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Journal of Magnetism and Magnetic Materials 306 (2006) 313–320

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Effect of zinc concentration on the structural, electrical andmagnetic properties of mixed Mn–Zn and Ni–Zn ferrites

synthesized by the citrate precursor technique

Anjali Verma�, Ratnamala Chatterjee

Department of Physics, Indian Institute of Technology, Hauz Khas, New Delhi-110016, India

Received 11 February 2006

Available online 4 April 2006

Abstract

Mixed manganese-zinc and nickel-zinc ferrites of composition Mn0.2Ni0.8�xZnxFe2O4 where x ¼ 0:4, 0.5 and 0.6 have been synthesized

by the citrate precursor technique. Decomposition of the precursor at temperatures as low as 500 1C gives the ferrite powder. The ferrites

have been investigated for their electrical and magnetic properties such as saturation magnetization, initial permeability, Curie

temperature, AC-resistivity and dielectric constant as a function of sintering temperature and zinc content. Structural properties such as

lattice parameter, grain size and density are also studied. The mixed compositions exhibited higher saturation magnetizations at sintering

temperatures as low as 1200 1C. While the Curie temperature decreased with zinc content, the permeability was found to increase. The

AC-resistivity ranged from 105–107O cm and decreased with zinc content and sintering temperature. The dielectric constants were lower

than those normally reported for the Mn–Zn ferrites. Samples sintered at 1400 1C densified to about 94% of the theoretical density and

the grain size was of the order of about 1.5 mm for the samples sintered at 1200 1C and increased subsequently with sintering temperature.

r 2006 Elsevier B.V. All rights reserved.

PACS: 75.50.Gg

Keywords: Manganese-zinc and nickel-zinc ferrites; Citrate precursor technique; Saturation magnetization; Initial permeability; Curie temperature;

Resistivity; Dielectric constant; Microstructure

1. Introduction

Manganese-zinc and nickel-zinc ferrites are well-knowntechnological magnetic materials finding applications invarious electrical devices [1,2]. The application range ofboth the ferrites, however, differs on account of thedifferences in their electrical and magnetic characteristics.Ni–Zn ferrites, for example, are usually employed for high-frequency applications on account of their high resistivitiesto counter eddy current losses while Mn–Zn ferrites aremore suitable at low frequencies in keeping with their lowresistivity and high permeability [3]. In an attempt toincrease the versatility of these ferrites for wider range of

- see front matter r 2006 Elsevier B.V. All rights reserved.

/j.jmmm.2006.03.033

onding author. Tel.: +91 11 26867627; fax: +9111 6862037/

ddress: anjali_verm@hotmail.com (A. Verma).

applications, in the present work, a study of mixedcombinations of these ferrites has been undertaken, asalso to understand the extent of modification of the variousproperties. Zinc is known to play a decisive role indetermining the ferrite properties [4], hence, the composi-tional variation in the ferrites has been brought about byvarying the zinc content. The redistribution of metal ionsover the tetrahedral and octahedral sites in the spinel latticeon incorporation of zinc is responsible for the modificationin ferrite properties. The concentration of manganese in thecompositions was restricted to 20mole percent as ourprevious study on the mixed Mn–Zn and Ni–Zn ferriteswith variations in Mn concentration revealed that ferriteswith 20mole percent Mn exhibited the best electrical andmagnetic properties [5]. Because the properties of ferritesare sensitive to their composition and microstructure,which in turn are sensitive to their processing variables, thekey to obtaining high-performance ferrites is their synthesis

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Fig. 1. X-ray diffractogram of Mn0.2Ni0.3Zn5Fe2O4 calcined at 500 1C.

Table 1

Lattice parameter of Mn0.2Ni0.8�xZnxFe2O4 ferrites sintered at 1200, 1300

and 1400 1C

Composition Lattice parameter ‘a’ (A)

x 1200 1C 1300 1C 1400 1C

0.4 8.401 8.402 8.393

0.5 8.402 8.411 8.402

0.6 8.406 8.434 8.425

A. Verma, R. Chatterjee / Journal of Magnetism and Magnetic Materials 306 (2006) 313–320314

by a specialized technique capable of giving composition-ally stoichiometric materials. The solid-state methodusually adopted for the preparation of ferrites suffers fromthe disadvantages of prolonged milling to mix the oxidesand high processing temperatures resulting in non-stoichio-metric compositions. In view of this a solution methodknown as the citrate precursor method that is capable ofgiving atomic-scale mixing of the constituent cations andformation of ferrite powders at low temperatures has beenused for the preparation of ferrites in the present work [6].There is ample literature available on the investigations ofthe individual Mn–Zn and Ni–Zn ferrites, but hardly anyon the mixed combinations [7–9]. This work, therefore,presents a systematic study of the structural, electrical andmagnetic properties of mixed Mn–Zn and Ni–Zn ferritesprocessed by the citrate precursor technique.

2. Experimental

Mixed Mn–Zn and Ni–Zn ferrites of the compositionMn0.2Ni0.8�xZnxFe2O4 where x ¼ 0:4, 0.5 and 0.6 wereprepared by the citrate precursor method according to theprocedure described in an earlier work [10]. The composi-tions are referred to as Zn0.4, Zn0.5 and Zn0.6 for x ¼ 0:4,0.5 and 0.6, respectively. Ferrite powder was obtained bycalcination of the dried citrate precursor at a temperatureof 500 1C in air. The powder was mixed with PVA binderand compacted into disc and torroidal shapes by applying apressure of 10 and 2.5 ton, respectively. The discs were ofdimensions 13mm diameter and 1.5mm thickness while thetorroidal rings had dimensions 10mm outer diameter,6mm inner diameter and thickness 3.0mm. The sampleswere sintered at different temperatures of 1200, 1300 and1400 1C for 30min duration. A 32 SWG copper wire waswound on the torroidal rings for magnetic measurementswhile silver electrodes were coated on the surface of thediscs for the electrical measurements. Dielectric constantand permeability were calculated from, respectively, thecapacitance and inductance measured on an impedanceanalyser (model HP 4192 A) up to a frequency of 13MHz.Saturation magnetization was measured using a vibratingsample magnetometer (EG & G Princeton AppliedResearch, model 155). X-ray diffractograms were recordedon a Philips PW 1730 diffractometer using Cu Ka sourcewhile the surface morphology of the samples was examinedusing a Cambridge Stereo Scan 360 Scanning ElectronMicroscope.

3. Results and discussion

The X-ray diffractogram recorded for the ferrite powderobtained on calcination at 500 1C corresponded to that ofthe single-phase cubic spinel structure and is shown inFig. 1 for the composition Mn0.2Ni0.3Zn0.5Fe2O4. The (hkl)values corresponding to the diffraction peaks are markedin the figure. The lattice parameters, ‘a’, calculated forsamples sintered at different temperatures are given in

Table 1. Lattice dimensional changes are observed withchanges in composition and sintering temperature. The ‘a’values are seen to increase with zinc content. This increaseis attributable to the higher ionic radius of Zn (0.82 A)compared to that of Ni (0.78 A). As zinc content increasesat the expense of Ni, the lattice seems to expand slightly toaccommodate the increased number of Zn ions of relativelylarger radius resulting in increased value of ‘a’. The latticeparameter is also observed to increase as the sinteringtemperature increases from 1200 to 1300 1C, but againdecrease for samples sintered at 1400 1C. The increase in ‘a’for samples sintered at 1300 1C indicates lattice expansionwhich may be attributed to the reduction of Fe3+ andformation of Fe2+ ions of larger ionic radius, whichprobably dissolve in the spinel lattice. At higher sinteringtemperature (1400 1C) the dissolved phase comes out asFe2O3 causing shrinkage of the lattice. The decrease in ‘a’for the 1400 1C sintered samples may also be attributed topartial oxidation of Mn2+ to Mn3+ of lower ionic radius[11] and also due to Zn loss and O2 release from the sample[12,13] leading to lattice shrinkage. Though ‘a’ is seento vary with Zn content and sintering temperature, thevalues are in accordance with those expected for the mixedferrites [14].Density was calculated from the weight and bulk

dimensions of the sintered samples and the values aregiven in Table 2. Density is observed to increase with zinccontent. Zinc is known to bring about densification and

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Table 2

Density and average grain size of Mn0.2Ni0.8�xZnxFe2O4 ferrites sintered at 1200, 1300 and 1400 1C

Composition Density (g cm�3) Grain size (mm)

x 1200 1C 1300 1C 1400 1C 1200 1C 1300 1C 1400 1C

0.4 4.25 4.70 4.60 1.38 3.40 5.10

0.5 4.50 4.75 4.65 1.50 3.50 17.0

0.6 4.80 4.70 4.70 1.61 5.0 19.5

Fig. 2. Scanning Electron photomicrographs of (a) Mn0.2Ni0.3Zn0.5Fe2O4 sintered at 1200 1C, (b) Mn0.2Ni0.4Zn0.4Fe2O4 sintered at 1400 1C,

(c) Mn0.2Ni0.2Zn0.6Fe2O4 sintered at 1400 1C and (d) Mn0.2Ni0.3Zn0.5Fe2O4 sintered at 1400 1C.

A. Verma, R. Chatterjee / Journal of Magnetism and Magnetic Materials 306 (2006) 313–320 315

grain growth [15], hence, higher the Zn content, higher thedensity expected. Samples sintered at 1300 1C are seen toexhibit the highest density. The slight decrease in densityfor samples sintered at 1400 1C may be attributed tomaterial loss during sintering. Evaporation of Zn from thesamples results in reduction of Fe3+ ions to Fe2+ and lossof oxygen to maintain charge neutrality in the sample [16].The role of Zn in densification is understood from themaximum density of 4.8 g cm�3 obtained for the composi-tion with Zn0.6 sintered at a temperature as low as 1200 1C.

The microstructure plays an important role in realizingmany application-oriented ferrite properties. Samplesexamined by scanning electron microscopy showed averagegrain sizes as given in Table 2 for different compositions.The grain sizes are seen to increase with sinteringtemperature. Samples sintered at 1200 1C posses compara-tively very small sized grains as shown in the photomicro-graph for the composition Mn0.2Ni 0.3Zn0.5Fe2O4,Fig. 2(a), while samples sintered at 1400 1C posses muchlarger grains. The higher grain size for the samples sinteredat 1400 1C may be attributed to the formation of Fe2+ ions,

which accelerate the growth rate of the grains [17,18].Increase in grain size with zinc content can be seen in thephotomicrographs shown in Figs. 2(b) and (c), respectivelyfor the samples of the compositions Mn0.2Ni0.4Zn0.4Fe2O4

and Mn0.2Ni0.2Zn 0.6Fe2O4 sintered at 1400 1C. The role ofzinc in enhancing the grain growth is clearly evident in thephotographs. Samples sintered at 1200 1C are devoid ofintragranular porosity while those sintered at 1400 1C showan inhomogeneous microstructure with intragranularpores. The pores are a result of zinc evaporation fromthe sample as visible in Fig. 2(d), which represents thesurface of the sample of the composition Mn0.2Ni0.3Zn0.5-Fe2O4.Table 3 shows the compositional dependence of Curie

temperature, Tc, obtained from the inductance versustemperature profiles of different compositions. The Curietemperature is observed to decrease with increasing zinccontent. Increase in the non-magnetic zinc ions reduces themagnetic moment of the A sublattice as the Zn ionspreferentially occupy the A sites of the spinel latticereducing thereby the A–B exchange interaction between

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Fig. 3. Temperature dependence of inductance of Mn0.2Ni0.2Zn0.6Fe2O4

sintered at 1400 1C.

Table 3

Curie temperature and saturation magnetization of Mn0.2Ni0.8�xZnxFe2O4

ferrites sintered at 1200, 1300 and 1400 1C

Composition Curie temperature, Tc

(1C)

Saturation magnetization, Ms

(emu/g)

x 1400 1C 1200 1C 1300 1C 1400 1C

0.4 310 89 76 75

0.5 215 88 67 64

0.6 162 54 50 52

A. Verma, R. Chatterjee / Journal of Magnetism and Magnetic Materials 306 (2006) 313–320316

A and B sublattices. This results in a reduction in Curietemperature. Tc of Ni–Zn ferrites are reported to be higherthan those of the Mn–Zn ferrites [4]. The Tc valuesobtained in the present work for the mixed ferrites are, asexpected, in between those reported for the individualferrites. Fig. 3 shows a typical inductance-temperatureprofile for the Mn0.2Ni0.2Zn0.6Fe2O4 composition. Theabrupt fall in permeability indicates the approaching of theCurie temperature. The profile also shows a Hopkinson’speak near the Curie temperature, a behaviour normallyexhibited by cubic structures like those of the samplesanalysed here [19].

The saturation magnetization, Ms, values obtained forthe different compositions are given in Table 3. Theobserved magnetization is a result of the simultaneousinfluence of several extrinsic and intrinsic factors such asdensity, anisotropy, grain size and the A–B exchangeinteractions. The concentration of manganese being con-stant in the ferrite compositions, the changes in Ms may beattributed to changes in Ni and Zn concentrations in thespinel lattice. Besides, Ms is also found to increase with

increase in density and grain size and reduction in themagnetocrystalline anisotropy. Zinc is known to increasethe density and grain size and reduce anisotropy [20].Hence Ms is expected to increase with increasing zinccontent. On the contrary, Ms shows a decreasing trend withincreasing zinc content. This can be attributed to theincreased migration of Fe3+ ions from the A- to B-siteswith increasing Zn content in order to accommodate theincreased number of Zn ions on A-sites as per their sitepreference. This migration results in an increase in Fe3+

concentration on B-sites which gives rise to antiparallelspin coupling and spin canting [21] resulting in weakeningof the A–B exchange interaction reducing thereby themagnetization. The magnetization values obtained in thepresent work for the mixed ferrites are, however, found tobe higher than those for the individual Mn–Zn and Ni–Znferrites. It is found that samples sintered at 1200 1C showthe maximum values of magnetization, which decrease athigher sintering temperatures. This shows that samplessintered at 1200 1C are compositionally more stoichio-metric owing to lower zinc loss at this temperature. Thisindicates the capability of the citrate precursor method inyielding ferrites of higher magnetization at sinteringtemperatures as low as 1200 1C in contrast to the solid-state method which requires higher sintering temperaturesand longer sintering durations for ferrite formation [22]. Attemperatures higher than 1200 1C loss of zinc due tovolatilization takes place, which, for charge compensationreasons, results in the formation of Fe2+ ions. This resultsin an increase in anisotropy [20] as well as decrease inmagnetic moment causing a decrease in magnetization.The initial permeability was found to vary with sintering

temperature and zinc content of the ferrites. Fig. 4 showsthe real (mi0) and imaginary (mi00) parts of the complexpermeability, mi ¼ m0iFjm00i , of different compositions sin-tered at 1400 1C. The increase in mi0 with zinc content isclearly evident. mi0 is observed to be nearly constant overseveral orders of frequency before it begins to decline. Thedecrease in permeability implies onset of ferromagneticresonance [23]. The fairly constant mi0 values over a largefrequency range shows the compositional stability andquality of the ferrites prepared by the citrate precursormethod. This is a desirable characteristic for variousapplications such as broadband pulse transformers andwide band read-write heads for video recording. Thefrequency above which, mi0 starts declining rapidly, is foundto decrease with zinc content. The flat mi0 region up to thefrequency where it starts declining rapidly is known as thezone of utility of the ferrite. The mi0 values obtained arefound to be higher than those of Ni–Zn ferrites of similarcompositions [24]. It may be noted that the incorporationof Mn to an extent as small as 20mole percent brings abouta significant increase in permeability which can be under-stood by comparing the mi0 value of 540 obtained for theMn0.2Ni0.3Zn0.5Fe2O4 ferrite and that reported, 240, for theNi0.5Zn0.5Fe2O4 ferrite without manganese [24]. This maybe ascribed to a decrease in the magnetocrystalline

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Fig. 4. Frequency variation of complex initial permeability of

Mn0.2Ni0.8�xZnxFe2O4 ferrites with x ¼ 0:4, 0.5 and 0.6, sintered at

1400 1C. Fig. 5. Effect of sintering temperature on the complex initial permeability

of Mn0.2Ni0.3Zn0.5Fe2O4 ferrite.

A. Verma, R. Chatterjee / Journal of Magnetism and Magnetic Materials 306 (2006) 313–320 317

anisotropy of Ni–Zn ferrites on substitution of manganese.It can be seen in Fig. 4 that there is not much differencebetween mi0 of ferrites of compositions with Zn0.5 and Zn0.6.Permeability is related to the saturation magnetization andanisotropy by the relation: m0i ¼Ms=Ha where Ha is theinternal anisotropy field. According to this relation, mi0

would increase with increase in Ms and decrease in Ha.Anisotropy is known to decrease with increase in zinccontent. In case of the composition with Zn0.6 Ha would belower due to the higher concentration of zinc but Ms is alsolow while for the composition with Zn0.5, though Ms isobserved to be higher, Ha is also expected to be highcompared to that for the composition with Zn0.6. Thus, dueto the conflicting behaviour of Ms and Ha with Zn content,mi0 of the two compositions is found to be nearly same.Fig. 5 shows the effect of sintering temperature on thecomplex permeability of Mn0.2Ni0.3Zn0.5Fe2O4 ferrites.Increase in permeability with sintering temperature canbe attributed to increase in density and grain size withsintering temperature [25]. Higher the density and grainsize, greater the grain-to-grain continuity in magnetic fluxleading to higher permeability. The high permeabilityMn–Zn ferrites are usually reported to show resonance atfrequencies less than 1MHz [26]. The occurring ofresonance at higher frequencies for the mixed ferritesshows extended zone of utility for these ferrites.

The losses and their frequency dependence determine theultimate operating frequency of magnetic devices. Thelosses in ferrites are associated with domain wall relaxationand rotational resonance. Domain wall relaxation isusually observed at lower frequencies while the rotationalresonance is observed in the megahertz range. The mi00

profiles, Fig. 4, show resonance peaks for the compositions

with Zn0.5 and Zn0.6 while for the composition with Zn0.4,the resonance peak seems to lie above 13MHz, which is themaximum frequency limit of the impedance analyser usedfor the measurements. The resonance frequency for Zn0.6 is5MHz while that for Zn0.5 is 7MHz. This behaviour is inaccordance with the law proposed by Globus [27]:fr �mi0 ¼ constant, where fr is the resonance frequency.The magnetic spins in the ferrites precess with a naturalfrequency known as the Larmor frequency. Resonanceoccurs due to the matching of the Larmor frequency of theelectron spins with the applied frequency and theconsequent absorption of energy. Thus, with variations inZn content, it is possible to tune the resonance frequency.The loss mi00 is observed to increase with zinc content.Though increase in the concentration of zinc is known toincrease the structural perfection [28] in ferrites, at thesame time, lattice defects caused due to increased Znevaporation tend to increase the loss. The increase of lossesand occurring of loss peaks at successively lower frequen-cies with increase in sintering temperature, Fig. 5, may beattributed to increase in grain size with sintering tempera-ture and consequently increase in the number and size ofmagnetic domains which contribute to loss due to delay indomain wall motion.High resistivity is a pre-requisite for high-frequency

applications of ferrites to counter the eddy current losses,which degrade the ferrite performance. AC-resistivity hasbeen calculated using the relation: r ¼ 1=oere0 tan de whereo is the angular frequency, er the observed dielectricconstant, e0 the permeability in free space and tan de theobserved dielectric loss factor, and is given in Table 4. Theresistivity is seen to vary with sintering temperature as well

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Table 4

AC-resistivity of Mn0.2Ni0.8�xZnxFe2O4 ferrites sintered at 1200, 1300 and

1400 1C

Composition AC-resistivity, O-cm

x 1200 1C 1300 1C 1400 1C

0.4 3.6� 107 1.7� 107 1.4� 107

0.5 2.1� 107 6.0� 105 2.0� 105

0.6 1.1� 106 2.2� 105 5.5� 104

Fig. 6. Variation of dielectric constant, e0 and dielectric loss, e00 withfrequency for Mn0.2Ni0.8�xZnxFe2O4 ferrites with x ¼ 0:4, 0.5 and 0.6

sintered at 1400 1C.

Fig. 7. Effect of sintering temperature on e0 and e00 as a function of

frequency for Mn0.2Ni0.4Zn0.4Fe2O4 ferrite.

A. Verma, R. Chatterjee / Journal of Magnetism and Magnetic Materials 306 (2006) 313–320318

as the zinc content of the ferrites. Decrease in resistivitywith increase in sintering temperature may be attributed toincreased zinc evaporation from the samples. Higher thezinc concentration, greater the probability of Zn loss andgreater the non-stoichiometry in the ferrites expected. Zincloss leads to the conversion of trivalent Fe ions to thedivalent state as mentioned above. The formation of Fe2+

ions gives rise to conduction in the ferrite due to electronhopping between the Fe3+ and Fe2+ ions co-existing at thecloser spaced B sites in the spinel lattice. The Fe ions at theA sites contribute little to conduction due to the largerdistances between them [16]. The decrease in resistivity withincreasing zinc content suggests the presence of anincreasing concentration of Fe2+ ions. Besides, partialoxidation of Mn2+ to Mn3+ may also add to conductivitydue to hopping between the two different oxidation statesof Mn. Resistivity is also influenced by the microstructuralfactors such as grain size, porosity and grain boundaryarea. Smaller the grain size more would be the number ofgrain boundaries. The ferrite grains being conducting, thebulk of the resistivity is contributed by the insulating grainboundaries [29]. Increase in sintering temperature results ingreater density and grain growth resulting in a correspond-ing decrease in porosity and the number of grainboundaries leading thereby to a decrease in resistivity.The DC-resistivity reported for Mn–Zn ferrites is of theorder of 100O cm while that reported for the Ni–Zn ferritesis of the order of 106O cm for samples prepared by theconventional solid-state method. The DC-resistivity forNi–Zn ferrites prepared by the citrate precursor methodranges from 105–1011O cm depending upon the composi-tion and sintering temperature [10]. The reasonably high-resistivity values obtained for the mixed Mn–Zn andNi–Zn ferrites, renders them suitable for high-frequencyapplications.

Dielectric constant in ferrites is also contributed byseveral structural and microstructural factors. Fig. 6 showsthe dielectric constant, e0 versus frequency plots of differentcompositions sintered at 1300 1C. The space-charge polar-ization resulting from electron displacement on applicationof electric field and the subsequent charge build-up at theinsulating grain boundary is a major contributor to thedielectric constant in ferrites. Therefore, more the numberof Fe2+ ions in the ferrite more would be the space chargepolarization expected due to ease of electron transferbetween Fe3+ and Fe2+ ions and consequently higher the

dielectric constant. For the composition with Zn0.4, e0 isquite low while that for the compositions with Zn0.5 andZn0.6 the low-frequency dielectric constants are muchhigher, with pronounced dispersion. In general, theincrease in e0 with increasing Zn content can be attributedto the simultaneous influence of increase in density, grainsize and Fe2+ concentration with Zn content. The high-frequency dielectric constants are contributed mainly bythe atomic and electronic polarizations [30] and hence arealmost independent of frequency. Fig. 7 shows the

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variation in dielectric constant with sintering temperaturefor the composition with Zn0.4. e0 is seen to increase withsintering temperature, which can again be attributed to theinfluence of increase in density, grain size and Fe2+ ionswith sintering temperature. Samples sintered at 1200 1Cexhibit quite low values of e0 suggesting thereby low Fe2+

concentration and higher porosity in the samples resultingin lower polarization. Mn–Zn ferrites are reported topossess very high dielectric constants of the order of 105 atlow frequencies [31] caused by strong intrinsic polarizationand grain–grain boundary interface space charge polariza-tion. High dielectric constants decrease the penetratingdepth of the electromagnetic waves by increasing the skineffect. Hence, the much lower dielectric constants obtainedfor the mixed ferrites warrant their applications at highfrequencies.

Dielectric loss is an important part of the total core loss[32] in ferrites. Hence for low core losses, low dielectriclosses are desirable. The dielectric loss e00 as a function offrequency for the different compositions and at differentsintering temperatures is depicted in Figs. 6 and 7. The lossprofiles are similar to those of the real part, e0. The lossestend to increase at higher frequencies with indication of apeak, which probably occurs at frequencies higher than13MHz. This behaviour can be correlated to resonancearising as a result of the matching of the natural oscillatingfrequencies of the electrons and that of the applied electricfield [30,33]. Loss peaks for the Mn–Zn and Ni–Zn ferritesare reported to occur at much lower frequencies [34]. Thusthe present ferrites with relatively lower losses could beuseful at frequencies higher than those of the individualferrites.

4. Conclusions

Mixed manganese-zinc and nickel-zinc ferrites of variouscompositions are synthesized by the citrate precursortechnique and the electromagnetic properties investigatedfor ferrites sintered at various temperatures. Some of thecharacteristics of the mixed ferrites are found to be betterthan those of the individual Mn–Zn and Ni–Zn ferrites.For example, the resistivity and dielectric constant of themixed ferrites are higher and lower respectively than thoseusually reported for the Mn–Zn ferrites while the initialpermeability of the mixed ferrites is higher than that of theNi–Zn ferrites. The zone of utility is seen to extend tohigher frequencies with decrease in zinc concentration inthe ferrites. Saturation magnetization of the mixed ferritesobtained at a relatively low sintering temperature of1200 1C is higher than that reported for the individualferrites. Thus a combination of the Mn–Zn and Ni–Znferrites results in better materials with positive attributes ofboth ferrites. The increase in saturation magnetization andpermeability without appreciable reduction in resistivitymakes the mixed ferrites practically useful over a widerfrequency range than the individual ferrites. The resultsshow that with the citrate precursor technique it is possible

to process the ferrites at relatively lower temperatures andmuch shorter duration than that required in the conven-tional solid-state route. As zinc loss leads to deteriorationin ferrite properties, a short sintering duration is advanta-geous for preparing low-loss high-performance ferrites.

Acknowledgements

The authors are grateful to the Department of Scienceand Technology, Ministry of Science and Technology,Government of India for funding the project (DST No. SP/S2/M-07/2000) under which this work has been carried out.

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