Microstructural, magnetic and electric properties of mixed CseZn ferrites prepared by solution combustion method Manik Gupta, B.S. Randhawa * Department of Chemistry, UGC Sponsored-Centre for Advance Studies-I, Guru Nanak Dev University, Amritsar 143001, India article info Article history: Received 21 August 2011 Received in revised form 28 January 2012 Accepted 10 April 2012 Available online 18 April 2012 Keywords: Ferrites Magnetic materials Solution combustion method Magnetic studies Electrical studies abstract Nanosized zinc substituted ferrites with composition Cs 0.5-x/2 Zn x Mn 0.05 Fe 2.45-x/2 O 4 (x ¼ 0e0.5) were prepared by solution combustion route. The ferrites obtained have been characterized by powder XRD, Mössbauer spectroscopy and Transmission Electron Microscope (TEM). Magnetic and electrical proper- ties have also been studied. Powder X-ray diffraction analysis shows the formation of single phase cubic spinel structure. The saturation magnetization (Ms) initially exhibits an upward trend followed by a regular decrease with increasing diamagnetic Zn content. Curie temperature shows a downward trend with Zn content. The Mössbauer spectra display transition from ferrimagnetic to super-paramagnetic phase with increasing ‘x’ value. The temperature dependence resistivity shows regular decrease with temperature reflecting semiconductor behaviour of the ferrite samples. The permittivity ( 3 0 ) and tangent loss (tan d) measured at room temperature as a function of frequency shows the expected ferrite behaviour. TEM studies indicate the formation of nanosized ferrite particles. These results demonstrate promising features of CseZn ferrites in microwave applications. Ó 2012 Elsevier Masson SAS. All rights reserved. 1. Introduction The ability to prepare nanostructures with defined morphol- ogies and sizes in large scale is an essential requirement for applications in nanomaterials. As a result, extensive efforts have been devoted to develop synthetic capabilities to produce nano- materials with tailored magnetic and electrical properties. The present-day microwave industry demands high-performance mixed ferrite materials capable of operating at high frequencies [1,2]. The mixed ferrites are the ferrimagnetic oxide materials exhibiting high resistivity, permeability and low eddy current los- ses. These novel materials are extensively used in radio, TV, radar, audioevideo and digital recording, bubble devices, memory cores of computer and microwave devices [3e5]. An outstanding quan- tity of theoretical and experimental work has been carried out by engineers and physicists to understand the microstructural, elec- trical, dielectric and magnetic properties of spinel ferrites suitable for high-frequency applications [6e9]. Out of various existing spinel ferrites, alkali metal based ferrites have gained interest particularly for high-frequency telecommunication devices and in other high-frequency applications like circulator and filters because of their high resistivity and hence low eddy current losses, high saturation magnetization and low-cost fabrication. Although several methods have been developed for the synthesis of mixed ferrites [10e13], solution combustion route has the advantage of obtaining nanosized and pure ferrites at lower temperature and in shorter time as compared to other conventional methods [14]. Several investigations have been reported on the synthesis and characterization of mixed lithium, sodium, potassium and rubidium ferrites [15,16] but nothing seems to be reported in literature on respective mixed CseZn ferrites. In this paper, we therefore report the synthesis, characterization and magnetic/ electric properties of nanosized CseZn ferrites prepared by novel solution combustion method. 2. Materials used and method The starting materials, such as caesium nitrate (99%, Spectrochem, AR), manganese nitrate (98.5%, Alfa Aesar), iron (III) nitrate (99.9%, Agro Organics), zinc nitrate (99.9%, Merck) and ethylene glycol (99.5%, Merck) were weighed in stoichiometric proportions. Aqueous solutions of respective metal nitrates prepared by using deionized water were mixed followed by the addition of ethylene glycol in desired molar ratio dropwise with vigorous stirring and the reaction mixture was combusted in muffle furnace at 600 C for 30 mins. The powdered product i.e. Cs 0.5x/2 Zn x Mn 0.05 Fe 2.45x/2 O 4 (x ¼ 0e0.5) * Corresponding author. Tel.: þ91 1832256284; fax: þ91 1832258819. E-mail address: [email protected](B.S. Randhawa). Contents lists available at SciVerse ScienceDirect Solid State Sciences journal homepage: www.elsevier.com/locate/ssscie 1293-2558/$ e see front matter Ó 2012 Elsevier Masson SAS. All rights reserved. doi:10.1016/j.solidstatesciences.2012.04.010 Solid State Sciences 14 (2012) 849e856
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Solid State Sciences 14 (2012) 849e856
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Solid State Sciences
journal homepage: www.elsevier .com/locate/ssscie
Microstructural, magnetic and electric properties of mixed CseZn ferritesprepared by solution combustion method
Manik Gupta, B.S. Randhawa*
Department of Chemistry, UGC Sponsored-Centre for Advance Studies-I, Guru Nanak Dev University, Amritsar 143001, India
a r t i c l e i n f o
Article history:Received 21 August 2011Received in revised form28 January 2012Accepted 10 April 2012Available online 18 April 2012
1293-2558/$ e see front matter � 2012 Elsevier Masdoi:10.1016/j.solidstatesciences.2012.04.010
a b s t r a c t
Nanosized zinc substituted ferrites with composition Cs0.5-x/2ZnxMn0.05Fe2.45-x/2O4 (x ¼ 0e0.5) wereprepared by solution combustion route. The ferrites obtained have been characterized by powder XRD,Mössbauer spectroscopy and Transmission Electron Microscope (TEM). Magnetic and electrical proper-ties have also been studied. Powder X-ray diffraction analysis shows the formation of single phase cubicspinel structure. The saturation magnetization (Ms) initially exhibits an upward trend followed bya regular decrease with increasing diamagnetic Zn content. Curie temperature shows a downward trendwith Zn content. The Mössbauer spectra display transition from ferrimagnetic to super-paramagneticphase with increasing ‘x’ value. The temperature dependence resistivity shows regular decrease withtemperature reflecting semiconductor behaviour of the ferrite samples. The permittivity ( 30) and tangentloss (tan d) measured at room temperature as a function of frequency shows the expected ferritebehaviour. TEM studies indicate the formation of nanosized ferrite particles. These results demonstratepromising features of CseZn ferrites in microwave applications.
� 2012 Elsevier Masson SAS. All rights reserved.
1. Introduction
The ability to prepare nanostructures with defined morphol-ogies and sizes in large scale is an essential requirement forapplications in nanomaterials. As a result, extensive efforts havebeen devoted to develop synthetic capabilities to produce nano-materials with tailored magnetic and electrical properties. Thepresent-day microwave industry demands high-performancemixed ferrite materials capable of operating at high frequencies[1,2]. The mixed ferrites are the ferrimagnetic oxide materialsexhibiting high resistivity, permeability and low eddy current los-ses. These novel materials are extensively used in radio, TV, radar,audioevideo and digital recording, bubble devices, memory coresof computer and microwave devices [3e5]. An outstanding quan-tity of theoretical and experimental work has been carried out byengineers and physicists to understand the microstructural, elec-trical, dielectric and magnetic properties of spinel ferrites suitablefor high-frequency applications [6e9]. Out of various existingspinel ferrites, alkali metal based ferrites have gained interestparticularly for high-frequency telecommunication devices and inother high-frequency applications like circulator and filters because
: þ91 1832258819.(B.S. Randhawa).
son SAS. All rights reserved.
of their high resistivity and hence low eddy current losses, highsaturation magnetization and low-cost fabrication. Althoughseveral methods have been developed for the synthesis of mixedferrites [10e13], solution combustion route has the advantage ofobtaining nanosized and pure ferrites at lower temperature and inshorter time as compared to other conventional methods [14].Several investigations have been reported on the synthesis andcharacterization of mixed lithium, sodium, potassium andrubidium ferrites [15,16] but nothing seems to be reported inliterature on respective mixed CseZn ferrites. In this paper, wetherefore report the synthesis, characterization and magnetic/electric properties of nanosized CseZn ferrites prepared by novelsolution combustion method.
2. Materials used and method
Thestartingmaterials, suchascaesiumnitrate (99%,Spectrochem,AR), manganese nitrate (98.5%, Alfa Aesar), iron (III) nitrate (99.9%,Agro Organics), zinc nitrate (99.9%, Merck) and ethylene glycol(99.5%,Merck)wereweighed in stoichiometric proportions. Aqueoussolutions of respective metal nitrates prepared by using deionizedwater were mixed followed by the addition of ethylene glycol indesiredmolar ratio dropwise with vigorous stirring and the reactionmixture was combusted inmuffle furnace at 600 �C for 30mins. Thepowdered product i.e. Cs0.5�x/2ZnxMn0.05Fe2.45�x/2O4 (x ¼ 0e0.5)
M. Gupta, B.S. Randhawa / Solid State Sciences 14 (2012) 849e856850
obtained was stored in a dessiccator. Ethylene glycol used in thismethod acts as a fuel (capping agent) for the combustion synthesis offerrites.
2.1. Instrumentation
X-ray investigations on the powders obtained were carried outby X-ray powder diffractometer (Rigaku made diffractometer, RINK2000) using a Cu Ka radiation (l ¼ 1.54059 Å) in a wide range ofBragg angles 2q (20� � 2q � 80�) with step size of 0.0170 and scanstep time of 20.0286 s�1. The size and shape of ferrite particles wereanalysed by transmission electron microscope (TEM, Hitachi H-7500). The elemental analysis of the samples was performed usingthe EDXRF spectrometer. The exciter source consisted of a 3 kWlong-fine-focus Mo-anode X-ray diffraction tube along with a 4 kWX-ray generator procured from PanAnalytic, The Netherlands. A Si(Li) detector (100 mm2 � 5 mm, 8 mm Be window, FWHM ¼ 180 eVat Mn Ka X-rays, Canberra, US) in the horizontal configurationcoupled with a PC based multichannel analyser was used to collectthe fluorescent X-ray spectra. Spectra were taken using the setupwith the X-ray tube operating voltage 38 kV and a combination ofthe selective absorbers based on the 30Zn, 35Br and 38Sr elements(K-shell jump ratios w7) in the incident beam. Infrared studieswere carried out on Varian 660, FTIR system after preparing pelletswith KBr.
57Fe Mössbauer spectra were recorded on Wissel (Germany),Mössbauer spectrometer. A 57Co (Rh) g-ray source was employedand the velocity scale was calibrated relative to 57Fe in Rh matrix.Mössbauer spectral analysis software WinNormos for Igor Pro hasbeen used for the quantitative evaluation of the spectra. Isomershift values were reported with respect to pure metallic ironabsorber. Curie temperature for the CseZn ferrite samples was
determined by using a simple experimental setup based on gravityeffect in the laboratory.
Saturation magnetization values were measured by usingVibrating Sample Magnetometer (Lake Shore’s new 7400 series).The electric properties (dielectric constant and tangent loss) weremeasured with 8714ET precision LCR metre. Resistivity of thesamples was measured by using two probe Kethley high sensitiveresistivity metre.
3. Results and discussion
3.1. Microstructural analysis
Fig. 1 shows the X-ray diffraction patterns for different compo-sitions of Cs0.5�x/2ZnxMn0.05Fe2.45�x/2O4. The diffraction peaks(220), (311), (400), (422), (511) and (440) reveal the existence ofsingle phase cubic spinel ferrites and are comparable to thosereported for respective lithium ferrites [5]. Table 1 and Fig. 2 (a)shows the variation of lattice constant as a function of ‘x’. The valueof lattice constant ‘a’ increases with increasing Zn content (x) in thecomposition. The lattice constant ‘a’ can be calculated theoreticallyby the following relation.
a ¼�8=3
ffiffiffi3
p �hðrA þ rOÞ þ
ffiffiffi3
pðrB þ rAÞ
i
where rO is the radius of oxygen ion, rA and rB are the ionic radii oftetrahedral (A) and octahedral (B) site respectively. This relationclearly indicates that there exists a correlation between the ionicradii and the lattice constant. This is attributed to the substitutionof larger Zn2þ cation (0.083 nm) for smaller Fe3þ cation (0.067 nm).Caesium ferrite being inverse spinel, have all the Csþ ions in octa-hedral position along with half of the Fe3þ ions and remaining Fe3þ
ions occupy tetrahedral site. The addition of Zn2þ ions which havestrong affinity for tetrahedral site, only Fe3þ ions present at tetra-hedral site get replaced resulting in an increase in lattice parameter.
The theoretical or X-ray density (dxRD) of the various composi-tions of CseZn series has been calculated by using the relationship[17]:
dXRD ¼ 8M=Na3
whereM is Molecular weight of the ferrite, N is Avogadro’s numberand ‘a’ is lattice constant obtained from the different XRD patterns.In Fig. 2 (b) the theoretical/X-ray density (dxRD) and experimentaldensity shows a regular decrease with increasing ‘x’ value due toa decrease in molecular weight of the ferrite. The magnitude ofobserved and calculated densities have been found to be compa-rable. Both parameters show a downward trend with increasingmagnitude of ‘x’. However, the X-ray density for any givencomposition is higher than that of the experimental density andthis difference is primarily due to the porosity of the material.
The percentage porosity for all the compositions was calculatedby using the equation:
0.0 0.1 0.2 0.3 0.4 0.58.38
8.39
8.40
8.41
8.42
8.43
8.44
Lat
tice
Con
stan
t a
Zn content
0.0 0.1 0.2 0.3 0.4 0.5
5.4
5.5
5.6
5.7
5.8
5.9
6.0
6.1
Den
sity
Zn content
dXRD
dExp
a b
Fig. 2. a. Variation of lattice constant ‘a’ with Zn content. b. Variation of theoretical density (dXRD) and experimental density (dExp) with Zn content (x).
M. Gupta, B.S. Randhawa / Solid State Sciences 14 (2012) 849e856 851
�1� dExp=dXRD
�� 100
The calculated value of the porosity (Table 1) has been found tobe quite low which is a characteristic requirement of good qualityferrite materials.
The average particle size, D, of the ferrite product estimatedfrom the XRD pattern using the Scherrer formula comes out to be
Fig. 3. a. TEM micrograph for Cs0.5�x/2ZnxMn0.05Fe2.45�x/2O4 with x ¼ 0.1. b. TEM microgZnxMn0.05Fe2.45�x/2O4 with x ¼ 0.5.
15e20 nm. The size and shape of CseZn ferrite particles synthe-sized by the solution combustion route were also analysed bytransmission electronmicroscope (TEM). An average particle size ofw20 nm has been estimated for the nanocrystalline Cs0.5�x/2
ZnxMn0.05Fe2.45�x/2O4 powder as shown in Fig. 3(aec). The smallerparticle size may be attributed to the combustion synthesisinvolving molecular level heating resulting into no thermal
raph for Cs0.5�x/2ZnxMn0.05Fe2.45�x/2O4 with x ¼ 0.3. c. TEM micrograph for Cs0.5�x/2
Fig. 4. a. EDXRF spectrum for Cs0.5�x/2ZnxMn0.05Fe2.45�x/2O4 with x ¼ 0.3. b. EDXRF spectrum for Cs0.5�x/2ZnxMn0.05Fe2.45�x/2O4 with x ¼ 0.5.
Fig. 5. A. Mössbauer spectrum for composition with x ¼ 0. B. Mössbauer spectrum forcomposition with x ¼ 0.1. C. Mössbauer spectrum for composition with x ¼ 0.5.
M. Gupta, B.S. Randhawa / Solid State Sciences 14 (2012) 849e856852
gradients and requiring much smaller time than the conventionaldouble sintering ceramic technique. The elemental analysis of thedifferent composition has been done with the help of EDXRFstudies (Fig. 4a,b) and is found to be in agreement with theexpected value with acceptable instrumental error (�5% error).
3.2. Mössbauer studies
Fig. 5(AeC) showsMössbauer spectra for compositions x¼ 0, 0.1and 0.5. Room temperature spectrum for the composition withx ¼ 0 (Fig. 5A) exhibits two well-resolved Zeeman sextets arisingdue to the Fe3þ ions present at both tetrahedral and octahedral sites(A and B sites). The relative intensity for two sextet components inthe spectrum is expected to be equal based upon inverse spinelstructure of ferrite. With the addition of Zn2þ ions, Mössbauerspectrum shows superimposition of a central paramagnetic doubletover the sextet pattern (x ¼ 0.1, Fig. 5B) and its intensity increaseswith further increase in Zn2þ ion concentration as shown in Figs. 5Cand 6. The relative intensities for two sextets and doublet compo-nent with quadrupole splitting of about 0.35 mm/s is an indicationthat most of the Zn2þ ions form non-magnetic phase around Fe3þ
ions and prevent them to participate in long-range magneticordering. So there is a transition from ferrimagnetic to super-paramagnetic phase with addition of diamagnetic zinc content.For diamagnetically substituted ferrites, the existence of a centraldoublet superimposed on well-resolved magnetic sextets has beenreported for a number of systems [14e16]. The isomer shift for theoctahedral site is slightly greater than that of tetrahedral site, whichmay be attributed to difference in Fe3þeO2� distance implyingdifference in covalency of FeeO bond. It is generally suggested thatthe Fe3þeO2� bonding distance is about 15% smaller for the tetra-hedral (A) sites than for the octahedral (B) sites in spinel ferrites,whichmeans a greater degree of covalent bonding for Fe3þAeO2� intetrahedral sites [17]. The value of nuclear hyperfine field at octa-hedral site is greater than the value for tetrahedral site, which isexpected for the lower coordination number around Fe cation inthe tetrahedral sites. The nuclear hyperfine field at A and B sitesshow a decrease with increase in the value of x, which is attributedto non-magnetic substitution. The smaller value of quadrupole shiftof the A and B magnetic patterns in all the samples confirms that
the local symmetry of the ferrite is close to cubic. Mössbauerparameters for all the compositions are listed in Table 2.
IR spectra for all the samples display twomain bands v1 and v2 atregion 560 and 430 cm�1 attributed to stretching vibrations ofFeeO bond in tetrahedral and octahedral sites respectively [18,19].
0.0 0.1 0.2 0.3 0.4 0.5
0
5
10
15
20
25
Para
mag
netic
Cha
ract
er (
%)
Zn content
Fig. 6. Variation of paramagnetic character with Zn content.
0.0 0.1 0.2 0.3 0.4 0.5
20
25
30
35
40
45
50
55
Ms
(em
u/g)
Zn content
Fig. 7. Variation of saturation magnetization with increasing value of ‘x’.
M. Gupta, B.S. Randhawa / Solid State Sciences 14 (2012) 849e856 853
3.3. Magnetic studies
3.3.1. Saturation magnetizationMagnetic measurements reveal that all the samples show
a typical MeH curve in which the magnetization rise sharply asapplied field increases from zero in either direction and then slowlyapproaches to saturation. This is a typical behaviour of nanosizedmagnetic materials where residual superparamagnetism relaxa-tions lead to rise in wings and ferrimagnetic part contributes to thehysteresis loop. Fig. 7 shows the variation of saturation magneti-zation as a function of Zn content. It has been observed that satu-ration magnetization increases initially up to a certain level ofsubstitution and then follows the reverse trend. The observedvariation can be explained on the basis of exchange interactions
x ¼ 0.1 0.31 0.02 49.56 60.46 (oct.)0.31 �0.01 45.38 37.47 (tet.)0.32 0.63 e 2.07 (C.D.)
x ¼ 0.2 0.30 �0.001 49.38 47.32 (oct.)0.29 �0.009 46.24 45.99 (tet.)0.34 0.35 e 6.69 (C.D.)
x ¼ 0.3 0.31 0.04 49.31 58.88 (oct.)0.30 �0.06 43.37 30.10 (tet.)0.33 0.36 e 11.02 (C.D.)
x ¼ 0.4 0.30 �0.002 49.24 64.39 (oct.)0.28 0.15 42.31 16.89 (tet.)0.34 0.30 e 18.72 (C.D.)
x ¼ 0.5 0.30 �0.01 49.16 73.47 (oct.)0.27 �0.06 41.35 2.93 (tet.)0.35 0.34 e 23.60 (C.D.)
a w.r.t. pure metallic iron absorber, oct. ¼ octahedral site, tet. ¼ tetrahedral site,C.D. ¼ central doublet.
[20] and canted spin model of Yafet and Kittel [21]. In ferrites, themagnetic ions occupy the tetrahedral (A) and octahedral (B) sites ofthe spinel lattice. The saturation magnetization is taken as thedifference between the magnetization of B and A sublattices. Sincenon-magnetic Zn2þ ion has a strong affinity for A site, its substi-tution reduces the magnetization of A sub lattice (MA) and therebyincreases the net MS value. This can be observed from the experi-mental results which show an initial increase in saturationmagnetization reaching to a maximum at x ¼ 0.3, and afterwardsreverses its course (Table 3).
3.3.2. Curie temperatureThe variation in Curie temperature (TC) with the substitution of
non-magnetic zinc content (x) in the basic compositional formulahas been studied. The observed variation of Curie temperaturewithx has been displayed in Fig. 8 that shows a regular decrease in Curietemperature with increase in zinc content (x). This variation inCurie temperature can be explained on the basis of exchangeinteractions.
Neel [20] considered three types of exchange interactionsbetween the unpaired electrons of magnetic ions in the crystallattice of ferrimagnetic ferrite materials:
AeA interactions (between ions in tetrahedral sites)BeB interactions (between ions in octahedral sites)AeB interactions (between ions in tetrahedral and octahedralsites)
Table 3Variation of saturation magnetization, Curie temperature & dc resistivity withcomposition ‘x’.
Fig. 8. Variation of Curie temperature with increasing value of ‘x’.
M. Gupta, B.S. Randhawa / Solid State Sciences 14 (2012) 849e856854
AeB interactions are greater than AeA and BeB interactions.
It is well established that the replacement of amagnetic ion, Fe3þ
ion on either site of the crystal lattice by diamagnetic ionswill resultin the reduction of the number of magnetic linkages and conse-quently a fall in Curie temperature [15,16,18,22,23]. With increasingvalue of ‘x’ in the different compositions, Zn2þ ions occupy the A site.Thus a decrease in the Curie temperature is the consequence of thesubstitution of non-magnetic ions in the crystal lattice. The Curietemperatures of different samples are listed in Table 3. The magni-tude of Curie temperatures for CseZn ferrites has been found to behigher than that of respective NaeZn and KeZn ferrites [15].
4. Electrical studies
4.1. dc-electrical resistivity
The electrical property in CseZn ferrites has been attributed toelectron hopping between the two valence states of iron,
Fig. 9. a. Variation of dc-resistivity with Zn content. b. Temperature dependence of dc-ele
Fe2þ 4 Fe3þ, on octahedral sites. The Fe2þ ion concentration isa characteristic property of a given ferrite material and dependsupon several factors such as sintering time, temperature andatmosphere, annealing time, etc., including the grain structure.Some amount of Fe2þ ions is also formed due to possible evapo-ration of metal ions during sintering as reported in lithiumferrites [24]. The variation of room temperature dc resistivity asa function of composition is presented in Table 3. It showsa regular increase with Zn content as shown in Fig. 9a and can beexplained on the basis of Verwey mechanism of electron hoppingbetween two valence states distributed randomly on equivalentlattice sites [25]. According to this model, ferrites form closedpacked oxygen lattice with metal ions located at tetrahedral (Asite) and octahedral (B site) sites and conduction may beconsidered due to hopping of Fe2þ and Fe3þ at B site. Since AeBdistance is greater than the BeB distance therefore, dominantmode of conduction due to hopping of Fe2þ and Fe3þ occurs atBeB site. The higher value of dc resistivity obtained may also becontributed to improved nanosized ferrite particles [26] andbetter compositional stoichiometry with reduced Fe2þ formationobtained by solution combustion method. Samples with smallergrain consist of more number of grain boundaries, which acts asbarrier to the flow of electrons. Another advantage of small grainsize is that it helps in reducing Fe2þ ions [27] as oxygen movesfaster in small grains, thus keeping iron in Fe3þ state. Thetemperature dependence of dc resistivity was also studied in thetemperature range 308e398 K as displayed in Fig. 9b that showsan almost linear decrease in resistivity with temperature sug-gesting semiconductor behaviour of the ferrite materials inaccordance with the Arrhenious relation:
r ¼ raexp�Er=KT
�
where r ¼ Resistivity
ra ¼ Resistivity extrapolated to T ¼ a
Er ¼ Activation energyk ¼ Boltzmann’s constantT ¼ Absolute temperature
The high dc-resistivity values obtained for the solutioncombustion route-processed CseZn ferrites make them suitable forhigh-frequency applications.
ctrical resistivity for Cs0.5�x/2ZnxMn0.05Fe2.45�x/2O4 with composition x ¼ 0.1, 0.3, 0.5.
102 103 104 105 106 107
0
1
2
3
4
5
6
7
8
Los
s ta
ngen
t (t
an δ
)
Frequency (Hz)
0.1 0.2 0.3 0.4 0.5
Fig. 11. Variation of tangent loss (d) for different compositions with frequency.
M. Gupta, B.S. Randhawa / Solid State Sciences 14 (2012) 849e856 855
4.2. Permittivity (dielectric constant) and tangent loss studies
Dielectric properties for different ferrite samples were studiedin the frequency range 102e107 Hz (Fig. 10). The frequencydependence of the dielectric constant ( 30) shows a continuousdecrease with increase in frequency with pronounced dispersionat lower frequency and it remains almost independent of appliedexternal field at high-frequency domain. It is clear from theFigs. 9a and 10 that the variation of dc-resistivity and dielectricconstant as a function of Zn content exhibit opposite trends witheach other. Similar trends have been reported by several workers[28,29] suggesting a strong relationship between conductionmechanism and dielectric behaviour of ferrites. The existence ofdielectric dispersion can be explained on the basis of Koop’s two-layer model [30] and MaxwelleWagner polarization theory[31,32], in which relatively good conducting grains and insulatinggrain-boundary layers of ferrite material can be understood asbeing given by an inhomogeneous dielectric structure. Since anassembly of space-charge carriers in the inhomogeneous dielectricstructure requires finite time to line up their axes parallel to analternating electric field, the dielectric constant ( 30) naturallydecreases. It was found that dielectric constant ( 30) value decreaseswith increase in zinc ion content. Initially with lower zinc ionconcentration (x ¼ 0), the Fe2þ ions are maximum, and hence, it isquite possible for these ions to polarize to the maximum extentcausing 30 to decrease, later Zn2þ ion substitution reduces Fe2þ ionconcentration, thereby hindering the interaction between Fe2þ
and Fe3þ ions. The overall low values of dielectric constantobserved may be attributed to nanosized grain size of the ferritesobtained making these materials suitable for higher-frequencyapplication. Fig. 11 shows an initial increase in the value oftangent loss (d) to attain a maxima followed by a regular decreasewith frequency. Such peak behaviour occurs when jump frequencyof electron exchange between Fe2þ and Fe3þ becomes equal to theapplied field [33,34]. It can also be noted that the height of thepeak increases with Zn2þ ions substitution at x ¼ 0.1, and then itshows a subsequent decrease with increase of Zn2þ ion concen-tration. The decrease of the height of the peak of tan d withincreasing Zn2þ ions substitutionmay be attributed to the additionof diamagnetic Zn2þ ions in place of Fe3þ ions that limits thedegree of conductivity by blocking hopping conduction mecha-nism thus resulting in an increase of resistivity.
102 103 104 105 106 107
0
1x104
2x104
3x104
4x104
5x104
6x104
7x104
8x104
Per
mit
tivi
ty (
ε')
Frequency (Hz)
0.1 0.2 0.3 0.4 0.5
Fig. 10. Variation of dielectric constant for different compositions with frequency.
5. Conclusion
The experimental results show that the solution combustionmethod adopted for the synthesis of ferrites is capable of controllingthe stoichiometry, phase and particle size by controlling thetemperature of the reaction. The magnetic as well as electric prop-erties of the ferrites obtained are both particle size and temperaturedependent. Mössbauer results show a transition from ferrimagneticto super-paramagnetic phase with increase in non-magnetic zinccontent. The results obtained with superior magnetic and electricalproperties (high saturation magnetization, Curie temperature,resistivity and low dielectric constant etc.) make these ferrites, thepotential materials for operating at microwave frequencies.
Acknowledgement
The financial support provided by CSIR, New Delhi is highlyacknowledged.
The authors are thankful to Dr. J.M. Greneche (France) for fruitfuldiscussion on Mössbauer studies.
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