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Journal of Physics and Chemistry of Solids 71 (2010) 375–380

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Journal of Physics and Chemistry of Solids

0022-36

doi:10.1

n Corr

E-m

journal homepage: www.elsevier.com/locate/jpcs

Improvement in electrical and magnetic properties of mixed Mg–Al–Mnferrite system synthesized by citrate precursor technique

Gagan Kumar a,n, Jagdish Chand a, Anjana Dogra b, R.K. Kotnala b, M. Singh a

a Department of Physics, Himachal Pradesh University, Shimla 171005, Indiab National Physical Laboratory, New Delhi 110012, India

a r t i c l e i n f o

Article history:

Received 22 April 2009

Received in revised form

5 December 2009

Accepted 14 January 2010

Keywords:

A. Ceramic

A. Magnetic material

C. X-ray diffraction

D. Magnetic properties

97/$ - see front matter & 2010 Elsevier Ltd. A

016/j.jpcs.2010.01.003

esponding author. Tel.: +91 9816418069; fax

ail address: bhargava_phy_hpu@yahoo.co.in (

a b s t r a c t

In the present work, mixed magnesium–manganese ferrites of composition Mg0.9Mn0.1Al0.3CozFe1.7�zO4

where z=0.3, 0.5 and 0.7 have been synthesized by the citrate precursor technique. X-ray diffraction

patterns of the samples confirmed the formation of single-phase spinel structure. The ferrites have been

investigated for their electric and magnetic properties such as dc resistivity, Curie temperature,

saturation magnetization, initial permeability and relative loss factor (RLF). Fairly constant value of

initial permeability over a wide frequency range (0.1–20 MHz) and low values of the relative loss factor

of the order of 10�4–10�5, in the frequency range 0.1–30 MHz, are the cardinal achievements of the

present investigation. In addition to this, initial permeability was found to increase with an increase in

temperature while RLF was observed to be low at these temperatures. The dc resistivity and Curie

temperature were found to increase with an increase in cobalt content. The mechanisms contributing to

these results are discussed in detail in this paper.

& 2010 Elsevier Ltd. All rights reserved.

1. Introduction

Ferrites have been the emerging focus of recent scientificresearch both from the synthesis and applications point of view[1–6]. Mg–Mn ferrites are well known technological magneticmaterials finding applications in various electrical and magneticdevices because of their high magnetic permeability and low corelosses. Properties of ferrites are known to be sensitive to theircomposition and the processing technique used to synthesizethem [7]. A small deviation in the compositional stoichiometry ofthe ferrite affects its properties greatly [8]. Mg–Mn ferrites,synthesized by conventional techniques, were studied by so manyworkers [9–11], but very little literature is available on the studyof these ferrites by non-conventional methods. The conventionalprocess is known to have some inherent drawbacks such as poorcompositional control, chemical inhomogeneity and introductionof various impurities during the ball milling [12]. In the case offerrites prepared by the conventional method, there is invariablysome possibility of introducing iron impurities into the ferrite dueto abrasion of steel balls during milling [13]. The introduction ofimpurities leads to inhomogeneity in the ferrite structure, whichhas an adverse effect on some of the magnetic properties. Inaddition milling also introduces lattice stress and strains. whichresult in distorted structures. Therefore, in the present study, anon-conventional technique known as the citrate precursor

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G. Kumar).

method [14,15] is employed for the preparation of ferrite series.This method does not require any ball milling to mix the materialsas required in the conventional method, so there is no possibilityof impurity and hence of non-stoichiometry. It has been reportedearlier that in Mg–Mn ferrites if Fe ions are replaced by Al3 + [16]and Co2 + [17], the electrical and magnetic properties aremodified. Thus, in the present work suitable combinations ofaluminium and cobalt were formed and investigated for theirelectric and magnetic properties. Although the electric andmagnetic properties of Mg–Mn ferrites have been studiedextensively [18,19] literature on the magnetic properties at highfrequencies is scarce. The dc resistivity of a ferrite is an importantproperty, since it determines its performance at high frequencies,where eddy current losses may be high, resulting in a significantloss of energy. Therefore, the ferrites have been investigated forthe variation of dc resistivity with temperature, composition,activation energy and Curie temperature. Temperature depen-dence of initial permeability is a very important factor to beconsidered in designing any magnetic component; thus, in thispaper we report the variations of initial permeability (mi) andrelative loss factor (RLF=tan d/mi) with frequency at differenttemperatures of Co2 + doped Mg–Al–Mn ferrite system.

2. Experimental procedure

The ferrite series with the composition Mg0.9Mn0.1Al0.3Coz-Fe1.7�zO4 (z=0.3, 0.5 and 0.7) was prepared using hydrated

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Fig. 1. X-ray patterns of Mg0.9Mn0.1Al0.3CozFe1.7�zO4, z=0.3, ferrite powder

sintered at 1200 1C.

Fig. 2. Scanning electron micrographs (SEM) of fractured surfaces for

Mg0.9Mn0.1Al0.3CozFe1.7�zO4, z=0.3, ferrites sintered at 1200 1C.

G. Kumar et al. / Journal of Physics and Chemistry of Solids 71 (2010) 375–380376

nitrates of the constituent elements. The chemicals used were (a)Mg(NO3)2 �6H2O (498%, Merck India), (b) Mn(NO3)2 �4H2O (498%,Merck Germany), (c) Al(NO3)3 �9H2O (498%, Merck Germany),(d) Co(NO3)3 �6H2O (498%, Merck Germany) and (e) iron was inthe form of iron citrate C6H5O7Fe �3H2O (498%, Merck Germany).The solid state reactions followed for the preparation of thesamples were

0:9½MgðNO3Þ2 � 6H2O�þ0:1½MnðNO3Þ2 � 4H2O�þ0:3½AlðNO3Þ3 � 9H2O�

þz½CoðNO3Þ3 � 6H2O�þð1:7�zÞ½C6H5O7Fe � 3H2O�

-½Mg0:9Mn0:1Al0:3CozFe1:7�zO4�; z¼ 0:3; 0:5 and 0:7

The quantities of the reagents required for each compositionwere calculated from the above solid state reactions. To preparethe samples, the required quantity of iron citrate was dissolved indistilled water by heating at 40 1C with constant stirring to obtainthe solution S1. The required amounts of magnesium nitrate,manganese nitrate, aluminium nitrate and cobalt nitrate weredissolved in distilled water by heating to 40 1C and constantstirring to obtain the solution S2. The solutions S1 and S2 weremixed together, ensuring no loss of material. The mixture washeated to 40 1C with constant stirring to obtain a dried product inthe form of brown coloured glassy material containing theconstituent metal ions homogeneously mixed together at theatomic level. The dried mixture was calcined for 2 h at 500 1C atthe rate of 250 1C/h to obtain the ferrite powder. The calcinedpowder obtained was mixed with polyvinyl alcohol (2% byweight) as a binder and pressed into pellets under a pressure of�10 tons. These specimens were sintered at 1200 1C for 1–2 hand subsequently cooled. The torroidal rings of these ferrites ofthickness 2.2 mm and the ratio of outer diameter to innerdiameter equal to 1.5 were formed under a pressure of �5 tons.For measurements of initial permeability and RLF, the torroidswere wound with about 55 turns of 32 SWG enamelled copperwire. Initial permeability of the samples was calculated using therelation mi=L/Lo [20,21], where L is the measured inductance ofthe sample and Lo the air core inductance, which can be expressedas Lo=4.6N2d log(OD/ID)�10�9 H, N being the number of turnsand d the thickness of toroid in meters. OD and ID are the outerand inner diameters of the toroids, respectively. The measure-ments were carried out using Precision LCR meter 4285A in thefrequency range 0.1–30 MHz. The temperature dependence of dcresistivity of all the samples was studied using the two-probemethod. The single-phase nature of the prepared samples waschecked by X-ray diffraction studies, which were made by Riga KuGeiger Flex 3 kW diffractometer and the microstructures of thefractured surfaces of the samples were studied using a CambridgeStereo Scan 360 scanning electron microscope (SEM). M–H studieswere carried out by VSM.

3. Results and discussion

3.1. Structural study

The XRD pattern of Mg0.9Mn0.1Al0.3Co0.3Fe1.4O4 ferrite samplesintered at 1200 1C is shown in Fig. 1. The observed diffractionlines were found to correspond to those of standard patterns ofMg–Mn ferrite with no extra lines, indicating thereby the singlephase cubic spinel structure of the sample with no unreactedconstituents. In order to confirm the homogeneity of the samplesscanning electron micrographs were taken. Fig. 2 shows typicalSEMs for the ferrite composition Mg0.9Mn0.1Al0.3Co0.3Fe1.4O4. Theparticles are very small, on average �0.5 mm.

3.2. Electrical resistivity and activation energy

The variation of resistivity as a function of temperature isshown in Fig. 3. A change in slope is markedly observed in all thesamples. Such a change is either due to Curie temperature [22] ordue to change in conduction mechanism [23]. This indicates thesemiconducting nature of ferrites. The resistivity of the samplesdecreases with increase in temperature according to the relation

r¼ ro expðDE=kTÞ ð1Þ

where DE is the activation energy [24], which is the energyneeded to release an electron from the ion for a jump to theneighbouring ion, thus giving rise to electrical conductivity, k theBoltzmann constant and T the absolute temperature. Each sampleshows a break near the Curie temperature, which is attributed tothe transition from ferrimagnetic to paramagnetic region. Theactivation energies are calculated from the slopes of the para-magnetic and ferrimagnetic region according to the relation

DE¼ ½0:198� 10�3dðlogrÞ�=dð1=TÞ ð2Þ

The calculated values are given in Table 1 along with Curietemperature. From the table it is evident that the value ofactivation energy in the ferrimagnetic region is lower than in the

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1.4 1.6 1.8 2.0 2.2 2.4 2.6

4

6

8

10

12

14

16lo

g ρ

(ohm

-cm

)

1000/T (K-1)

z=0.3 z=0.5 z=0.7

Fig. 3. Variation of resistivity with temperature for Mg0.9Mn0.1Al0.3CozFe1.7�zO4

ferrites.

Table 1Activation energies and Curie temperature for Mg0.9Mn0.1Al0.3CozFe1.7�zO4

ferrites.

Composition Activation energy (eV) Tc from resistivity (K)

Ef Ep DE

z=0.3 0.28 0.67 0.39 583

z=0.5 0.24 0.85 0.61 613

z=0.7 0.29 1.24 0.95 658

0.6

1.2

1.8

2.4

3.0

3.6

4.2

30

45

60

75

90

105

10

15

20

25

30

35

40

Rete

ntiv

ity (e

mu/

gm)

Composition (z)Co

erci

vity

(Gau

ss)

Satu

ratio

n m

agne

tizat

ion

(em

u/gm

)

50°C 100°C 150°C 175°C

50°C 100°C 150°C 175°C

50°C 100°C 150°C 175°C

0.3 0.4 0.5 0.6 0.7

Fig. 4. Compositional variation of (a) saturation magnetization, (b) coercivity and

(c) retentivity for Mg0.9Mn0.1Al0.3CozFe1.7�zO4 ferrites at different temperatures.

Fig. 5. M–H curve for Mg0.9Mn0.1Al0.3CozFe1.7�zO4 (z=0.5) ferrite at different

temperatures.

G. Kumar et al. / Journal of Physics and Chemistry of Solids 71 (2010) 375–380 377

paramagnetic region. The lower activation energy in theferrimagnetic region is attributed to the magnetic disordering[25] due to a decrease in concentration of current carriers [26],while the change in activation energy is attributed to the changein conduction mechanism [27,28]. The change in activationenergy for different compositions is attributed to the hopping ofpolarons. The values of activation energy above 0.2 eV clearlyindicate the polaron hopping in the system [29]. Further,resistivity was found to increase with increasing substitution ofcobalt ions. This is because of the fact that Co2 + ions reside mainlyon B-sites [30]. Hence the Co2 + ions impede electron exchangebetween divalent and trivalent iron ions on B-sites, resultingthereby in higher value of resistivity [31]. The increase ofelectrical resistivity for the investigated system gives apromising property for communication systems as the eddycurrent loss decreases.

3.3. Saturation magnetization and magneto-crystalline anisotropy

Fig. 4 shows the variations in saturation magnetization withincreasing substitution of cobalt ions. It was found that saturationmagnetization increases with an increase in cobalt content. Thismay be because of the fact that Co2 + ions are known to give largeinduced anisotropy due to relatively high orbital contribution tothe magnetic moment [32]. Fig. 5 shows the M–H curve for z=0.5at different temperatures. The variations in magneto-crystallineanisotropy constant (K1) with increasing substitution of cobaltions are shown in Fig. 6. The values of K1 were calculated from thefollowing relation [33]:

K1 ¼ cMsHa ð3Þ

where c is 12 for K140 and �3

4 for K1o0 and Ha is the anisotropicfield. In the calculation of K1 of the present ferrites, c was taken as�3

4 since K1 for Mg–Mn ferrite is known to be negative [34,35]. Itis seen from Fig. 6 that K1 increases with increasing substitutionof cobalt ions. The variations of K1 can be qualitatively explainedon the basis of the single-ion anisotropy model, which shows thatFe3 + ions present at A- as well as at B-sites contribute to theanisotropy energy. The net value of K1 is given by the relativecontribution of positive anisotropy of Fe3 + ions at tetrahedralsites, which is compensated by the negative anisotropy of Fe3 +

ions at octahedral sites. As the concentration of Co2 + ions isincreased, the cation distribution of Fe3 + ions gets modified,which yields a different number of Fe3 + ions present at both the

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0.3 0.4 0.5 0.6 0.7

-4.8

-4.4

-4.0

-3.6

-3.2

-2.8

-2.4

-2.0

-1.6

K 1/104 (e

rg/g

m)

Composition (z)

50°C 100°C 150°C 175°C

Fig. 6. Compositional variation of magneto-crystalline anisotropy for

Mg0.9Mn0.1Al0.3CozFe1.7�zO4 ferrites at different temperatures.

0.1 1 5 10 25450

600

750

900

600

750

900

1050

600

800

1000

1200

1400

μ i

Frequency (M Hz)

μ iμ i

50°C 100°C 150°C 175°C

50°C 100°C 150°C 175°C

50°C 100°C 150°C 175°C

Fig. 7. Variation of initial permeability with frequency: (a) Mg0.9Mn0.1Al0.3Co0.3-

Fe1.4O4, (b) Mg0.9Mn0.1Al0.3Co0.5Fe1.2O4 and (c) Mg0.9Mn0.1Al0.3Co0.7FeO4 ferrites at

different temperatures.

G. Kumar et al. / Journal of Physics and Chemistry of Solids 71 (2010) 375–380378

sites, which in turn affect K1. With most ions like Mn2 +, Fe2 +, etc.the orbital moment is quenched and the spin–orbit interactionintroduces only a relatively small anisotropy. In Co2 +, however,the crystal field is not able to remove orbital degeneracy and theorbital moment is of the same order of magnitude as the spinmoment. This is regarded as the cause of the large anisotropy ofcobalt [36].

Table 2Anisotropic field and resonant frequency for Mg0.9Mn0.1Al0.3CozFe1.7�zO4 ferrites

at 50 1C.

Composition Anisotropic field (Ha) (G) Resonant frequency fr (GHz)

z=0.3 1731 2.4

z=0.5 1931 2.7

z=0.7 2185 3.05

3.4. Initial permeability

The typical variations of initial permeability with frequency atdifferent temperatures for Mg0.9Mn0.1Al0.3CozFe1.7�zO4 withz=0.3, 0.5 and 0.7 are shown in Fig. 7. The result shows fairlyconstant values of mi over a wide frequency range, implyingcompositional stability and high quality of the ferrites preparedby the citrate precursor method. This is a desirable characteristicfor various applications such as broadband pulse transformersand wide band read–write heads for video recording. However,the rapid increase in mi after 17 MHz is indicative of the onset ofresonance. The magnetic spins in the ferrites precess with anatural frequency known as the Larmor frequency. The resonanceoccurs due to the matching of the Larmor frequency of theelectron spins with the applied frequency and the consequentabsorption of energy. The frequency of onset of resonance varieswith temperature; it decreases with an increase in thetemperature. The increase in temperature reduces the magneticanisotropy, which effectively reduces the magnitude of the localmagnetic field experienced by the precessing spins. As a result,the Larmor precessional frequency decreases and the resonance isobserved at a lower frequency. This observation is in agreementwith Snoek’s law [37]. According to Snoek’s law the resonancefrequency (fr) and the initial permeability (mi) are related as

mifr ¼ constant ð4Þ

This indicates that the higher the permeability values, the lowerthe resonance frequency. It was not, however, possible to observethe complete resonance peaks as they seem to appear atfrequencies beyond 30 MHz, the upper limit of frequencies usedin our studies. The resonance of rotation magnetization caused bythe action of the anisotropic field (Ha) can be calculated using thefollowing formula [38]:

fr ¼Han=2p ð5Þ

where n is the gyromagnetic constant given by

n¼ 8:791� 106g ðOe�1 s�1Þ ð6Þ

where g is the gyromagnetic ratio. The theoretically calculatedvalues of resonant frequency due to domain rotation forMg0.9Mn0.1Al0.3CozFe1.7�zO4, z=0.3, 0.5 and 0.7, ferrites at 50 1Care given in Table 2. As the resonance occurs at higher frequenciesfor the series of mixed Mg–Al–Mn ferrites, which leads to anextended zone of utility for these ferrites and also increasesthe operational frequency range in applications where therequirement of high permeability is not rigid. Further, theresults show an increase in mi with increasing temperature. Thisincrease in initial permeability with temperature can beattributed to increases in density and grain size of thespecimen, which facilitate the movement of the spins as thenumber of pores that impede the wall motion is reduced. Higherthe density and grain size, larger the grain-to-grain continuity inmagnetic flux leading to higher permeability. The increase intemperature also results in a decrease in magnetic anisotropy bydecreasing the internal stresses and crystal anisotropy, resultingthereby in the increased value of mi [39]. The variations in theinitial permeability with temperature at frequency 30 MHz areshown in Fig. 8. It is observed that initial permeability increases toa peak value up to a certain temperature and then decreasesgradually. The increase in permeability with temperature is due to

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50 100 150 200

500

750

1000

1250

1500

μ i

Temperature (°C)

z=0.3 z=0.5 z=0.7

Frequency=30 M Hz

Fig. 8. Variation of initial permeability with temperature for Mg0.9Mn0.1Al0.3Coz-

Fe1.7�zO4 ferrites at frequency 30 MHz.

0.1 1 5 10 250.0

7.0x10-5

1.4x10-4

2.1x10-4

0.0

6.0x10-5

1.2x10-4

1.8x10-4

0.0

5.0x10-5

1.0x10-4

1.5x10-4

tanδ

/μi

Frequency (M Hz)

tanδ

/μi

tanδ

/μi

50°C 100°C 150°C 175°C

50°C 100°C 150°C 175°C

50°C 100°C 150°C 175°C

Fig. 9. Variation of relative loss factor with frequency for (a) Mg0.9Mn0.1Al0.3Co0.3-

Fe1.4O4, (b) Mg0.9Mn0.1Al0.3Co0.5Fe1.2O4 and (c) Mg0.9Mn0.1Al0.3Co0.7FeO4 ferrites at

different temperatures.

7.2x10-5

8.0x10-5

nδ/μ

i

z=0.3 z=0.5 z=0.7

Frequency=30 M Hz

G. Kumar et al. / Journal of Physics and Chemistry of Solids 71 (2010) 375–380 379

the fact that anisotropy decreases faster with temperature. Themaximum initial permeability corresponds to zero anisotropyfield [40]. It is observed that the zero points in crystal anisotropyshift towards higher temperature with increasing substitutionof cobalt ions, which resembles the results in Ref. [41]. Further,this result shows that at each temperature initial permeabilitydecreases with increasing substitution of Co2 + ions. This can beexplained by the following dependence of initial permeability[42]:

miaMs2Dm=K1 ð7Þ

where Dm is the average grain diameter, K1 the magneto-crystalline anisotropy constant and Ms the saturationmagnetization. As the magneto-crystalline anisotropy increaseswith increasing cobalt content mi is expected to decrease withincreasing substitution of Co2 + ions.

50 100 150

5.6x10-5

6.4x10-5ta

Temperature (°C)

Fig. 10. Variation of relative loss factor with temperature for Mg0.9Mn0.1Al0.3Coz-

Fe1.7�zO4 ferrites at frequency 30 MHz.

3.5. Relative loss factor

The variations in magnetic loss in the form of relative lossfactor (RLF), versus frequency at different temperatures forMg0.9Mn0.1Al0.3CozFe1.7�zO4, z=0.3, 0.5 and 0.7, ferrites are shownin Fig. 9. Relative loss factor is expressed as the ratio of magneticloss to the initial permeability (tan d/mi). The value of RLF isobserved to decrease initially with frequency, reaching aminimum value, and then rise sharply thereafter. The frequencyat which RLF is minimum, called the threshold frequency, isobserved to vary with temperature. It decreases with increasingtemperature for every composition, which resembles the resultsin Ref. [43]. In general, low values of RLF, of the order of 10�4–10�5, show that these samples are useful for inductor andtransformer applications [44]. The variation of RLF withtemperature, at 30 MHz frequency, for Co2 + doped Mg–Al–Mnferrite series is shown in Fig. 10. The loss is attributed toimperfections in the lattice. It is observed that samples at lowtemperatures, 50–150 1C, exhibit lower loss values. This showsthat at lower temperatures, defects that influence the wallmoment are avoided. Another factor contributing to the low-loss values is relatively higher purity of the samples obtained bythe present method. The values of initial permeability and losstangent are known to depend on various factors such as porosity,grain size, density, magneto-crystalline anisotropy and Fe3 +

concentration. The increase in RLF with increasing temperatureis due to the combined effects of all these factors.

4. Conclusions

The present study shows that with the citrate precursortechnique we can process ferrites at relatively lower tempera-tures; thus, due to lower processing temperature we have moreuniform grain growth and atomic scale mixing at an earlier stageof preparation of ferrite samples results in perfect crystal growth.As compositional stoichiometry plays a crucial role in modifyingthe ferrite properties, the citrate precursor technique can beespecially advantageous in processing ferrites for high frequencyapplications. Resistivity and Curie temperature were observed toincrease with increasing substitution of cobalt ions. The samplesare found to show high initial permeability and low values ofrelative loss factor, which are essential parameters for anypresent-day electronic and electromagnetic applications.

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G. Kumar et al. / Journal of Physics and Chemistry of Solids 71 (2010) 375–380380

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