Microstructural, magnetic and electric properties of mixed Cs–Zn ferrites prepared by solution combustion method

Solid State Sciences 14 (2012) 849e856

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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

a r t i c l e i n f o

a b s t r a c t

Article history: Received 21 August 2011 Received in revised form 28 January 2012 Accepted 10 April 2012 Available online 18 April 2012

Nanosized zinc substituted ferrites with composition Cs0.5-x/2ZnxMn0.05Fe2.45-x/2O4 (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 properties 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 (30 ) 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.

Keywords: Ferrites Magnetic materials Solution combustion method Magnetic studies Electrical studies

1. Introduction The ability to prepare nanostructures with defined morphologies 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 nanomaterials 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 losses. 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 quantity of theoretical and experimental work has been carried out by engineers and physicists to understand the microstructural, electrical, 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

* Corresponding author. Tel.: þ91 1832256284; fax: þ91 1832258819. E-mail address: [email protected] (B.S. Randhawa). 1293-2558/$ e see front matter Ó 2012 Elsevier Masson SAS. All rights reserved. doi:10.1016/j.solidstatesciences.2012.04.010

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. Cs0.5x/2ZnxMn0.05Fe2.45x/2O4 (x ¼ 0e0.5)

850

M. Gupta, B.S. Randhawa / Solid State Sciences 14 (2012) 849e856 Table 1 Variation of various 2ZnxMn0.05Fe2.45x/2O4.

311 Counts

440 400

220

422

511

X=0.5 X=0.4

XRD

parameters

with

composition

‘x’

for

Cs0.5x/

Composition Molecular (x) weight

Density Density (dXRD) g/cm3 (dExp) g/cm3

Porosity Lattice (%) parameter ‘a’

0 0.1 0.2 0.3 0.4 0.5

6.0974 6.0180 5.9335 5.8477 5.7644 5.6698

6.45 5.89 5.35 5.03 4.06 4.58

270.45 267.54 264.63 261.72 258.51 255.90

5.7019 5.6659 5.6158 5.5543 5.5298 5.4103

8.3845 8.3909 8.3999 8.4097 8.4186 8.4333

X=0.3 X=0.2 X=0.1 X=0

determined by using a simple experimental setup based on gravity effect in the laboratory. Saturation magnetization values were measured by using Vibrating Sample Magnetometer (Lake Shore’s new 7400 series). The electric properties (dielectric constant and tangent loss) were measured with 8714ET precision LCR metre. Resistivity of the samples was measured by using two probe Kethley high sensitive resistivity metre. 3. Results and discussion 3.1. Microstructural analysis

20

30

40

50

60

70

Position[2 theta](Copper(Cu)) Fig. 1. X-ray powder diffraction pattern for Cs0.5x/2ZnxMn0.05Fe2.45x/2O4 with x varies from 0 to 0.5.

obtained was stored in a dessiccator. Ethylene glycol used in this method acts as a fuel (capping agent) for the combustion synthesis of ferrites. 2.1. Instrumentation X-ray investigations on the powders obtained were carried out by X-ray powder diffractometer (Rigaku made diffractometer, RINK 2000) using a Cu Ka radiation (l ¼ 1.54059 Å) in a wide range of Bragg angles 2q (20  2q  80 ) with step size of 0.0170 and scan step time of 20.0286 s1. The size and shape of ferrite particles were analysed by transmission electron microscope (TEM, Hitachi H7500). The elemental analysis of the samples was performed using the EDXRF spectrometer. The exciter source consisted of a 3 kW long-fine-focus Mo-anode X-ray diffraction tube along with a 4 kW X-ray generator procured from PanAnalytic, The Netherlands. A Si (Li) detector (100 mm2  5 mm, 8 mm Be window, FWHM ¼ 180 eV at Mn Ka X-rays, Canberra, US) in the horizontal configuration coupled with a PC based multichannel analyser was used to collect the fluorescent X-ray spectra. Spectra were taken using the setup with the X-ray tube operating voltage 38 kV and a combination of the selective absorbers based on the 30Zn, 35Br and 38Sr elements (K-shell jump ratios w7) in the incident beam. Infrared studies were carried out on Varian 660, FTIR system after preparing pellets with KBr. 57 Fe Mössbauer spectra were recorded on Wissel (Germany), Mössbauer spectrometer. A 57Co (Rh) g-ray source was employed and the velocity scale was calibrated relative to 57Fe in Rh matrix. Mössbauer spectral analysis software WinNormos for Igor Pro has been used for the quantitative evaluation of the spectra. Isomer shift values were reported with respect to pure metallic iron absorber. Curie temperature for the CseZn ferrite samples was

Fig. 1 shows the X-ray diffraction patterns for different compositions of Cs0.5x/2ZnxMn0.05Fe2.45x/2O4. The diffraction peaks (220), (311), (400), (422), (511) and (440) reveal the existence of single phase cubic spinel ferrites and are comparable to those reported for respective lithium ferrites [5]. Table 1 and Fig. 2 (a) shows the variation of lattice constant as a function of ‘x’. The value of lattice constant ‘a’ increases with increasing Zn content (x) in the composition. The lattice constant ‘a’ can be calculated theoretically by the following relation.

a ¼




i pffiffiffih pffiffiffi 8=3 3 ðrA þ rO Þ þ 3ðrB þ rA Þ

where rO is the radius of oxygen ion, rA and rB are the ionic radii of tetrahedral (A) and octahedral (B) site respectively. This relation clearly indicates that there exists a correlation between the ionic radii and the lattice constant. This is attributed to the substitution of larger Zn2þ cation (0.083 nm) for smaller Fe3þ cation (0.067 nm). Caesium ferrite being inverse spinel, have all the Csþ ions in octahedral position along with half of the Fe3þ ions and remaining Fe3þ ions occupy tetrahedral site. The addition of Zn2þ ions which have strong affinity for tetrahedral site, only Fe3þ ions present at tetrahedral site get replaced resulting in an increase in lattice parameter. The theoretical or X-ray density (dxRD) of the various compositions of CseZn series has been calculated by using the relationship [17]:

dXRD ¼ 8M=Na3 where M is Molecular weight of the ferrite, N is Avogadro’s number and ‘a’ is lattice constant obtained from the different XRD patterns. In Fig. 2 (b) the theoretical/X-ray density (dxRD) and experimental density shows a regular decrease with increasing ‘x’ value due to a decrease in molecular weight of the ferrite. The magnitude of observed and calculated densities have been found to be comparable. Both parameters show a downward trend with increasing magnitude of ‘x’. However, the X-ray density for any given composition is higher than that of the experimental density and this difference is primarily due to the porosity of the material. The percentage porosity for all the compositions was calculated by using the equation:

M. Gupta, B.S. Randhawa / Solid State Sciences 14 (2012) 849e856

a

851

b

8.44

6.1

dXRD dExp

6.0 5.9

8.42

Density

Lattice Constant a

8.43

8.41

8.40

5.8 5.7 5.6 5.5

8.39

5.4

8.38 0.0

0.1

0.2

0.3

0.4

0.5

Zn content

0.0

0.1

0.2

0.3

0.4

0.5

Zn content

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).



 1  dExp =dXRD  100

The calculated value of the porosity (Table 1) has been found to be quite low which is a characteristic requirement of good quality ferrite materials. The average particle size, D, of the ferrite product estimated from the XRD pattern using the Scherrer formula comes out to be

15e20 nm. The size and shape of CseZn ferrite particles synthesized by the solution combustion route were also analysed by transmission electron microscope (TEM). An average particle size of w20 nm has been estimated for the nanocrystalline Cs0.5x/2 ZnxMn0.05Fe2.45x/2O4 powder as shown in Fig. 3(aec). The smaller particle size may be attributed to the combustion synthesis involving molecular level heating resulting into no thermal

Fig. 3. a. TEM micrograph for Cs0.5x/2ZnxMn0.05Fe2.45x/2O4 with x ¼ 0.1. b. TEM micrograph for Cs0.5x/2ZnxMn0.05Fe2.45x/2O4 with x ¼ 0.3. c. TEM micrograph for Cs0.5x/2 ZnxMn0.05Fe2.45x/2O4 with x ¼ 0.5.

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M. Gupta, B.S. Randhawa / Solid State Sciences 14 (2012) 849e856

Fig. 4. a. EDXRF spectrum for Cs0.5x/2ZnxMn0.05Fe2.45x/2O4 with x ¼ 0.3. b. EDXRF spectrum for Cs0.5x/2ZnxMn0.05Fe2.45x/2O4 with x ¼ 0.5.

gradients and requiring much smaller time than the conventional double sintering ceramic technique. The elemental analysis of the different composition has been done with the help of EDXRF studies (Fig. 4a,b) and is found to be in agreement with the expected value with acceptable instrumental error (5% error).

the local symmetry of the ferrite is close to cubic. Mössbauer parameters for all the compositions are listed in Table 2. IR spectra for all the samples display two main bands v1 and v2 at region 560 and 430 cm1 attributed to stretching vibrations of FeeO bond in tetrahedral and octahedral sites respectively [18,19].

3.2. Mössbauer studies Fig. 5(AeC) shows Mössbauer spectra for compositions x ¼ 0, 0.1 and 0.5. Room temperature spectrum for the composition with x ¼ 0 (Fig. 5A) exhibits two well-resolved Zeeman sextets arising due to the Fe3þ ions present at both tetrahedral and octahedral sites (A and B sites). The relative intensity for two sextet components in the spectrum is expected to be equal based upon inverse spinel structure of ferrite. With the addition of Zn2þ ions, Mössbauer spectrum shows superimposition of a central paramagnetic doublet over the sextet pattern (x ¼ 0.1, Fig. 5B) and its intensity increases with further increase in Zn2þ ion concentration as shown in Figs. 5C and 6. The relative intensities for two sextets and doublet component with quadrupole splitting of about 0.35 mm/s is an indication that most of the Zn2þ ions form non-magnetic phase around Fe3þ ions and prevent them to participate in long-range magnetic ordering. So there is a transition from ferrimagnetic to superparamagnetic phase with addition of diamagnetic zinc content. For diamagnetically substituted ferrites, the existence of a central doublet superimposed on well-resolved magnetic sextets has been reported for a number of systems [14e16]. The isomer shift for the octahedral site is slightly greater than that of tetrahedral site, which may be attributed to difference in Fe3þeO2 distance implying difference in covalency of FeeO bond. It is generally suggested that the Fe3þeO2 bonding distance is about 15% smaller for the tetrahedral (A) sites than for the octahedral (B) sites in spinel ferrites, which means a greater degree of covalent bonding for Fe3þAeO2 in tetrahedral sites [17]. The value of nuclear hyperfine field at octahedral site is greater than the value for tetrahedral site, which is expected for the lower coordination number around Fe cation in the tetrahedral sites. The nuclear hyperfine field at A and B sites show a decrease with increase in the value of x, which is attributed to non-magnetic substitution. The smaller value of quadrupole shift of the A and B magnetic patterns in all the samples confirms that

Fig. 5. A. Mössbauer spectrum for composition with x ¼ 0. B. Mössbauer spectrum for composition 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) 849e856

853

55

25

20

45

Ms (emu/g)

Paramagnetic Character (%)

50

15

10

40 35 30 25

5

20

0

0.0

0.1

0.2

0.3

0.4

0.5

Zn content 0.0

0.1

0.2

0.3

0.4

0.5

Fig. 7. Variation of saturation magnetization with increasing value of ‘x’.

Zn content Fig. 6. Variation of paramagnetic character with Zn content.

3.3. Magnetic studies 3.3.1. Saturation magnetization Magnetic measurements reveal that all the samples show a typical MeH curve in which the magnetization rise sharply as applied field increases from zero in either direction and then slowly approaches to saturation. This is a typical behaviour of nanosized magnetic materials where residual superparamagnetism relaxations lead to rise in wings and ferrimagnetic part contributes to the hysteresis loop. Fig. 7 shows the variation of saturation magnetization as a function of Zn content. It has been observed that saturation magnetization increases initially up to a certain level of substitution and then follows the reverse trend. The observed variation can be explained on the basis of exchange interactions Table 2 Mössbauer parameters for various compositions of ‘x’ in Cs0.5x/2ZnxMn0.05Fe2.45x/2O4 recorded at 300 K. Composition

da

D mm/s

Magnetic hyperfine field Tesla

Distribution of Fe3þ ions (%)

0.01 0.008 0.02 0.01 0.63 0.001 0.009 0.35 0.04 0.06 0.36 0.002 0.15 0.30 0.01 0.06 0.34

49.71 46.76 49.56 45.38 e 49.38 46.24 e 49.31 43.37 e 49.24 42.31 e 49.16 41.35 e

45.51 54.49 60.46 37.47 2.07 47.32 45.99 6.69 58.88 30.10 11.02 64.39 16.89 18.72 73.47 2.93 23.60

mm/s x¼0 x ¼ 0.1

x ¼ 0.2

x ¼ 0.3

x ¼ 0.4

x ¼ 0.5

0.32 0.31 0.31 0.31 0.32 0.30 0.29 0.34 0.31 0.30 0.33 0.30 0.28 0.34 0.30 0.27 0.35

(oct.) (tet.) (oct.) (tet.) (C.D.) (oct.) (tet.) (C.D.) (oct.) (tet.) (C.D.) (oct.) (tet.) (C.D.) (oct.) (tet.) (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, the magnetic ions occupy the tetrahedral (A) and octahedral (B) sites of the spinel lattice. The saturation magnetization is taken as the difference between the magnetization of B and A sublattices. Since non-magnetic Zn2þ ion has a strong affinity for A site, its substitution reduces the magnetization of A sub lattice (MA) and thereby increases the net MS value. This can be observed from the experimental results which show an initial increase in saturation magnetization reaching to a maximum at x ¼ 0.3, and afterwards reverses its course (Table 3). 3.3.2. Curie temperature The variation in Curie temperature (TC) with the substitution of non-magnetic zinc content (x) in the basic compositional formula has been studied. The observed variation of Curie temperature with x has been displayed in Fig. 8 that shows a regular decrease in Curie temperature with increase in zinc content (x). This variation in Curie temperature can be explained on the basis of exchange interactions. Neel [20] considered three types of exchange interactions between the unpaired electrons of magnetic ions in the crystal lattice 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 octahedral sites)

Table 3 Variation of saturation magnetization, Curie temperature & dc resistivity with composition ‘x’. Composition (x)

Saturation magnetization Ms (emu/g)

Curie temperature ( C)

Resistivity (Ohm-cm)

0 0.1 0.2 0.3 0.4 0.5

22 34 44 52 39 28

425 360 320 260 210 126

8.02 9.5 1.23 1.35 1.67 1.80

     

105 105 106 106 106 106

854

M. Gupta, B.S. Randhawa / Solid State Sciences 14 (2012) 849e856

450

400

Tem perature 0C

350

300

250

200 150

100 0.0

0.1

0.2

0.3

0.4

0.5

Zn content Fig. 8. Variation of Curie temperature with increasing value of ‘x’.

AeB interactions are greater than AeA and BeB interactions. It is well established that the replacement of a magnetic ion, Fe3þ ion on either site of the crystal lattice by diamagnetic ions will result in the reduction of the number of magnetic linkages and consequently a fall in Curie temperature [15,16,18,22,23]. With increasing value of ‘x’ in the different compositions, Zn2þ ions occupy the A site. Thus a decrease in the Curie temperature is the consequence of the substitution of non-magnetic ions in the crystal lattice. The Curie temperatures of different samples are listed in Table 3. The magnitude of Curie temperatures for CseZn ferrites has been found to be higher than that of respective NaeZn and KeZn ferrites [15]. 4. Electrical studies

Fe2þ 4 Fe3þ, on octahedral sites. The Fe2þ ion concentration is a characteristic property of a given ferrite material and depends upon several factors such as sintering time, temperature and atmosphere, annealing time, etc., including the grain structure. Some amount of Fe2þ ions is also formed due to possible evaporation of metal ions during sintering as reported in lithium ferrites [24]. The variation of room temperature dc resistivity as a function of composition is presented in Table 3. It shows a regular increase with Zn content as shown in Fig. 9a and can be explained on the basis of Verwey mechanism of electron hopping between two valence states distributed randomly on equivalent lattice sites [25]. According to this model, ferrites form closed packed oxygen lattice with metal ions located at tetrahedral (A site) and octahedral (B site) sites and conduction may be considered due to hopping of Fe2þ and Fe3þ at B site. Since AeB distance is greater than the BeB distance therefore, dominant mode of conduction due to hopping of Fe2þ and Fe3þ occurs at BeB site. The higher value of dc resistivity obtained may also be contributed to improved nanosized ferrite particles [26] and better compositional stoichiometry with reduced Fe2þ formation obtained by solution combustion method. Samples with smaller grain consist of more number of grain boundaries, which acts as barrier to the flow of electrons. Another advantage of small grain size is that it helps in reducing Fe2þ ions [27] as oxygen moves faster in small grains, thus keeping iron in Fe3þ state. The temperature dependence of dc resistivity was also studied in the temperature range 308e398 K as displayed in Fig. 9b that shows an almost linear decrease in resistivity with temperature suggesting semiconductor behaviour of the ferrite materials in accordance with the Arrhenious relation:



r ¼ ra exp Er =KT



where r ¼ Resistivity

ra ¼ Resistivity extrapolated to T ¼ a

Er ¼ Activation energy k ¼ Boltzmann’s constant T ¼ Absolute temperature

4.1. dc-electrical resistivity The electrical property in CseZn ferrites has been attributed to electron hopping between the two valence states of iron,

The high dc-resistivity values obtained for the solution combustion route-processed CseZn ferrites make them suitable for high-frequency applications.

Fig. 9. a. Variation of dc-resistivity with Zn content. b. Temperature dependence of dc-electrical resistivity for Cs0.5x/2ZnxMn0.05Fe2.45x/2O4 with composition x ¼ 0.1, 0.3, 0.5.

M. Gupta, B.S. Randhawa / Solid State Sciences 14 (2012) 849e856

8

Dielectric properties for different ferrite samples were studied in the frequency range 102e107 Hz (Fig. 10). The frequency dependence of the dielectric constant (30 ) shows a continuous decrease with increase in frequency with pronounced dispersion at lower frequency and it remains almost independent of applied external field at high-frequency domain. It is clear from the Figs. 9a and 10 that the variation of dc-resistivity and dielectric constant as a function of Zn content exhibit opposite trends with each other. Similar trends have been reported by several workers [28,29] suggesting a strong relationship between conduction mechanism and dielectric behaviour of ferrites. The existence of dielectric dispersion can be explained on the basis of Koop’s twolayer model [30] and MaxwelleWagner polarization theory [31,32], in which relatively good conducting grains and insulating grain-boundary layers of ferrite material can be understood as being given by an inhomogeneous dielectric structure. Since an assembly of space-charge carriers in the inhomogeneous dielectric structure requires finite time to line up their axes parallel to an alternating electric field, the dielectric constant (30 ) naturally decreases. It was found that dielectric constant (30 ) value decreases with increase in zinc ion content. Initially with lower zinc ion concentration (x ¼ 0), the Fe2þ ions are maximum, and hence, it is quite possible for these ions to polarize to the maximum extent causing 30 to decrease, later Zn2þ ion substitution reduces Fe2þ ion concentration, thereby hindering the interaction between Fe2þ and Fe3þ ions. The overall low values of dielectric constant observed may be attributed to nanosized grain size of the ferrites obtained making these materials suitable for higher-frequency application. Fig. 11 shows an initial increase in the value of tangent loss (d) to attain a maxima followed by a regular decrease with frequency. Such peak behaviour occurs when jump frequency of electron exchange between Fe2þ and Fe3þ becomes equal to the applied field [33,34]. It can also be noted that the height of the peak increases with Zn2þ ions substitution at x ¼ 0.1, and then it shows a subsequent decrease with increase of Zn2þ ion concentration. The decrease of the height of the peak of tan d with increasing Zn2þ ions substitution may be attributed to the addition of diamagnetic Zn2þ ions in place of Fe3þ ions that limits the degree of conductivity by blocking hopping conduction mechanism thus resulting in an increase of resistivity.

7

4

8x10

0.1 0.2 0.3 0.4 0.5

4

Permittivity ( ε ')

7x10

4

6x10

4

5x10

4

4x10

4

3x10

4

2x10

4

1x10

0 2

10

3

10

4

10

5

10

6

10

7

10

Frequency (Hz) Fig. 10. Variation of dielectric constant for different compositions with frequency.

Loss tangent (tan δ)

4.2. Permittivity (dielectric constant) and tangent loss studies

855

0.1 0.2 0.3 0.4 0.5

6 5 4 3 2 1 0 2

10

3

10

4

10

5

10

6

10

7

10

Frequency (Hz) Fig. 11. Variation of tangent loss (d) for different compositions with frequency.

5. Conclusion The experimental results show that the solution combustion method adopted for the synthesis of ferrites is capable of controlling the stoichiometry, phase and particle size by controlling the temperature of the reaction. The magnetic as well as electric properties of the ferrites obtained are both particle size and temperature dependent. Mössbauer results show a transition from ferrimagnetic to super-paramagnetic phase with increase in non-magnetic zinc content. The results obtained with superior magnetic and electrical properties (high saturation magnetization, Curie temperature, resistivity and low dielectric constant etc.) make these ferrites, the potential materials for operating at microwave frequencies. Acknowledgement The financial support provided by CSIR, New Delhi is highly acknowledged. The authors are thankful to Dr. J.M. Greneche (France) for fruitful discussion on Mössbauer studies. References [1] M. Pardavi-Horvath, J. Magn. Magn. Mater. 215 (2000) 171. [2] N. Gupta, M.C. Dimri, S.C. Kashyap, D.C. Dube, Ceram. Int. 31 (2005) 171. [3] V.K. Sankaranarayanan, O. Parkash, R.P. Pant, M. Islam, J. Magn. Magn. Mater. 252 (2002) 7. [4] M. Tabuchi, K. Ado, H. Sakaebe, C. Masquelier, H. Kageyema, O. Nakamumo, Solid State Ionics 79 (1995) 220. [5] H. Vincent, S. Nicopoulos, J. Solid. State Chem. 98 (1992) 386. [6] P. Peshev, M. Pecheva, Mater. Res. Bull. 15 (1980) 1199. [7] P.I. Slick, E.P. Wholfarth (Eds.), Ferromagnetic Materials, North Holland Publishing Co., Amsterdam, 1980, p. 9. [8] D.C. Khan, M. Mishra, Bull. Mater. Sci. 7 (1985) 253. [9] J.G. Na, T.D. Lee, S.J. Park, IEEE Trans. Magn. 28 (1992) 2433. [10] A. Ahniyaz, T. Fujiwara, S. Song, M. Yoshimura, Solid State Ionics 151 (2002) 419. [11] G. Bonsdorf, H. Langbein, K. Knese, Mater. Res. Bull. 30 (1995) 175. [12] Z.C. Xu, J. Appl. Phys. 3 (2003) 4746. [13] A. Costa, E. Tortella, M.R. Morelli, M. Kaufman, R.H. Kiminami, J. Mater. Sci. 37 (2002) 3569. [14] B.S. Randhawa, H. Dosanjh, Hyperfine Interact. 183 (2008) 45. [15] B.S. Randhawa, M. Gupta, H. Dosanjh, N. Kumar, Ceram. Int. 37 (2011) 2207. [16] M. Gupta, B.S. Randhawa, Mater. Chem. Phys. 130 (2011) 513. [17] R.E. Watson, A.J. Freeman, Phys. Rev. 123 (1961) 2027. [18] R.D. Waldron, Phys. Rev. 99 (1955) 1727. [19] J. Preudhomme, P. Tarte, Spectrochim. Acta A 27 (1971) 961. [20] L. Neel, Ann. Phys. 3 (1948) 137. [21] Y. Yafet, C. Kittel, Phys. Rev. 90 (1952) 295. [22] K.H. Rao, N.K. Gaur, K.J. Aggarwal, J. Appl. Phys. 53 (1982) 1122. [23] V. Anjali, C.J. Ratnamala, J. Magn. Magn. Mater. 306 (2006) 313.

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