. For all
current applications, as well as within this study, the alloy used for AFM layers is IrMn3.
This material has a value of TN 690K, at which point diffusion of the multilayer would
occur. It is important to note, however, that field cooling from temperatures as low as
475K has been reported to result in the setting of IrMn layers. This is due to the thermal
activation of the orientation of the AFM lattice within each grain58
. Secondly, due to the
AFM grains being thermally unstable, the state of order of the AFM can change during
measurement, as shown by Fulcomer and Charap 51
. This means that for measurements of
the parameters and c results are highly non-reproducible. Thirdly, when a
conventional magnetic measurement is made the AFM layer itself gives no signal.
Chapter 5
68
Fig 5. 2: a) Schematic diagram and b) measurement steps of the York protocol. 14
O‘Grady et al. 14
proposed that through the careful management of the thermal
and magnetic history of the AFM a uniform initial state could be produced. This would
allow for reproducible results and therefore comparability between multiple
measurements. O‘Grady et al. 14
also suggested an interesting solution to the blindness of
the AFM layer to an applied field. Through changes in the state of the AFM grains
changes in the properties of the adjacent FM layer occur. Thus through the careful control
of the magnetic and thermal history one can infer changes in the AFM grains through
observations in the changes of the FM layer.
To control the magnetic and temperature history of the samples a set of protocols
were used 14
. Firstly, the AFM layer was set in a reproducible manner. This was done by
Chapter 5
69
applying a magnetic field in the direction of the known FM layer easy axis that was
sufficient to saturate the FM layer. Then the sample was heated to a maximum
temperature ( ) at which interfacial diffusion did not occur. For this study the setting
of the samples was undertaken using both a hand built ‗annealing‘ furnace and an ADE
Vector Model 10 VSM with a value of of 90mins and a maximum temperature of
498K. Secondly, the temperature at which no thermal activation occurs ( ) was
established. This was done by cooling down the sample, with the setting field still
applied, to which was determined by first cooling down the sample to a trial .
The sample was then held in the state of magnetisation for a short period (1min) and a
hysteresis loop was measured. The process was then repeated however with the FM layer
reversed for a period of 30mins before a measurement was taken. If the hysteresis loop
was not reproduced then thermal activation had occurred and a lower value of would
need to be used. It is important to take note that two hysteresis loops should be taken at
each stage to eliminate the training effect. The training effect occurs only for the first
hysteresis loops and is thought to be due to spin-flop coupling which is removed by the
first hysteresis loop 104
. In the case of this study all samples used were thermally inactive
at room temperature so > 298K. This process is shown schematically in Fig 5.2 a) and
the precise measurement sequence in Fig 5.2 b) 14
.
5.2 Results and Discussion
5.2.1 Grain Size Analysis
The image shown in Fig 5.3 gives a good example of a high resolution TEM
image taken at a magnification of 80,000x. The grains can be clearly seen as black spots
of varying sizes.
Chapter 5
70
Fig 5. 3: TEM Image
These grains were counted using the Zeiss grain size analyzer, as described above,
and then plotted to a lognormal distribution as shown in Fig 5.4. As can be quite clearly
seen there is an equivalently equal increase in average grain size with the increase in bias
voltage as well as an increase in the standard deviation of the grain sizes for larger bias
voltages. However it is important to note the mediocre fit of the log-normal distribution
curve to the data as well as the difference in height between the 400V and 600/800V
samples. Although the CPM was utilized it is highly likely that too few grains were
measured. To obtain a good fit between the theory and data 150-200 more grains should
have been measured.
Chapter 5
71
Fig 5. 4: Graph showing the log-normal distribution of grain sizes.
400V
600V 800V
Chapter 5
72
Curve fitting was obtained by the equation 2.5 and rewritten below as
This data, in comparison with the recorded sputter rates along with the calculated
average grain sizes, is shown in Table 5.1. As expected there is a near constant increase
between each bias voltage. This is in line with the theory as described by Vopsaroiu et al.
57 in which it was found that the mean grain size depends on both the sputter rate and
nucleation rate. As mentioned an increase in standard deviation was observed, which
again confirms the theory in 57
, where it is shown that as sputter rate increases the range
of possible grain sizes increases. This increase in standard deviation could also be used to
explain the flatness of the 600/800V curves. However this is unlikely as although
flattening would be expected it would not occur at such a magnitude.
Table 5. 1: The results obtained for the average grain size at different bias voltages.
Bias Voltage
Sputtering Rate
of AFM (nm/s)
Average Grain
Size (nm)
Standard
Deviation
400V 0.10 4.9 0.54
600V 0.12 5.8 0.42
800V 0.14 7 0.35
5.2.2 Exchange Bias
The hysteresis loop, as shown in Fig 5.5, demonstrates a clear example of an
exchange biased system measured using the York protocol. The values for and c are
straightforwardly obtained using the standard methods: is the offset of the loop centre
from zero fields; c is half its width. Errors in these calculations were calculated by
observing the nearest data point to zero magnetisation and making a reasonable estimate
as to the possible variance in value.
Chapter 5
73
Fig 5. 5: Typical hysteresis loop obtained using the York protocol.
The effects on both and c of a 2Å Mn dusting layer at the AFM/FM interface for
different bias voltages are shown in Table 5.2. The most striking feature is that in the
600/800V cases an increase in of ~10% is observed, accompanied by an enhancement
in by ~13%. This in agreement with the work of Tsunoda et al.105
in which the
interfacial layers of 0-1nm was studied. They believed that the cause of the enhancement
of and c was due to the modification of the AFM spin structure at the interface. This
is a fair assessment; however results in the following section raise more questions as to
the effect.
.
Table 5. 2: The results obtained for and c at different bias voltages.
Bias
Voltage
400V
Control
400 Mn
doped
600V
Control
600 Mn
doped
800V
Control
800 Mn
doped
Hex
(±10.0)
(Oe)
-532
-569 -429 -496 -411 -479
Hc
(±10.0)
(Oe)
-271
-241 222 256 212.3 252
Chapter 5
74
In the 400V case there appears to have been no real change in , however an
enhancement in C by ~11% is observed. The lack in enhancement is odd judging a
similar increase in with that of the 600 and 800V cases is seen. Due to the 200V
samples being thermally unstable at room temperature it is reasonable to assume that the
400V sample suffered from a slight instability.
5.2.3 Blocking Temperature
The effect of heating a sample under a field opposite to that of the set
magnetisation is shown in Fig 5.6. The loop shift is clearly identifiable, from which the
value for can be easily obtained by York protocol 14
; is the temperature at which
goes to zero under a reverse field. Errors in these calculations were estimated by
taking into account the errors as shown in Table 5.1 as these were used, in association
with other results, to calculate the blocking temperatures.
Chapter 5
75
Fig 5. 6: Typical loop shift for different temperatures under a constant reverse field.
Chapter 5
76
The effects on of a 2Å Mn dusting layer at the AFM/FM interface for different
bias voltages and reverse fields are shown in Table 5.2. The most obvious feature is that
in all situations there was ~5% decrease in . This result seems to go against the first
impression of what should occur. With the value of being determined by the number
of AFM grains that are both set and thermally stable, an increase in implies that there
are a larger number of AFM grains contributing to the total value. This could be
interpreted as either more grains have become set or thermally stable. In the former
situation should increase; however in the latter situation should decrease. As such,
the decrease in could be due to the high moment of the Mn holding the grains
previously too small to be set in place.
Fig 5. 7: Measurement of Blocking Temperature (TB) by York Protocol
Chapter 5
77
Table 5. 3: The results obtained for TB at different reverse fields.
600V
Control
0.5T
600V
Control
2T
600V
Mn
doped
0.5T
600V
Mn
doped
2T
800V
Control
0.5T
800V
Control
2T
800V
Mn
doped
0.5T
800V
Mn
doped
2T
TB
(K) 421.9
417.21
409.7 396.9 412.6 408.3 388.4 380.0
A less obvious, but even more interesting feature, is that in both the 600/800V Mn
doped samples there was ~1-2% decrease in blocking temperature between the 0.5T and
2T reverse fields. This result is intriguing as at such fields the FM layer is completely
saturated and in theory the AFM shouldn‘t be able to see the field. A possible source for
this feature could be dirt or other contaminants at the interfacial layer, however further
studies are needed to confirm this.
5.3 Training Effect
The training effect of IrMn/CoFe bilayer system with different seed layers Cu and
NiCr is studies. Training effect have dependence on many factors e.g. temperature,
thickness of the FM and AFM layers, grain structure, crystallinity as well as the
orientation on FM/AFM on the interface. However complete mechanism is still under
debate. Only theoretical model at present is described by Hoffman62
showing that the
training effects can be seen for AFM with biaxial anisotropy. Few researchers have
shown the particle or grain size dependence of the training effect106
. Recently Kevin
O′grady successfully proposed a theoretical model describing various features of
exchange bias. In this work the training effect of sputtered IrMn/CoFe bilayer is
described by relating to the average grain size. It is found that training effect if maximum
for smaller grain size and vice versa for Cu and NiCr seed layers used in this study. Also
the dependence of training effect on seed layer is also discussed.
Chapter 5
78
5.3.1 Sample Preparation
All samples studied in this work were produced by sputtering using a HiTUS
sputtering system described by O′grady et al93
and also described in section 3.1.1.
A dc field of 500 Oe was applied during the deposition to pin the direction of
IrMn/CoFe system. The sputtering conditions were kept same for all the spattered films.
Each sample had 5nm NiCr as a seed layer and caped with 10 nm Ta layer to avoid any
oxidation. For comparison similar samples with 5nm Cu seed layers was also prepared
under the same condition. Each series consists of 6 samples grown with variable bias
voltage (200,400,600,800, 980 and 1000V) whilst keeping the composition other than
the difference in type of the seed layer. The magnetic properties of all the samples were
determined with VSM. The training effect of the hysteresis loops was measured by
cycling the applied field continuously starting from pinned direction anti-parallel to
pinned direction. Only first two cycles were recorded. All the measurements are taken
from 2000 Oe to -2500 Oe so that the ascending and descending arm of the loop feel
the same amount of applied field during the measurement.
Fig 5. 8: Schematic diagram of sample structure
Exchange bias system with the structure shown schematically in Fig 5.8 which is used
in the current studies. This is a standard exchange bias system which is already reported
and discussed in the previous sections of this thesis
Chapter 5
79
5.4. Results and Discussion
Fig 5.9 shows the normalized hysteresis loops of samples produced with different
seed layer namely NiCr and Cu fabricated with HiTUS and characterized according to
York protocol. Only first two loops are taken to observe the training effect in our samples
to compare with size and seed layer structure. A clear shift in the descending curve is
pronounced, while almost no shift in the ascending curves. The decrease in coercivity and
exchange field can be observed and also presented in the table 5.4.
Fig 5. 9: Training effect with a) NiCr seed layer b) Cu seed layer
Chapter 5
80
Table 5.4 and Fig 5.10 shows a comparison between the Bias voltage and the
observed training effect. The bias voltage is in turn related to the grain size due to the
characteristic fabrication process by HiTUS. This is also reported in the literature and
further confirmed in Fig 5.10 b) by plotting training amount vs. the grain size for NiCr
seed layer. The grain size follows lognormal distribution which is discussed in section
5.2.3 with median grain sizes shown in Table 5.4. It can be seen that the grain size is
maximum at an operating bias voltage of 200V and decreases with the increase bias
voltage. However the grain size for 1000V bias voltage again decreases unexpectedly.
This might be due to the fact that Ar plasma used for sputtering was not stable at 1000V
and the sputtering rate was too fast. For confirmation another sample was fabricated with
bias voltage 980V with the same and before sputtering plasma was allowed to stabilize
for 5-10 minutes. The grain size for the samples fabricated at 980V is in good agreement
and in sequence with the grain sizes of the samples grown on bias voltage 200,400,600
and 800V.
Fig 5. 10: a) comparison of bias voltage vs. training effect for NiCr and Cu under layer b)
grain size vs. training effect for NiCr under layer
Chapter 5
81
Table 5. 4: Amount of training for Cu and NiCr underlayer
Bias Voltage
(V) Training Field ( 10 Oe) Grain size (nm)
NiCr under layer Cu under layer NiCr underlayer
200 28.5 73.2 3.9
400 21.1 53.2 4.2
500 20 52.5 4.4
600 17.5 50.9 5.2
800 5.3 42.3 5.9
980 3.8 40.3 6.2
1000 11.3 52 5.2
The training effect found in the sputtered films in the 1st two loops might be due
to presence of metastable biased spins configuration in the AFM layer. During each cycle
these AFM spins turn into the energetically favorable spins. This change in initial spin
distribution brings a decrease in the exchange anisotropy of AFM layer. In
nanostructured crystalline material large numbers of spins are located at the grain
boundaries. The disordered structure of the spins results in fluctuations in the magnetic
anisotropies which alternately affects the exchange strength. Thus strength of the
training effect is more pronounced in small grain size107
.
Fig 5.10 b) shows a plot between bias voltage which alternately predict the grain
sizes and the training effect. It is found that the training effect is found more pronounced
for the films grown on the Cu seed layer and weak for the films grown on the NiCr seed
layer. However both plots follow the same trend, again confirming the dependence of
training effect on the grain size.
Marian Fecioru-Morariu108
has reported that a better quality growth of the (110)
FeMn has less structural defects as compared to the (001) oriented film. He concluded
that rearrangement takes place during field cycling of the AFM domain structure. This
rearrangement arises from the presence of defects in the AFM lattice, which is more
Chapter 5
82
pronounced in (001) grown FeMn as compared to (110) FeMn. This explains why the
training effect is stronger for the (001) sample as compared to the (110) sample.
Nick et al.109
XRD scans found that there is strong peak from the FCC IrMn (111)
planes when growing a similar sample structure. There was also a possible peak from the
BCC CoFe (110) planes. For the samples with Cu seed layers no peaks were observed
indicating that there is no strong texture and poor crystallinity in general. This indicates
that the IrMn when grown on NiCr has a strong in-plane (111) texture where the (111)
planes lie parallel to the substrate. It is therefore concluded that in the set of samples with
Cu seed layer provides poor crystallinity which alternately provides more structural
defects and grain boundaries of the polycrystalline IrMn is responsible for the more
pronounced increase in the training effect value.
Summary
The effects of placing a 2Å Mn dusting layer at the AFM/FM interface on the
exchange bias, coercivity and blocking temperature of IrMn/CoFe bilayers were
investigated. The purpose of placing Mn dusting layer was to modify AFM/FM interface
on which all the above properties depend. The study was taken in search of enhancement
in TB. of the bilayer was mildly enhanced from (-411±10) Oe to (-479.7±10) Oe,
while was increased from (222.3±10) Oe to (256.5±10) Oe. It is believed that the
enhancement of and is due to the modification of the AFM spin structure at the
interface.
However as the blocking temperature was reduced from 408 K to 380 K with the
addition of Mn, it is believed that grains that were previously too small to be thermally
stable were somehow set. This could be due to either a chemical reaction with the Mn at
the interface or due to exchange effects.
Chapter 5
83
Training effect is found more pronounced in smaller grains, which is due to the
more disordered structure of the spins at the grain boundaries. Also it is found that
training effect have a significant influence on the crystallinity of AFM at the interface.
Chapter 6
84
Chapter 6
Transition Metal Doped TiO2 Thin Films by AACVD
This chapter is devoted to synthesis of Ni, and Co doped TiO2 thin films, optimization of
the parameters of the AACVD grown thin films, the study of structural, morphological
and magnetic properties. The optimization of the process parameters (temperature, carrier
gas flow rate, and deposition time) for the thin film deposition by AACVD technique was
defined. The growth temperature for thin films on silicon substrates were 450°C for Ni
doped TiO2 and 650°C for Co doped TiO2,The carrier gas flow rate was kept 120mL/ min
and a deposition time of 20 minutes for all the films grown in current studies. Anatase
phase formation was obtained for Ni doped TiO2 thin films as shown in XRD patterns.
For Co doped TiO2 thin films; at a synthesis temperature around 650°C oxygen deficient
rutile phase namely magneli phases (Ti3O5, Ti4O7, TiO etc) were found. SEM images
show that as grown Ni and Co doped TiO2 thin films in this study were polycrystalline.
SEM images also confirm that the films were highly crystalline. Rutherford Back
Scattering (RBS) was carried out to confirm the TM doping concentration and to find the
thickness of the films. The thickness of the films found to be in the range 200-260nm
except sample doped with 2wt. % Ni has film thickness of ~800nm. The SQUID
measurements of TM doped TiO2 thin films used to show room temperature
ferromagnetism in the films.
6.1 Introduction
A range of metal oxide semiconductors are reported in the literature including
ZnO, TiO2 in the form of thin films and nanoparticles for various applications12, 110-128
.
Cynthia Edusi et al129
optimized the conditions in controlling the TiO2 phase. They have
shown that at 400 C anatase phase of the TiO2 can be achieved on the glass substrate.
However there are only a handful of reports on transition metal doped TiO2 thin films
prepared by AACVD technique130-131
.
Chapter 6
85
In this section Ni doped TiO2 thin films are reported synthesized by AACVD. The
optimized conditions and choice of precursor are made to get the most suitable anatase
phase for DMS applications. Suitable precursor to get the proper stoichiometry is
designed by modifying the synthesis procedure adopted by Tahir et. al.132
.
6.2 Nickel Doped TiO2 Thin Films
There are only a handful of reports in literature on Ni doped TiO2 thin films
which may be due to difficulties to prevent secondary phases and precise understanding
in RTFM in this system. However, there are few reports on Ni doped TiO2 films and
particles with various synthesis techniques and results. Earlier Hong et al. 133
reported
RTFM in Ni doped TiO2 anatase and rutile phases grown on SrTiO3 and LaAlO3 by laser
ablation. Cho et al.134
reported highly resistive NixTi1-xO2 films by solgel method. They
concluded with two different mechanisms for justification of RTFM in their report i.e.
due to formation of Ni and NiO clusters. However possibility of carrier mediated
ferromagnetism was ruled out due to high resistivity in their samples134
.
D.L. Hou et al.135
reported some exiting results in NixTi1-xO2 anatase films grown
by reactive magnetron sputtering on amorphous SiO2 substrates. MFM results shown
regular and clear magnetic domains for the first time thus predicting intrinsic
ferromagnetism in Ni doped TiO2 anatase films135
.
Fig 6. 1: a)Topography image and corresponding b) MFM images of Ni doped TiO2 thin
films135
Chapter 6
86
Few other reports also show variation in RTFM in Ni doped TiO2 structures. Uhm and
coworkers136
prepared Ni:TiO2 powder by mechanical alloying by varying Ni
concentration 0-12wt.%. All the three phases were found in XRD data and an increase in
ferromagnetic behavior upto 8 wt.% of Ni concentration. Cabrera et al.137
reported TM
(Fe, Ni, Mn and Co) doped TiO2 powder obtained by ball milling. However Ni clusters
were found magnetically ordered in their report. More recently Hoa et al.138
reported
intrinsic RTFM in Ni doped TiO2 nanowires synthesized via solvothermal technique.
Coesivity value of ~125Oe confirmed the intrinsic nature of RTFM in Ni doped TiO2
nanowires. Owing to higher Ms value in undoped samples also suggested surface defects
playing important role in ferromagnetism. Badaur et al.139
reported RTFM solgel derived
Ni doped TiO2 powder. Any contribution in RTFM from any secondary phase was ruled
out by using HRTEM and XPS analysis. So far there is no report with any TM doped
TiO2 thin film by AACVD for DMS applications. It is important because various
properties are strongly influenced by the synthesis routes adopted as can be seen in above
paragraph and discussed in section 6.1.
6.2.1 Experimental
Thin films of Ti1-xNixO2 (x=0.02-0.15) were prepared by Aerosol Assisted
Chemical Vapor Deposition (AACVD). Ni doped TiO2 films were deposited by AACVD
on silicon substrates. The precursor [Ni2Ti2(OEt)2(l-OEt)6(acac)4] synthesized for current
studies has already been reported by Asif et al.132
. The stated precursor was synthesized
for 1:1 molar ratio of Ni to Ti. For current studies precursor was prepared to achieve
2,4,6,8 and 15 wt.% Ni concentration. In detailed synthesis process 1.00 g (4.36mmol) of
Ti(OEt)4 to 15 ml toluene and 1.11g (4.36mmol) Ni (acac)2 was dissolved in 15 ml of
toluene separately. Then stoichiometric amount of Ni (acac)2 solution added to Ti(OEt)4
solution to obtain 2,4,6,8 and 15 wt.% Ni. The solution thus formed homogeneous
mixture of [Ni2Ti2(OEt)2(l-OEt)6(acac)4] and Ti(OEt)4. AACVD was performed using the
aforementioned precursor.
Chapter 6
87
Substrates were washed with acetone, isoproponol and ethanol ultrasonically. The
substrate kept in ethanol before use. Modified AACVD apparatus was used as reported
by Asif et al.132
. Instead of using reactor chamber, hot plate was used to decompose the
precursor on Si substrates. PIFCO air humidifier was used to generate the aerosols and
Argon gas was used as a carrier gas in order to prevent any possible reaction of precursor
with air before deposition. All the films were grown on 450 C and the Argon flow rate
was kept on 120 mL/min with the help of a micro controller. Films were deposited for 20
min each. After deposition hot plate and Argon flow turned off until the room
temperature was achieved. Scotch tap test was conducted to check the adhesion of the
films.
6.2.2 Results and Discussion
6.2.2.1 XRD analysis
The crystallinity, phase and preferred orientation of the films was investigated
using PANalytical X-ray diffractometer model XPert PRO with primary monochromatic
high intensity Cu-K (λ = 1.54184 Å) radiation. Data was acquired over the range of 2θ
from 10 to 80 with a slow scanning speed of 1 step/s and each step was 0.02 . The
results show anatase up to 8 wt.% Ni concentration. In general, Oxygen deficient
environment is favorable for the growth of rutile phase due to 3 times smaller c/a ratio of
rutile (2.52) to that of anatase (1.54). Formation of predominantly anatase phase here is
due to the fact that acac ligand and chelating ethoxide used for the synthesis of films
contain oxygen atoms which are coordinatively saturating both Ni and Ti as shown in
Fig. 2.6. 132
This oxygen thus eliminates need of any additional oxygen for the formation
of anatase TiO2. Post deposition cooling in air also pronounced anatase phase.
XRD shows an overall increase in crystallinity with increasing Ni contents, plus a
greater peak intensity in the (101) reflection. This indicates an increase in the c-axis
crystal orientation perpendicular to the substrate surface with increased doping
concentration. Further there is a slight shift in the 2θ values towards lower values,
Chapter 6
88
specially, considerable in (101) plane suggesting that Ni contents are doped within the
lattice of TiO2.
The small shifting in (101) peak towards lower angle suggest that Ni+2
has
replaced Ti+4
rather Ni+3
. It can be explained by taking comparison of the ionic radii of
Ti4+
(0.745 Å) ion with Ni2+
(0.830 Å) and Ni3+
(0.740 Å) ions.140
Further there is no
additional peak of Ni or NiO from 2-8wt.% Ni doping.
The crystallite size using Sherrer formula for 2,4,6,8 and 15 wt% Ni doped
samples are 25.1nm, 29.6nm, 25.8nm, 23.4nm and 13.4nm respectively as calculated
from maximum intense peak (101).
Fig 6. 2. XRD pattern of Ni (2%) doped TiO2
Chapter 6
89
Fig 6. 3. XRD pattern of Ni (4%) doped TiO2
Fig 6. 4. XRD pattern of Ni (6%) doped TiO2
Chapter 6
90
Fig 6. 5. XRD pattern of Ni (8%) doped TiO2
Fig 6. 6: XRD pattern of Ni (15%) doped TiO2 thin films
Chapter 6
91
Fig 6. 7: Crystal structure of [Ni2Ti2(OEt)2(l-OEt)6(acac)4]132
XRD shows an overall increase in crystallinity with increasing Ni contents, plus a
greater peak intensity in the (101) reflection. This indicates an increase in the c-axis
crystal orientation perpendicular to the substrate surface with increased doping
concentration. Further there is a slight shift in the 2θ values towards lower values,
specially, considerable in (101) plane suggesting that Ni contents are doped within the
lattice of TiO2.
The small shifting in (101) peak towards lower angle suggest that Ni+2
has
replaced Ti+4
rather Ni+3
. It can be explained by taking comparison of the ionic radii of
Ti4+
(0.745 Å) ion with Ni2+
(0.830 Å) and Ni3+
(0.740 Å) ions.140
Further there is no
additional peak of Ni or NiO from 2-8wt.% Ni doping.
Chapter 6
92
Table 6. 1. Lattice parameters, Cell volume and crystallite size calculated from XRD data
composition a(Å) c(Å) Lattice Distortion (c/a)
Cell volume V (Å)3
Crystallite Size (nm)
Reference (83-2243)
3.7842 9.5146 2.5142 136.25 --------
0.02 3.765 9.4396 2.507198
133.8085 25.2
0.04 3.7842 9.4390 2.494318 135.1681 30.2
0.06 3.766 9.4368 2.505789 133.8398 26
0.08 3.769 9.456 2.508888 134.3259 26
Reference (83-0198, 33-960)
5.0311 13.7960 ----------- 302.42 ---------
0.15 4.9989 13.785 2.757607 344.4734 14.4
The crystallite size using Sherrer formula for 2,4,6,8 and 15 wt% Ni doped
samples are 25.1nm, 29.6nm, 25.8nm, 23.4nm and 13.4nm respectively as calculated
from maximum intense peak (101) and given in Table 6. 1.
At a Ni concentration of 15wt.% NiTiO3 phase appeared. However no Ni or NiO
peaks are observed within sensitivity range of XRD. These phases might exist in
amorphous phases, or NiO has reacted with TiO2 to form NiTiO3 at 15wt.% Ni
concentration.
A small peak of TiO2 rutile phase can be seen in all samples. Ni would have
increased the number of oxygen vacancies which might be responsible for the
transformation of anatase to rutile and NiTiO3. 141
6.2.2.2 Rutherford Back Scattering
Rutherford backscattering spectrometry (RBS) measurement of Ni doped TiO2
samples were performed using Accelerator Facility at Experiential Physics Labs, NCP.
RBS is a non-destructive analysis technique for thickness and composition of elements of
materials.
A collimated 2.0 MeV He+ beam produced by 5UDH-2 Pelletron was used for
Chapter 6
93
RBS channeling measurements. The sample was mounted on a high precision (0.01 )
five-axis goniometry in a vacuum chamber, so that the orientation of this sample relative
to the He+ beam could be precisely controlled. The backscattered particles were collected
by Silicon Surface Barrier (SSB) detector (FWHM 11 keV and area 50 mm2) using
energy resolution of 25 keV placed at angle of 170 .
Simulation was performed by SIMNRA software for RBS data from Accelerator.
RBS spectrum of Ni doped TiO2 thin films (black line) along with simulation spectrum
(red line) is shown in Fig 6.2
The overlaid spectra of Ni doped TiO2 films with all five compositions are also
shown in the Fig 6.3. The peaks at high energy (channel number) correspond to the
scattering from the Ti and Ni. However no Ti or Ni peaks corresponding to the scattering
from Si are observed. By measuring the intensities (yield) of the Ti and Ni signals and
correcting from the scattering crossection of the Ti and Ni exact ratio of the Ni/Ti is
determined and shown in the Table 6.1. From width of the RBS peaks thickness and
composition of the films were determined and shown in the table 6.1. The composition of
the oxide films and the Ni concentration were determined by comparing the experimental
RBS spectra with those obtained by simulation.
Chapter 6
94
Fig 6. 8: Comparison of experimental and simulated RBS spectra on Ni doped TiO2 thin
film
Both spectra indicate that the Ni doped TiO2 films are stoichiometric. The small
projection at the edge of the Ti portion of the spectra may be due to the close atomic
weight of the Ni and Ti the peaks could not be resolved due to the limitations of the RBS
studies.
0 500 1000 1500 2000
Channel
0
5
10
15
20
25
30
35
Nor
mal
ized
Yie
ld
0.5 1.0 1.5 2.0
Energy (MeV)
Ni1(2.085MeV)
Ni
Ti
Ca & Si from substrateO
Chapter 6
95
Fig 6. 9: RBS spectra of Ni doped TiO2 thin films with various concentrations
Table 6. 2: Ni concentration and film thickness as calculated from RBS spectra
Sample Doping concentration (wt.%)
Calculated Measured
Films thickness
(nm)
Ni-2 2 1.9 0.1 789 5
Ni-4 4 3.8 0.1 228 5
Ni-6 6 5.9 0.1 217 5
Ni-8 8 7.7 0.1 255 5
Ni-15 15 14.3 0.1 208 5
Chapter 6
96
6.2.2.3 Scanning electron microscopy
SEM images of Ni doped TiO2 thin films deposited by AACVD (Fig 6.4) show
compact and smooth film morphologies with homogenously dispersed grains. Individual
grains are well defined and clear grain boundaries can be seen. The packing density of the
microstructure and the grain sizes apparently seem to be affected by variation of the Ni
concentration SEM images. Further it can be seen that all the films are nanoporous with a
very uniform grain size distribution.
The average grain size lies in the range of (40-60nm) which is 2-3 times greater
than crytallite size determined by XRD data which is righly justified. Shape of the grains
changed with increase in Ni concentration as shown in Fig. 6.4. The change is the
structure of the grains is attributed to the growth stress of the films, radius and the
concentration of the Ni ions.
Apparantly the films prepared by different concentrations of Ni are of different
colors, with a thickness of as calculated by the RBS. Apparently the films turns into
yellow green, with the increase in the Ni concentration. As widely known, the color of
Ni+2
ion is generally green, which corresponds with the colors of film, which illustrates
that the Ni+2
may have come into the films142
. In case of sample with 2wt.% Ni, a
compact dense morphology was obtained. The grains are of flower like shape. At 4 wt.%
Ni concentration grains have retained their flower like shape and the roughness of film is
pronounced with increased porosity. A deeper examination shows that even small grains
are aggloromated to form a bigger grain. The SEM image of the films with 6wt.% Ni
concentration shows that the growth of particles is turned into elongated shape. There is a
significant enhancemend in the porosity of the films. Another change in the shape of the
grains is observed in the films with 8wt.% Ni concentration. The film seems to be very
compact. With the increase in the number of the grains the porosity is decreased as can be
seen in Fig 6.4 e). In 15wt.% Ni concentration the shape of the grains is changed into leaf
like structures.
Chapter 6
97
Fig 6. 10: SEM images of Ni doped TiO2 thin films with a) 2wt.% Ni b) 4wt.% Ni c)
6wt.% Ni d) 8wt.% Ni and e) 15wt.% Ni doping
a) b)
c) d)
e)
Chapter 6
98
6.2.2.4 Magnetic properties
Room temperature magnetic hysteresis loops of Ti1-xNixO2 for x=0.02, 0.04, 0.06,
0.08 and x=0.15 are shown in Fig 6.5 respectively. All the samples exhibit
ferromagnetism both at 100K and 300K. The ferromagnetic behavior is found to have a
strong dependence on Ni concentration and the synthesis route. Fig. 6.5 shows saturation
magnetization, Ms, variation with Ni concentration at two different temperatures i.e.
100K and 300K. It can be seen that the magnetization increases with increasing Ni
concentration from 2wt.% Ni concentration to 8wt.% Ni concentration. However, for the
sample with 15wt.% Ni concentration the sample is not saturated and paramagnetic effect
is dominant which is attributed to the secondary phases present in the sample like
NiTiO3.
There are many possible mechanisms which can establish magnetic coupling in
semiconducting TiO2 i) super exchange, ii) double exchange, iii) RKKY interaction, and
iv) impurity band exchange. Carrier mediated interactions occurs in situation where high
carrier concentration (electrons or holes) exists. These interactions include RKKY and
double exchange. Transition metals are normally doped in low concentration due to the
unfavorable precipitation problems in the form of secondary phases in diluted magnetic
oxides. Now, if TM doping concentration is low than percolation threshold (xp) which is
essential for nearest neighbor coupling; super exchange and double exchange interaction
becomes irrelevant for the explanation of ferromagnetic ordering. In oxides xp is
typically 25-30% which can be calculated from xp~2/Z, where Z is the cation
coordination number.79
However, in relevance to our synthesis technique and doping concentration
formation of bound magnetic polaron143
is most likely the reason in establishing the long
range ordering. Since the Argon gas is used as the carrier gas which may have resulted in
the oxygen vacancies. It is believed that local carrier are involved in RTFM in our
Chapter 6
99
synthesized Ni doped TiO2 thin films. Further formation of oxygen deficient magneli
phases in small amount cannot be ruled out since films are synthesized in inert
atmosphere, although, there is no such clear evidence in XRD. However the oxygen
vacancies are the most likely source of RTFM. Bound Magnetic Polaron (BMP) model143
more appropriate for elucidation of magnetic behavior for current study. From analysis of
XRD it is concluded that Ni+2
may have replaced Ti+4
on octahedral site. In order to
maintain the charge neutrality, substitution of Ni+2
results in generation of positively
charged oxygen vacancies. Theses oxygen vacancies capture donor electrons and
constitute an F-centre. The strong electron-phonon interactions establish a strong
polaronic effect in TiO2 which enhances the carrier effective mass.144-145
Polaronic
electron captured in such F-centers tends to spend their time in hydrogen like orbital
which effectively overlap the d shells of the neighboring magnetic atoms. Thus, F-center
bound magnetic polaron formed by an electron trapped by an oxygen vacancy,
surrounded by magnetic impurity ions (Ni+2
in this study) is a possible mechanism for the
ferromagnetism 143
.
These F-centre BMPs grow at low temperature resulting in long range ordering.143
This may be the reason that at low temperature 100K the moment in all the samples has
increased. In sample with 15wt.% Ni concentration is not saturated. XRD results show
NiTiO3 and TiO2 rutile phases at 15wt.% Ni concentration with a low crystallinity.
NiTiO3 shows antiferromagnetic behavior below 23K as evident from neutron
diffraction and magnetic susceptibility.146
Also NiTiO3 is paramagnetic at room
temperature. However rutile phase may have contained some amount of Ni doped in
amorphous form, which shows hint of ferromagnetism.
Chapter 6
100
Fig 6. 11: Magnetic moment of Ni doped TiO2 thin films at 100K
Fig 6. 12: Magnetic moment of Ni doped TiO2 thin films AT 300K
Chapter 6
101
Thus we can summarize the BMP model as the interaction of the trapped electron
with the host lattice that lies within its orbit ferromagnetically. This leads to a bound
polaron with a large magnetic moment. If the density of BMP is less, then they do not
strongly interact resulting in an insulating paramagnetic phase. However, for a certain
polaron density they couple ferromagnetically.
Table 6. 3: Magnetic moment of Ni doped TiO2 thin films at 100K and 300K
Sample Moment (emu/cc)
100K 300K
Ni-2 3.9776 3.348
Ni-4 10.277 7.7278
Ni-6 26.721 17.6859
Ni-8 32.50 30.5075
Ni-15 No saturation
6.3 Cobalt Doped TiO2 Thin Films
6.3.1 Experimental
Thin films of Ti1-xCoxO2 (x=0.02-0.15) were prepared by Aerosol Assisted
Chemical Vapor Deposition (AACVD). Co doped TiO2 films were deposited by AACVD
on silicon substrates. The precursor for Co doped TiO2 films was synthesized by using
Titanium Isopropoxide Ti(OCH(CH)3)2)4 and Co(acac)2 as the starting materials. We
have used this precursor by varying the Co concentration 2,4,6,8 and 15 molar ratio. In
detailed synthesis process 1.00 g (4.36mmol) of Ti Isopopoxide and 1.02g (4.36mmol)
Co(acac)2 was dissolved in 15 ml of toluene separately. Then stoichiometric amount of
Co(acac)2 solution added to Ti Isopropoxide solution to obtain 2,4,6,8 and 15 wt% Co.
The precursor thus form is clear solution suggesting that both the starting materials are
mixed uniformly. AACVD was carried on by using this precursor.
Substrates were washed with acetone, isoproponol and ethanol ultrasonically. The
Chapter 6
102
substrate kept in ethanol before use. AACVD apparatus used as shown in the chapter 2 as
discussed above. All the films are grown on 650°C and the Argon flow rate was kept on
120 mL/min with the help of a micro controller. Films were deposited for 20 min each.
After deposition hot plate turned off until the room temperature achieved while Argon
flow was kept on. Scotch tap test was conducted to check the adhesion of the films.
6.3.2 Results and Discussion
6.3.2.1 XRD Analysis
The XRD results show the existence of different phases including the non
stoichiometric Titanium oxide magneli phases, which follow the integer formula TinO2n-1
(n<4<10) (putt formula here). At 2wt.% Co concentration, peaks for Ti6O, TiO, Co3Ti3O,
(TiO1.20)3.12, and TiO2 rutile phases are shown at their respected angles (however it is
very difficult to identify them exactly from XRD). For the films with 4wt.% Co
concentration Ti3O5, TiO2 rutile, Ti5O9 and TiO1.95 phases are pronounced with a sharp
peak for rutile TiO2 phase. Further increase in Co concentration up to 6wt.% have the
same peaks but over all crystallinity is lost. For 8wt.% Co concentration the magneli
phases disappeared. Only the Rutile TiO2 phase was obtained with further decrease in the
crystallinity. Sample with 15wt.% Co concentration a highly crystalline rutile phase is
obtained.
Many reaction mechanism studies147-150
carried out on Ti(OCH(CH)3)2)4. Authors
proposed reaction mechanisms at various temperatures. Fictorie et al.149
employed
temperature programmed reaction spectroscopy and molecular beam spectroscopy to
investigate reaction kinetics of Ti(OCH(CH)3)2)4 decomposition in inert carrier gas. They
observed TiO2, acetone CH3COCH3, propane C3H6 and hydrogen (H2) decomposition
products of Ti(OCH(CH)3)2)4 at temperature higher that 550 C as shown in equation
Ti(OCH(CH)3)2)4 (g) TiO2 (s) + CH3COCH3 (g) + H2(g) + C3H6(g) (6.1)
Chapter 6
103
Since Co doped titanium oxide film are deposited at 650°C (well above 550°C) in
inert environment, the above equation fits well in describing the reaction mechanism.
Further, reaction products H2 and carbon from the CH3COCH3 and C3H6 can reduce T+4
to Ti+3
and even into Ti+2
and Ti+1
states. Liu et. al.151
proposed three types of H2 and
TiO2 interactions with increasing temperature in their report. They proposed electrons are
transferred from H2 to oxygen present in TiO2 lattice; reacts with H2 to form H2O, leaving
behind an oxygen vacancy. When temperature is above 560 C, electrons present in the
oxygen vacancies are transferred to Ti+4
to form Ti+3
. Above arguments successfully
discuss the reason behind the formation of unstable magneli phases from 2-6 wt.% Co
doped TiO2 films in current studies.
Besides above stated chemical reaction, another reaction may be proposed in
which H2 produced during the decomposition of Titanium isopropoxide may react with
oxygen present in the Co(acac)2. With the increase in Co(acac)2 amount to get 8wt.% and
15wt.% Co doped samples, this reaction becomes more prominent. This might be the
reason for stable rutile phases at higher doping concentration.
From XRD data it is concluded that 2-8wt.% Co concentration has stabilized the
unstable oxygen deficient magneli phases. However there was a decrease in the
crystallinity from 2-8wt.% Co concentration which is attributed towards the formation of
new phases. With further increase in Co concentration up to 15wt.% pure rutile phase is
confirmed with very high crystallinity. In all the samples no Co or Co oxide peaks were
found. A slight shift of rutile peak is observed at 2θ 20 . It is known that Co has the
larger atomic radius (0.72Å) than Ti atomic radius (0.68 Å). When an atom with larger
atomic radius replaces a small atom in the lattice, it is observed that peak is shifter
towards the lower 2 value. Therefore a slight shift on rutile peak at 20 suggests that Co
is incorporated within the TiO2 matrix.
Chapter 6
104
Fig 6. 13: XRD pattern of Ni doped TiO2 thin films with a) 2wt.% Ni b) 4wt.% Ni c)
6wt.% Ni d) 8wt.% Ni and e) 15wt.% Ni doping
Chapter 6
105
6.3.2.2 Rutherford Back Scattering
Rutherford backscatering spectrometry (RBS) mearurements of epilayer Co doped
TiO2 samples are presented in Fig 6.8 and table 6.3. All the measurements were taken
with the same method as discussed above for the Ni doped TiO2.
Fig 6.8 shows RBS for Co doped TiO2 thin films.
Fig 6. 14: RBS spectra of Co doped TiO2 thin films with various concentrations
The peaks at high energy correspond to the scattering from the Ti and Co. There is no
evedience for the formation of silicate with Ti or Co . The Co concentration calculated
by RBS analysis is slightly low than the experimental value. This may be due to low
solubility of Co(acac)2 in taluene. Films thicknesses were found in the range of 150-
250nm as measured from RBS spectra and shown in the table 6.3
Chapter 6
106
Table 6. 4: Co concentration and film thickness as calculated from RBS spectra
Sample Doping concentration(wt.%)
Stiociometric Calcutated
Films thickness
(nm)
Co-2 2 1.85 166
Co-4 4 3.77 219
Co-6 6 5.4 204
Co-8 8 7.1 246
Co-15 15 13.22 160
. Both spectra indicate that the Co doped TiO2 films are non-stoichiometric. This
result is in agreement to the result found from XRD analysis. Arguments for non-
stoichiomery in the Co doped TiO2 films are established in section 6.3.2.1.
6.3.2.3 Scanning electron microscopy
SEM images of Co doped TiO2 thin films deposited by AACVD (Fig 6.9) shown
compact and smooth film morphologies with homogenously dispersed particles.
Individual grains are well defined and clear grain boundaries can be seen. The packing
density of the microstructure and the grain sizes apparently seem to be affected by
variation of the Co concentration. SEM images further reveal that all the films are
nanoporous structure with a very uniform grain size distribution. The average grain size
lies in the range of (40-60nm). Shape of the grains changed with increase in Co
concentration as shown in Fig. 6.9. The change is the structure of the grains is attributed
to the growth stress of the films and radius and the concentration of the Co ions.
In the case of sample with 2wt.% Co a compact dense morphology was obtained.
The grains are of spherical in shape. Average grain size is 25-50nm and the size
Chapter 6
107
distribution throughout the film is very uniform. Pore size in the film is found only a few
nanometers. At 4 wt.% Co concentration shape of the grains is changes into elongated
pallet like with a length between 50-100nm and width is only a few nanometer. Gains are
uniformly distributed with uniform grain size distribution. However there are some
flakes or the larger aglaromated grains can be seen with a small concentration. The SEM
image of the films with 6wt.% Co concentration shows that the growth of particles has
retained their shape but the grain size terned into smaller range . A decrease in the
porosity can also be seen clearly. Another change in the shape of the grains is observed in
the films with 8wt.% Co concentration. However some large size grains can also be seen
with average grain size in the range of 100-200nm. In 15wt.% Co concentration the shape
of the grains is changed into beed like structures.
The change in the grain shape in can be explained by analyzing XRD data. From
XRD data it can be seen that there is considerable variation in the maximum intense peak,
indicating preffered growth direction are different for each phase formed with increase in
doping concentration. Also decomposition reaction kienetics at the substrate surface
define the grain shape and size.
Chapter 6
108
Fig 6. 15: SEM images of Co doped TiO2 thin films with a) 2wt.% Co b) 4wt.% Co c)
6wt.% Co d) 8wt.% Co and e) 15wt.% Co doping
a) b)
c) d)
e)
Chapter 6
109
6.3.2.4 Magnetic properties
Room temperature magnetic hysteresis loops of Ti1-xCoxO2 for x=0.02, 0.04, 0.06,
0.08 and x=0.15 are shown in Fig 6.10
Fig 6. 16: Hysteresis loop of Co doped TiO2 thin films with various Co doping
concentrations
Chapter 6
110
All the samples exhibit ferromagnetism (300K) as shown in the Fig. 6.10
Magnetisation has increased with increasing Co concentration. But at 8wt.% Co
concentration anomalous behavior is observed. There can be number of possibilities
which can cause RTFM in Co doped Oxygen deficient TiO2 structures as follows:
1. Co clusters in metallic form and secondary phases
2. Ti+3
and/or Ti+2
interstitial defects
3. Ti+3
doped TiO2 (self doping)
4. Ti+3
and/or Ti+4
replaced by Co+2
5. Oxygen vacancies
No clustering from metallic Co and other ferromagnetic phases observed in XRD
analysis. Further magnetic saturation in samples with 2-8wt.% Co doping concentration
occurs well below than that for cobalt metal films (H~1.5-2 Tesla).18
Therefore
formation of BPMs and their overlap is one of the possible source of RTFM in the films.
Further magneli phases with lower stoichiometry (TiO1.75, TiO1.80 and TiO1.83 etc.)
are antiferromagnetic with Neel temperature ~130K and paramagnetic at higher
stoichiometry(Ti3O7 and Ti4O9 etc.)152
. Therefore contribution in RTFM from these
magneli phases can be ruled out at this level.
As discussed in XRD section, presence of T+3
and Ti+2
cannot be ruled out. Ti+3
and Ti+2
may itself source of ferromagnetism due to their unfilled d-shell with electronic
configuration as 3d1 and 3d
2, however it these ions are isolated then there may only be a
paramagnetic effect.153
Recently Hua et al.138
reported RTFM in Ti+3
doped TiO2 nanowires prepared by
solvothermal method, The Ms value was found to be ~23.6 memu/g. However in the
presence of dopant, it is very difficult to distinguish whether RTFM is from Ti or Co
doping.
Chapter 6
111
RTFM in current study can be explained with the help of BPM model for both
cases i.e. Co+2 and Ti+3 doping. In this case Co+2
can replace Ti+4
as well as Ti+3
. In both
cases it can introduce positively charged vacancy. However, Co+2
most likely replace Ti+3
due to comparable oxidation state as compared to Ti+4
. These vacancies can capture an
electron in quasi hydrogenic orbit surrounded by Co+2
ions and construct a BMP. Co+2
ions may couple via donor electron ferromagnetically. As doping concentration increases,
BMPs concentration increases throughout the samples. The overlap of BMPs establishes
long range ferromagnetic ordering. Co-Co, Ti-Ti and Ti-Co may interact via BPMs.
Thus net magnetization may be the combined result of ferromagnetic interaction of
different types of BMPs.
At 8wt.% Co doping concentration ferromagnetic signal coupled with a large
paramagnetic signal is observed. Since unstable oxygen deficient magneli phases152
are in
transition to stable rutile phase from 2-8wt.% Co doped samples and overall crystallinity
of sample is reduced in the process as can be seen in XRD patterns. It is therefore quite
possible that paramagnetic magneli phases are still present in amorphous form. An
increase in the moment observed in 15% Co doped film. XRD results shown that the
there is a pure rutile phase at this doping concentration. Magnetic moment seems to
increase with crystallinity and also in the stable rutile phase.
Table 6. 5: Magnetic moment of Co doped TiO2 at 300K
Sample Moment (emu/cc)
300K
Co-2 4.90
Co-4 6.29
Co-6 7.11
Co-8 No Saturation
Co-15 10.54
Chapter 6
112
Summary
In summary, Ni and Co doped TiO2 films were synthesized by AACVD under
oxygen deficient Argon environment. Working temperature was 450 C and 650 C for Ni
and Co doped films respectively. Analyses of the XRD suggests that anatase phase in the
dominant phase up to 8% Ni concentration, however at 15% Ni doping a change in the
phase observed. Due to the fact that Ni reacted with the TiO2 matrix NiTiO3 phase is
formed. With Co doping magneli phases are observed along with the rutile phase. At 15%
pure rutile phase with high crystallinity was observed. SEM has shown uniform
distribution on the grain sizes in both the cases. All samples are ferromagnetic at room
temperature i.e. 300K..
Chapter 7
113
Chapter 7
Conclusions and Future Work
This chapter is based on some important conclusions drawn from the thesis. The
thesis was comprised of two studies, both having the great impact in the field of
spintronics. First part was about the studies of Exchange Bias and its related
phenomenon. The second half was about the synthesis of Nickel and Cobalt Doped TiO2
by Aerosol Assisted Chemical Vapour Deposition (AACVD) keeping in view the
importance of finding new synthesis routes, as properties may vary significantly in such a
systems.
In section 7.1 conclusions drawn on the basis of experiments on Exchange Bias
phenomenon are summarized. Section 7.2 is devoted for the come out of research on
synthesis and structural and magnetic properties of Ni and Co doped TiO2 thin films.
7.1 Exchange Bias
In chapter 4 the effects of sample shape and fabrication process on the reversal
mechanism in Exchange Bias multilayer thin films were studied.
To study the effects, all samples studied in this work were produced by sputtering
using a HiTUS sputtering system. The average process pressure was 2.75×10−3mbar and
the RF power was held at 1.5kW. During fabrication of the samples a field of 300Oe was
applied to give an easy axis to the exchange coupled films. Five samples were grown
with variable bias voltage (200,400,600,800 and 1000V) whilst keeping the composition.
However sample grown on 800V was used to carry out further studies being the best
sample in the series. To study the effect of fabrication technique Samples were prepared
by three methods:
1. Samples were cut from a continuous film using a diamond scribe and then
cracked.
Chapter 7
114
2. The sample was sputtered through a thin stainless steel mask
3. And a sample was cut with ultrasonic cutter.
The effect of nucleation was described arising from both sample shape effects and the
process used to cut the sample. It was found that cutting techniques such as the use of
ultrasonic cutters leads to a large increase in nucleation which distorts the hysteresis loop.
Deposition through masks causes shadowing effects at the edges that also distort the
loops. Cutting with a diamond scribe appears to give the best outcome. Finally it was
concluded that
The sample edge roughness leads to a distribution of nucleation fields and hence
changes the shape of the hysteresis loop which has consequences for MRAM or
spintronics devices.
The best way to cut samples of nucleation controlled materials is by cracking.
The effect of using an ultrasonic cutter is catastropic with many chips along the
film leading to areas with a strong demagnetising field that will readily nucleate
multiple domain nucleation.
Shadow masks cause thinning at the edges which also affects nucleation.
The overall shape of a sample is less critical than edge roughness.
The geometric response in coils or other sensors in magnetometers has to be taken
into account and a calibration sample of similar size, moment and shape has to be
used.
A different coercivity is observed in the recoil loop for the sample deposited
through a mask.
Our understanding of the coercivity of exchange bias systems is not clear enough.
In chapter 5 The effects of inserting mono-atomic layers of Mn at the interface of
IrMn/CoFe bi-layers were investigated. The insertion of Mn alters spin structure at
AFM/FM interface which may bring an enhancement in TB. Samples were grown using
HiTUS sputtering technology at 200, 400, 600, 800 and 1000V bias voltages.
Chapter 7
115
The effects of placing a 2Å Mn dusting layer at the AFM/FM interface on the
exchange bias, coercivity and blocking temperature of IrMn/CoFe bilayers were
investigated. of the bilayer was mildly enhanced from (-411±42.4) Oe to (-
479.7±44.3) Oe, while was increased from (222.3±23.9) Oe to (256.5±37.4) Oe. It is
believed that the enhancement of and is due to the modification of the AFM spin
structure at the interface.The blocking temperature was reduced from 408.3 K to 380.0 K
with the addition of Mn; it is believed that grains that were previously too small to be
thermally stable were somehow set. This could be due to either a chemical reaction with
the Mn at the interface or due to exchange effects.
In chapter 5 effect of grain size on the training effect was also investigated. It is
an important parameter due to the fact that it is the measure of the stability of exchange
bias bilayer which is inside many devices.
The training effect is strongly reinforced by the reduced grain size of the AFM
layer.
It was confirmed that nanocrystalline structure have large ratio of spins located
at grain boundaries. These spins are disordered which bring out variation in the
magnetic anisotropy. This alternately changes the strength of the exchange bias
thus promoting the training effect.
Larger AFM grain-volumes give rise to thermally stable bias fields and
consequently smaller training effects.
7.2 Diluted magnetic semiconductors
Thin films of Til-XNiXO2 (x=0.02-0.15) were prepared by Aerosol Assisted
Chemical Vapor Deposition (AACVD) on Si substrates in Argon atmosphere on 450 C.
The precursor [Ni2Ti2(OEt)2(l-OEt)6(acac)4] was used for Ni doped TiO2 films. A
modified design of AACVD was used for deposition. XRD pattern shows that up to 8%
Ni doping and an overall increase in crystalline with increasing Ni contents, plus a
greater peak intensity in the (101) reflection. However No secondary phase from Ni or
Chapter 7
116
NiO obtained suggesting that the Ni contents are doped within the TiO2 matrix. At 15%
Ni concentration NiTiO3 phase appeared. This change in phase was due to the fact that Ni
would have increased the number of oxygen vacancies which might be responsible for
the transformation of anatase to rutile. RBS data showed the Ni and Ti composition was
close to the stoichiometry. The Depth profile showed that composition is very uniform
throughout the film thickness. Thickness for 2% to 8% Ni concentration is between 200-
260nm however for 15% Ni concentration it was more than 800nm. SEM images show
uniform distribution on the grains with clear grain boundaries. Average grain size was
measured between 40-60nm, however, for 15% Ni doped samples grains or the grain
boundaries are not clearly visible. Magnetic measurements taken from SQUID show that
all the samples exhibit ferromagnetism at room temperature (RTFM). It can be concluded
that this ferromagnetism is intrinsic as no Ni peaks can be seen within the XRD limit.
Further no oxide of nickel or bimetallic oxides of Ni and Ti are ferromagnetic.
RTFM is attributed to the oxygen vacancies. Oxygen vacancies were induced by the
oxygen deficient environment and due to the incorporation of Ni doping. Bound
Magnetic Polaron model is successful model to describe RTFM in Ni doped TiO2 films.
This can be further confirmed by taking SQUID measurements at low temperature
(100K). At low temperature BMPs formed due to the presence of oxygen vacancies grow
resulting in enhancement of RTFM.
Co doped TiO2 films were deposited with the same scheme as for Ni doped TiO2
films, however, on a higher temperature of 700 C. At higher temperature in the oxygen
deficient environment so called magneli phases of TiO2 appeared. However From 2-8%
Co concentration has stabilized the unstable oxygen deficient magneli phases. This may
be due to the inert atmosphere and high temperature during the synthesis of the films. A
slight shift of rutile peak is observed at 2θ 20 due to the fact that Co has the larger
atomic radius (0.72Å) than Ti atomic radius (0.68 Å).This shift is evident that Co ions are
incorporated within the TiO2 lattice. Grain size was uniform throughout the series as seen
by SEM. There was a slight decrease in concentration of Co than the stoichiometric
amount, which is due to less solubility of Co(acac)2 in toluene. All the samples exhibit
Chapter 7
117
RTFM. This RTFM is a combination of Co doping and the oxygen deficient magneli
phases.
7.3 Future Work
For further investigation few suggestions drawn from the research out of the thesis are:
1. To further investigate the effects of the interfacial layers it would be highly
beneficial to investigate the effect of varying the dopant layer thickness.
2. In the case of Mn it is vital to attempt to characterize the phenomena of the
decrease in with higher setting fields.
3. In case of Mn doping, the elimination of experimental error as a source of this
effect would be a result of further measurements.
4. Theoretic understanding of coercivity mechanism in exchange bias films, it is
therefore an area to investigate.
5. York protocol should be implied to investigate the theoretical understanding of
Training Effect. It would produce high impact on spintronics industry if the entire
exchange bias related phenomenon be related with the grain size.
6. Synthesis through AACVD provides a good opportunity to control number of
parameter like reaction environment, temperature, selection of precursor.
Presence of magneli phases alone can induce RTFM. It is worth to study TiO2
films at various temperatures without doping.
7. Number of transition metals and non-metallic doping cases are not studied with
AACVD. It is in the scientific to compare various transition metal doped DMS
properties synthesized with various routes. AACVD can be one of them
Chapter 8
118
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List of Publications
1. R. Hussain, B. Kaeswurm and K. O'Grady
Sample fabrication effects in exchange bias systems
J. Appl. Phy. 109 (7), 07E533-533 (2011).
2. S. T. Hussain, K. Khan and R. Hussain
Size control synthesis of sulfur doped titanium dioxide (anatase) nanoparticles, its
optical property and its photo catalytic reactivity for CO2 + H2O conversion and
phenol degradation
J.Nat. Gas Chem. 18, 383-391 (2009).
3. S. T. Hussain, N. Niaz, A., I. Amed and R. Hussain
New Synthesis Procedure for the Production of Trimetallic Composite for
Hydrogen Storage
Intl.l Rev. Chem. Engg. 1, 238-242 (2009).
4. N. A. Niaz, I.Ahmad, S. Nasir, Z.Wazir, R. Hussain, N. R. Khalid and S. T.
Hussain Synthesis and Characterization Of Mg-al Alloys For Hydrogen Storage
Applications Digest Journal of Nanomaterials and Biostructures. 8(1), 423-431
(2013).
5. A. Nisar, S.T. Hussain, M.A.Iqbal, H. Ayesha, M. Arshad, S. Akram, Z. Ali, N.
Ahmad, S. M. Abbas and R. Hussain
Optoelectronic properties of evaporated antimony tin sulfide thin films for solar
cell applications
Elixir Renewable Energy Engg. 55, 13129-13132 (2013).
6. N. Ahmad, S.T. Hussain, B. Muhammad, T. Mahmood, Z. Ali, N. Ali, S.M.
Abbas, R. Hussain and S.M. Aslam
Effect of the Reaction Conditions on Al-Pillared Montmorillonite Supported
Cobalt-Based Catalysts for Fischer Tropsch Synthesis
Digest Journal of Nanomaterials and Biostructures. 8(1), 347-358 (2013).