Analysis of Ferromagnetic-Multiferroic interfaces in Epitaxial Multilayers of LSMO and BFO

Student: Peter KnappResearch Advisor: Professor Jeremiah Abiade

OverviewI. Bilayers were fabricated from ferromagnetic

(FM) LSMO (La0.7Sr0.3MnO3) and anti-ferromagnetic (AFM) BFO (BiFeO3) via Pulsed Laser Deposition (PLD)

II. Layers were analyzed using TEM (Transmission Electron Microscopy), XRD (X-ray Diffraction), and XPS (X-ray Photoelectron Spectroscopy) in order to confirm composition and observe structural detiails

Motivation For Project• Need to control the structure of oxide thin films and

multilayers

• Understand effects of structure & layering on magnetic interaction

• Preliminary work for future experiments on properties of ferromagnetic/ferroelectric systems

Introduction to Multiferroic Bilayers• Materials where electric polarization

influences ferromagnetic polarization, allowing manipulation of electric/magnetic order1

• Contemporary research focuses on bilayers of FM and AFM materials

• These structures demonstrate exchange bias (EB), exchange enhancement (EE), and exchange coupling (EC)

Particular Interest in LSMO and BFO• On their own LSMO and

BFO possess useful characteristics

• Combined they clearly exhibit exchange interactions that characterize multiferroic systems

• Additional advantages include common perovskite structure and a close lattice parameter

(A) (B)

All Perovskites have the same basic chemical formula: ABO3

Interfacial Effects• Researchers know little about how interfacial

effects impact magnetic effects• It is known that there is lattice mismatch and

diffusion between LSMO and BFO layers.• It is necessary to understand how these

phenomena can effect film properties

Lattice Mismatch

Controlling Structure

• These experiments will focus on achieving structural control during deposition

• Substrate will be varied between LaAlO3 or SrTiO3

• The thickness of the layers will be varied

• Layer order will be varied

Potential Applications of Work

• Could help demonstrate novel uses for materials like LMSO and BFO in memory devices and sensors, for instance Hard Drives and SQUIDs (superconducting quantum interference devices)

• Development of novel heterostructures for unusual uses i.e. LMSO as electrode for ferroelectric films

• Tailor structures to realize multicomponent multiferroic systems (e.g. electrical control of magnetism)

Experimental Procedures

I. PLD for synthesis of the Bilayers.II. TEM to observe local characteristicsII. XRD to observe interlayer

interaction and structural characteristics

III. XPS to confirm composition

Pulsed Laser Deposition1. Physical Vapor Deposition

Technique2. High Powered (Excimer Laser)

focused on target (material to be deposited) in vacuum

3. Material is vaporized into plasma plume which extends from target

4. Proceeds to land on substrate forming a thin film

5. Highly Advantageous

Transmission Electron Microscopy

• Beam of Electrons fired through specimen

• Electrons interact with material in film

• Image created on photographic film or a CCD camera

II. X-Ray Reflectivity• Measurement: Specular reflection

as a function of angle of incidence.

• Result: electron density profile along substrate normal

• Thickness and average electron density of the film.

• Thickness and electron density can be used to infer roughness and structural defects like diffusion and lattice mismatch

• X-ray techniques can also be used to analyze strain in the films

Thin Film or Multilayer

Thin Film or

Multilayer

III. X-ray Photoelectron Spectroscopy• XPS = X-Ray Photoelectron

Spectroscopy• Kinetic Energy and

Intensity of electrons emitted from material irradiated with X-Rays is measured

• Yields elemental composition, empirical formula, chemical state, and electronic state XPS Mechanism

PLD Results: Films Deposited

• Target Substrate Distance=4.5 cm

• Deposition Temp=6500 Celsius

• O2 Background=0.02 Torr

• Pulse Frequency=5 Hz• Laser Fluence =1.5 Jcm-2

• Wavelength=248 nm• Used KrF Excimer Laser

Thickness LSM0 (nm)

Pulses for LSMO

Deposition

Thickness BFO (nm)

Pulses for BFO

Deposition

Order of layers on substrate (bottom/top)

0 0 150 10,580 BFO

150 10,580 150 10,580 BFO/LSMO

200 14,100 150 10,580 BFO/LSMO

250 17,630 150 10,580 BFO/LSMO

150 10,580 0 0 LSMO

150 10,580 150 10,580 LSMO/BFO

150 10,580 200 14,100 LSMO/BFO

150 10,580 250 17,630 LSMO/BFO

Films deposited on both LaAlO3 and SrTiO3

TEM Results – 150nm_BFO_LaAlO3

5 nm5 nm 5 nm5 nm inverse contrast

LaAlO3

BFO

LaAlO3

BFO

TEM Results – Contd.

film (40cm) 100 nm100 nm

300 nm

Clean Diffraction Pattern Indicates highly crystalline film

Growth rate of BFO twice what was expected

LaAlO3

BFO

Glue

Unknown

TEM - Results

1. PLD Allowed for deposition of films that are highly crystalline

2. At the interface there is a slight rotation (30o to 40o) between the crystalline plane of the substrate and film

3. Growth Rate of BFO is twice that of LSMO

XRD Preliminary WorkSlit Collimation GeometryS1 = 0.5 mm (h) 2 mm (v) S2 = 0.1 mm (h) 2 mm (v)S3 and X Replaced with Soller Slit to lock out reflection from excess crystal planes/substrate Sample : 5mmX5mmX0.5mm substrates

Rigaku-ATXG diffractometer

X

S3

S2

S1

Crystallinity Scans• Hold Omega at 0.5 degrees• Scan 2Theta from 20o to 600

• If resulting graph has – Single Peak Single Crystal– Multiple Peaks Polycrystalline– No clear Peaks Amorphous

Polycrystalline Nanocrystaline

20 25 30 35 40 45 50 55 600

50100150200250300350

150nm_BFO_LaAlO3

2Theta (deg)

Coun

ts P

er S

econ

d

20 25 30 35 40 45 50 55 600

20

40

60

80

100

150nm_BFO_SrTiO3

2Theta (deg)

Coun

ts P

er S

econ

d

20 25 30 35 40 45 50 55 600

5

10

15

20

25

150nm_LSMO_150nm_BFO_LaAlO3

2Theta (deg)

Coun

ts P

er S

econ

d

Amorphous

Sample Scans

Polycrystalline Nanocrystaline

20 25 30 35 40 45 50 55 600

50100150200250300350

150nm_BFO_LaAlO3

2Theta (deg)

Coun

ts P

er S

econ

d

20 25 30 35 40 45 50 55 600

20

40

60

80

100

150nm_BFO_SrTiO3

2Theta (deg)

Coun

ts P

er S

econ

d

20 25 30 35 40 45 50 55 600

5

10

15

20

25

150nm_LSMO_150nm_BFO_LaAlO3

2Theta (deg)

Coun

ts P

er S

econ

d

Amorphous

20 25 30 35 40 45 50 55 600

50

100

150

200

250

150nm_BFO_150nm_LSMO_SrTiO3

2Theta (deg)

Coun

ter P

er S

econ

d

Approaching Single Crystal

Crystallinity Scan Contd.

20 30 40 50 60 70 800

10

20

30

40

50

60

150nm_LSMO_LaAlO3

2Theta (deg)

Coun

ts P

er S

econ

d

20 25 30 35 40 45 50 55 600

10203040506070

150nm_LSMO_SrTiO3

2Theta (deg)

Coun

ts P

er S

econ

d

Amorphous Nanocrystalline or Amorphous

20 25 30 35 40 45 50 55 600

20

40

60

80

100

150nm_BFO_150nm_LSMO_LaAlO3

2Theta (deg)

Coun

ts P

er S

econ

d

20 25 30 35 40 45 50 55 600

50

100

150

200

250

150nm_BFO_150nm_LSMO_SrTiO3

2Theta (deg)

Coun

ter P

er S

econ

d

Nanoctystalline or Amorphous Polycrystaline

Crystallinity Scans Contd.

20 25 30 35 40 45 50 55 600

5

10

15

20

25

150nm_LSMO_150nm_BFO_LaAlO3

2Theta (deg)

Coun

ts P

er S

econ

d

20 25 30 35 40 45 50 55 600

5

10

15

20

25

30

150nm_LSMO_150nm_BFO_SrTiO3

2Theta (deg)

Coun

ts P

er S

econ

d

20 25 30 35 40 45 50 55 6005

101520253035

200nm_BFO_150nm_LSMO_LaAlO3

2Theta (deg)

Coun

ts P

er S

econ

d

20 25 30 35 40 45 50 55 600

20

40

60

80

100

120

200nm_BFO_150nm_LSMO_SrTiO3

2Theta (deg)

Coun

ts P

er S

econ

d

Amorphous Amorphous

Amorphous Amorphous or Nanocrystalline

Crystallinity Scans Contd.

20 25 30 35 40 45 50 55 600

5

10

15

20

25

200nm_LSMO_150nm_BFO_LaAlO3

2Theta (deg)

Coun

ts P

er S

econ

d

20 25 30 35 40 45 50 55 6002468

10121416

200nm_LSMO_150nm_BFO_SrTiO3

2Theta (deg)

Coun

ts P

er S

econ

d

20 25 30 35 40 45 50 55 6002468

10121416

250nm_BFO_150nm_LSMO_LaAlO3

2Theta (deg)

Coun

ts P

er S

econ

d

20 25 30 35 40 45 50 55 6002468

1012141618

250nm_BFO_150nm_LSMO_SrTiO3

2Theta (deg)

Coun

ts P

er S

econ

d

Amorphous Amorphous

Amorphous Amorphous

Crystallinity Scans Contd.

20 25 30 35 40 45 50 55 6005

101520253035

250nm_LSMO_150nm_BFO_LaAlO3

2Theta (deg)

Coun

ts P

er S

econ

d

20 25 30 35 40 45 50 55 600

20

40

60

80

100

120

250nm_LSMO_150nm_BFO_SrTiO3

2Theta (deg)

Coun

ts P

er S

econ

d

Amorphous Amorphous or Nanocrystalline

Results• Majority of Films are amorphous

• Several Films appear to be Polycrystalline or Nanocrystalline

• New BFO film created with alternate deposition parameters

Nanocrystaline Samples

Possible to determine the size of crystallites using the Scherrer Eqn.

cos

2LKB

B(2) = Peak Width (radians)λ = .1542 nmL = Crystallite Width (nm) = d-spacing (radians)K = Scherrer Constant (Assumed to be 1)

Film (radians)

B(2)(radians)

Crystallite Width(nm)

150nm_BFO_SrTiO3

0.263 0.111 5

150nm_LSMO_SrTiO3

0.256 0.0803 8

150nm_BFO_150nm_LSMO_LaAlO3

0.265 0.111 5

200nm_BFO_150nm_LSMO_SrTiO3

0.259 0.0986 6

250nm_LSMO_150nm_BFO_SrTiO3

0.254 0.116 5

New 150 nm BFO Film on SrTiO3• Used standard Laser

Fluence and Pulse Frequency

• Modified Annealing Process

• Deposition at 670o C at .02 Torr

• Cool to 390o C, anneal for 1 hour

• Cool to room temperature at 5o C/min

20 30 40 50 60 70 800

10

20

30

40

50

60

150 nm BFO on SrTiO3 Newly Prepared

2Theta (deg)

Coun

ts P

er S

econ

d

Data indicates Amorphous film. XPS analysis used to confirm

composition allowing us to draw a more accurate conclusion.

Crystallinity Scans - Results

• Majority of films are amorphous with some polycrystalline and nanocrystalline samples

• Likely due to diffusion of oxygen during annealing

• Indicated deposition process still requires optimization

X-Ray Reflectivity 150nm_BFO_150nm_LSMO_SrTiO3

GE111 Compressor CrystalS1 = 0.5 mm (h) 2 mm (v) S2 = 0.1 mm (h) 2 mm (v)S3 = 0.2 mm (h) 5 mm (v)X = 0.2 mm (h)Flux: ~ 2.1*106 photons/s Sample : 5mmX5mmX0.5mm substrates

Layer Thickness (Å) SLD (Real) SLD (Imaginary) Roughness (Å)

Air INF 0 0 0

Residue 84.3 4.27*10-6 3.32*10-8 30.1

BFO 1450 6.57*10-5 7.90*10-6 77.5

LSMO 1550 5.09*10-5 1.59*10-5 51.5

SrTiO3 Substrate INF 4.49*10-5 1.95*10-6 49.8

Conclusion - XRR

• Thickness and SLD data seems reasonable but contrasts with data on growth rate from TEM

• Unfitted drop results from having a high roughness film and low X-ray intensity during scanning

• Top residue Layer is Likely a Combination of organics and silver particles from adhesive

XPS Analysis

Peak Position BE (eV)

FWHM (eV)

Raw Area (CPS)

RSF Atomic Mass

Atomic Conc. (%)

Mass Conc. (%)

Bi 4f 157 2.881 1481870 9.140 208.98 24 74

Fe 2p 708 5.099 245382.5 2.957 55.846 13 11

O 1s 527 3.380 331482.5 0.780 15.99 63 15

Peak Position BE (eV)

FWHM (eV)

Raw Area (CPS)

RSF Atomic Mass

Atomic Conc. (%)

Mass Conc. (%)

Bi 4f 156 2.767 1302850 9.140 208.98 21 68

Fe 2p 708 4.572 378805.0 2.957 55.846 19 17

O 1s 527 3.162 318230.0 0.780 15.99 60 15

XPS Results for original 150nm_BFO_SrTiO3: Proper Stoichiometry Observed

XPS Results for New 150nm_BFO_SrTiO3: Proper Stoichiometry not Observed

XPS - Results

• Stoichiometry of films very similar to target material

• Currently no explanation for iron deficiency in the new BFO film

Summary/Conclusion

• While the constructed films were not epitaxial many were highly crystalline

• The Stoichiometry of films examined by XPS was consistent with the target material

• XRR indicated the films have a large roughness

• The deposition process for LSMO and BFO still requires optimization.

Acknowledgements

The financial support from the National Science Foundation, EEC-NSF Grant # 1062943 is gratefully acknowledged. I would like to thank Professors Jursich and Takoudis for organizing the REU Program. I would like to thank the LORE lab in general and Professor Jeremiah Abiade specifically for providing me with the opportunity to work in their lab.

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