8/12/2019 1-s2.0-S0025540812009750-main

1/5

Electrical and dielectric properties of nano crystalline NiCo spinel ferrites

V.L. Mathe *, R.B. Kamble

Novel Materials Research Laboratory, Department of Physics, University of Pune, Ganeshkhind, Pune 411 007, M.S., India

1. Introduction

Nanocrystalline magnetic materials have attracted large

number of researchers due to their unusual physical and chemical

properties compared to bulk. Nanomaterials, particularly with

magnetic properties, are of great interest for their applications in

home appliances, electronic products, communication and data

processing devices, radio, television, microwave and satellite

communication, etc. [15]. Magnetic nanomaterials also find

applications in a variety of areas such as magnetic refrigeration at

high temperature [6], high density information storage [7], color

imaging [8], ferrofluids [9], bioprocessing [10], medical diagnosis

[11], electromagnetic wave absorption [12], etc. Mixed ferrites

usually have better performance than simple ferrites and show

properties and stability depending on the stoichiometry and

nature of the metallic ions in the final composition.

NiCo ferrites have been used extensively since few decades in

the electrical and electronic industries. NiCo ferrites are soft

magnetic materials that have spinel configurations based on a face-

centered cubic lattice of the oxygen ions, with the unit cell

consisting of 8 formula unit of the type [Fe3+]tetra [Ni2+Fe3+]octa

O42 or [Fe3+]tetra [Co2+Fe3+]octa O4

2 or mixed Ni and Co ferrite. In

this formula tetra represents tetrahedral site and octa represents

octahedral site. The structural, electrical, dielectric and magnetic

properties of ferrite are very much sensitive to technique adopted

for the synthesis, preparative parameters, initial ingredients, heat

treatment, etc. Due to the above mentioned parameters there may

be change in cation distribution which may result into unexpected

electrical, dielectric and magnetic properties.

Ferrites are commonly produced by a ceramic process; which

consists of rather large and non uniform sized grains being

prepared at high temperatures [13]. These non uniform grains on

compaction result in the formation of voids leading to low density.

These materials also can be prepared by various chemical

techniques such as, chemical co-precipitation [14], solgel [15],

soft chemical route [16], etc. Interest in preparing nano crystalline

ferrites is avoiding energy losses associated with the synthesis of

bulk material. Also the need of obtaining the materials near to

theoretical density is raised so as to further improve physical

properties [5]. The chemical co-precipitation has been considered

as a good method for production of homogeneous, fine grained and

reproducible ferrite systems [17]. In this work, we have studied

synthesis of nanosized ferrimagnetic NixCo1xFe2O4 powder by co-

precipitation technique followed by heat treatment. The series of

NiCo ferrite having nominal compositions NiFe2O4, Ni0.8Co0.2-Fe2O4, Ni0.6Co0.4Fe2O4, Ni0.4Co0.6Fe2O4, Ni0.2Co0.8Fe2O4 and

CoFe2O4 are refereed as A, B, C, D, E and F respectively. These

Materials Research Bulletin 48 (2013) 14151419

A R T I C L E I N F O

Article history:

Received 5 June 2012

Received in revised form 6 December 2012

Accepted 9 December 2012

Available online 19 December 2012

Keywords:

A. Magnetic materials

C. X-ray diffraction

D. Dielectric properties

D. Electrical properties

A B S T R A C T

Nanocrystalline samples of NixCo1xFe2O4, where x = 1, 0.8, 0.6, 0.4, 0.2 and 0, were synthesized by

chemical co-precipitation method. The spinel cubic phase formation of NiCo ferrite samples was

confirmedbyX-raydiffraction (XRD) data analysis.All theBragglinesobservedin XRDpatternbelongto

cubic spinel structure of ferrite. Scanning Electron Microscopy (SEM) technique was used to study thesurface morphology of the NiCo ferrite samples. Nanocrystalline size of NiCo ferrite series was

observed in SEM images. Pellets of NiCo ferrite were used to study the electrical and dielectric

properties.The resistivitymeasurementswere carried outon thesamples in the temperature range 300

900 K. Ferrimagnetic to paramagnetic transition temperature (Tc) for all samples was noted from

resistivity data. The activation energy below and above Tcwas calculated. The dielectric constant (e0)

measurementswith increasing temperature showtwo peaks in the temperaturerange ofmeasurements

for all samples under investigation.The peaks observedshowfrequency andcompositional dependences

as a function of temperature. Electrical and dielectric properties of nanocrystalline NixCo1xFe2O4samples show unusual behavior in temperature range of 500750K. To our knowledge, nobody has

discussed such anomalies for nanocrystalline NixCo1xFe2O4 at high temperature. Here, we discuss the

mechanism responsible for electrical and dielectric behavior of nanocrystalline NixCo1xFe2O4samples.

2012 Elsevier Ltd. All rights reserved.

* Corresponding author. Tel.: +91 020 25692678; fax: +91 020 25691684.

E-mail addresses: vlmathe@physics.unipune.ernet.in,

dearvikas_2000@yahoo.com (V.L. Mathe).

Contents lists available at SciVerse ScienceDirect

Materials Research Bulletin

journal homepa ge : www.elsevier .co m/loc ate /matresb u

0025-5408/$ see front matter 2012 Elsevier Ltd. All rights reserved.

http://dx.doi.org/10.1016/j.materresbull.2012.12.019

http://dx.doi.org/10.1016/j.materresbull.2012.12.019http://dx.doi.org/10.1016/j.materresbull.2012.12.019http://dx.doi.org/10.1016/j.materresbull.2012.12.019http://dx.doi.org/10.1016/j.materresbull.2012.12.019http://dx.doi.org/10.1016/j.materresbull.2012.12.019http://dx.doi.org/10.1016/j.materresbull.2012.12.019http://dx.doi.org/10.1016/j.materresbull.2012.12.019http://dx.doi.org/10.1016/j.materresbull.2012.12.019http://dx.doi.org/10.1016/j.materresbull.2012.12.019http://dx.doi.org/10.1016/j.materresbull.2012.12.019http://dx.doi.org/10.1016/j.materresbull.2012.12.019mailto:vlmathe@physics.unipune.ernet.inmailto:dearvikas_2000@yahoo.commailto:dearvikas_2000@yahoo.comhttp://www.sciencedirect.com/science/journal/00255408http://www.sciencedirect.com/science/journal/00255408http://www.sciencedirect.com/science/journal/00255408http://dx.doi.org/10.1016/j.materresbull.2012.12.019http://dx.doi.org/10.1016/j.materresbull.2012.12.019http://www.sciencedirect.com/science/journal/00255408mailto:dearvikas_2000@yahoo.commailto:vlmathe@physics.unipune.ernet.inhttp://dx.doi.org/10.1016/j.materresbull.2012.12.019

8/12/2019 1-s2.0-S0025540812009750-main

2/5

compositions have been investigated for their structural, electrical

and dielectric properties.

2. Experimental

2.1. Material preparation

Chemical co-precipitation route was used for synthesis of

nanocrystalline powder of NixCo1xFe2O4 series, where x = 1, 0.8,

0.6, 0.4, 0.2, 0. Nickel nitrate [Ni(NO3)26H2O], ferric nitrate[Fe(NO3)39H2O] and cobalt acetate (CH3COO)2Co4H2O were taken

as ingredients in desired molar proportion and mixed together in

distilled water. It was then heated at 90 8C with constant stirring for

2 h to get the clear solution. Drop by drop addition of 1 M NaOH to

above solution resulted into precipitate. The co-precipitation

reaction was carried out at 90 8C and pH = 12. The precipitate was

washed several times using hot distilledwater and filtered until the

pH of the filtrate became 7. Finally the wet slurry was dried at 100 8C

for 3 h to obtain dry powder. All the samples under investigation

weresynthesized insimilarmanner. Thedriedpowderswere shaped

into pellets and subjected to various heat treatments as given in

Table 1. The powders were characterized at several stages using X-

raydiffraction technique. X-raydiffractometer Advanced Bruker D-8

machine

having

Cu-Ka radiation

of

wavelength

l =

1.5418

A wasused to record X-ray diffractogram.

2.2. Pellet formation

The powder obtained by drying the slurry at 100 8C for 3 h was

used to make pellets of desired size. An organic binder Poly Vinyl

Alcohol (PVA) was added to the fine powder, and this mixture was

crushed to mix PVA uniformly in the powder. The powder was then

used to make pellets with the die-punch having diameter 10 mm. A

hand press machine was used to apply pressure of about 5 ton.

Powder was pressed and compacted in a die to form a pellet. In order

to enhance actual density, measured experimentally, the samples in

the form of pellet were sintered at high temperature as given in

Table

1.

Scanning

Electron

Microscope

(Model

JEOL,

JSM-6360A)was used tostudy surface morphology. Two flat faces ofa pellet were

polished, and silver paint was applied to make ohmic contact. These

pellets were used for electrical and dielectric measurements. DC

electrical resistivity measurements were carried out using two

probe method, where Keithley multimeter model 2000 was used to

measure current at constant voltage, atdifferent temperatures in the

range 300900 K. The dielectric measurements were carried out

using HIOKI 3532-50 LCR Hi tester at fixed frequencies as a function

of temperature in the range 300900 K.

3. Results and discussion

3.1. X-ray diffraction analysis

X-ray diffractogram recorded on the powders co-precipitat-

ed and dried at 100 8C does not show any Bragg lines, which is

indication of amorphous nature of the material. All the powders

were calcined at 400 8C for 8 h and characterized using X-raydiffraction technique. Only samples A and F (i.e. NiFe2O4 and

CoFe2O4) show Bragg lines while other samples do not show any

Bragg line. Therefore, samples B, C, D and E were again calcined at

600 8C for 8 h and examined again using X-ray diffraction

technique. Now these samples show presence of Bragg lines. X-

ray diffraction patternsof all the samples are shownin Fig. 1(a)(f).

The XRD patterns were analyzed for the phase identification. Inter

planar distance d for all the diffraction lines was found in good

agreement with that ofJCPDSdataof NiFe2O4 [18]. Slight shift in d

values is observed with increasing Co content. Normally the

ferrites prepared by chemical method show presence of a-Fe2O3phase due to loss ofoneof thedivalent elements. In Fig.1, there is

a diffraction linehavingvery small intensity correspondingtoa-

Fe2O3 between

the

(2

2

0) and

(3

1

1)

reflections for CoFe2O4.However, the diffraction peak intensity of the peak belong to a

Fe2O3 is negligibly small in comparison with the ferrite peak.

This indicates there is insignificant loss ofNi2+ orCo2+ions. Thus

from X-ray diffraction pattern, it is clear that the sample under

study is monophase spinel cubic ferrite. The crystallite size was

calculated from the XRD line width of (3 1 1) line using Scherrer

formula, t = 0.9l/b cos u, where t crystallite size, l

wavelength of X-ray, b full width at half maximum and u

Bragg angle. X-ray density was calculated using the formula

(XD) = 8 M/Na3 forthe spinel ferrite, whereM molecularweight

of ferrite, N is Avagadro number and a is the lattice parameter.

Experimental density measurements were carried out using

Archimedes principle. The values of lattice constant a and

crystallite

size t , X-ray density,

experimental

density and

%porosity for nanocrystalline samples are given in Table 1. The

lattice parameter obtained is in closely agreement with the

literature [19].

Table 1

Sintering temperature, structural and morphological data on nanocrystalline NixCo1xFe2O4.

Sample

name

Sintering

temp. (8C)

Lattice

const. a (A)

Cryst. size

from XRD (nm)

Grain size

from SEM (nm)

X-ray

density (gm/cm3)

Experimental

density (gm/cm3)

Pore

fraction (f)

% porosity

A 400, 8h. 8.338 8.3 33 5.37 4.32 0.19 19

B 600, 8h. 8.347 12.8 40 5.35 2.94 0.45 45

C 600, 8h. 8.353 12.8 40 5.34 3.08 0.42 42

D 600, 8h. 8.362 13.9 50 5.32 3.28 0.38 38

E 600, 8h. 8.372 11.9 50 5.30 3.21 0.39 39

F 400, 16h. 8.381 8.3 33 5.29 3.38 0.36 36

20 30 40 50 60 70 80

B

C

D

E

_Fe23

F

Int

ensity(A.U.)

A

(222)

(440)

(511)

(422)

(400)

(311)

(220)

2(deg.)

Fig. 1. (a)(f) X-ray diffractograms for NixCo1xFe2O4, forx = 1, 0.8, 0.6, 0.4, 0.2and

0 AF respectively. The samples x = 0 & 1 were sintered at 400 8C and the samples

x = 0.2, 0.4, 0.6 & 0.8 were sintered at 600 8C for 8 h.

V.L. Mathe, R.B. Kamble/Materials Research Bulletin 48 (2013) 141514191416

8/12/2019 1-s2.0-S0025540812009750-main

3/5

3.2. Scanning electron microscopy

The micrographs of samples A F obtained using SEM

technique are shown in Fig. 2(a)(f) respectively. It is clearly seen

in the micrographs that the grain size of the ferrites is at nano scale.

The grain size and their distribution are found to be almost uniform

in all the samples. The average grain size determined from SEM

images is noted as 3350 nm for NiCo ferrites, which is large than

the crystallite size from XRD data. This is due to the agglomeration

of the particles during processing of the samples. The grain size

data for all the samples under investigation are given in Table 1.

3.3. Electrical properties

The electrical resistivity of the samples in the form of pellet was

measured using two probe method in the temperature range 300

900 K. It was observed from density data that the highest observed

density was less than the X-ray density of the material. Therefore, a

correction for pore fraction has to be applied to obtain the

crystalline value of electrical resistivity. This has been done using

the relation [20]:

r rp1 f 1 f2=3

1

1

(1)

where r is corrected value, rp is measured value of dc resistivity

and f is pore fraction as given in Table 1. Fig. 3 shows variation of

resistivity (ln r) as a function of the reciprocal of absolute

temperature

for

NiCo

ferrite

sample.

Plots

in

Fig.

3

mainly

showthree regions as 300425 K (first region), 425 Ktransition

temperature (second region), above transition temperature (third

region). This is analogous to resistivity behavior observed for spinel

ferrites in literature [21,22]. The first region from 300 to 425 K

showed almost no variation in r with temperature. The second

region, which extend from 425 K to transition temperature, shows

temperature dependence. The third region covers the rest of the

temperature range. In all linear region, it follows an Arrhenious

relation r = roexp(DE/kT), where ro= pre-exponent factor, DE =

activation energy, k = Boltzmann constant and T = absolute

Fig.

2.

(a)(f)

SEM

micrographs

of

NixCo1xFe2O4,

for

x

=

1,

0.8,

0.6,

0.4,

0.2

and

0

AF

respectively.

V.L. Mathe, R.B. Kamble /Materials Research Bulletin 48 (2013) 14151419 1417

8/12/2019 1-s2.0-S0025540812009750-main

4/5

8/12/2019 1-s2.0-S0025540812009750-main

5/5

around 570 K and other is around Tcof CoFe2O4. For all the samples

the peak around Tcis dominant. (iii) From Fig. 4(a) it is seen that the

peak temperature shows slight shift with change in frequency.

Feature (i) can be explained as: up to 700 K the thermal energy

given is insufficient to free the localized dipoles to be oriented in

the direction of applied electric field. Above 700 K a large number

of dipoles become free due to sufficiently high thermal energy and

the applied electric field aligned them in its direction. Suchbehavior is observed by Ahmad et al. in case of lanthanum

substituted NiZn ferrite [29]. In the second feature, the peak

observed at 773 K can be explained as: the migration of Fe3+ ions

increases electron exchange between thermally activated Fe3+

Fe2+; hole transfer between Co3+Co2+ and Ni3+Ni2+ at octahedral

site, which results into increased polarization and dielectric

constant. Afterwards, migration saturates resulting e0 peak with

temperature. This observation is analogous to resistivity data in

the range of temperature 425800 K. For the sample F this peak is

shifted to low side and observed at around 583 K. For CoFe2O4sample, the migration starts at low temperature and saturates

around 570 K as observed in case of Ni substituted CuFe2O4[30].

The peak above 800 K for the samples A to E is attributed to

ferrimagnetic (magnetically ordered) to paramagnetic (disor-dered) transition of the samples and for sample F this peak is

observed at around 783 K. This is in agreement with dielectric

behavior observed for other ferrites at Curie temperature [29,31

34]. The decrease in e0 above transition temperature is attributed to

decrease in internal viscosity of the system giving rise to more

degree of freedom to the dipoles, which causes increased disorder

in the system and hence decreased in e0. This is in agreement with

Ahmed et al. [29]. The third feature can be explained as: The

behavior observed in NiCo is due to collective contribution of two

types of charge carriers; p and n to polarization. The appearance

of p type carriers in the present case is due to hole transfer in

Co3+Co2+ and Ni3+Ni2+ while Fe2+Fe3+ gives rise to n type

charge carriers. The polarization of p type charge carriers is in the

opposite

direction

to

that

of

n type

charge

carriers.

Also

themobility of p type charge carriers is low as compared to that of n

type charge carriers. Therefore, the shift in peak depends upon the

majority charge carriers. In the present case, the majority charge

carriers are of p type, confirmed from Seebeck coefficient data

with temperature (not shown). The peak in e0 shifts to low

temperature side with increasing temperature.

4. Conclusions

NixCo1xFe2O4 ferrites of grain size 3350 nm have been

successfully synthesized usingchemical co-precipitationmethod

and characterized for their structural and morphological proper-

ties. The morphological and structural studies prove the

nanocrystalline nature of

the

samples.

The spinel

cubic structure

of NiCo series possesses lattice parameter a = 8.33 to 8.38 A as

confirmed from XRD data analysis. The unusual electric and

dielectric behavior at high temperature is explained taking into

consideration an aspect of cation distribution. Ferrimagnetic to

paramagnetic transition values for NixCo1xFe2O4 ferrites lie

between 863 and 793K.

Acknowledgments

Authors are thankful to Center For Advanced Studies (CAS) in

Materials Science, Department of Physics, University of Pune, Pune

for financial support. Also, financial help from University of Pune

under the Research Scheme RG 14 is greatly acknowledged. VLM is

thankful to DST, New Delhi for the financial support, FAST TRACK

young scientist fellowship and BOYSCAST fellowship.

References

[1] M.K. Shobana, V.Rajendran, K.Jeyasubramanian, N. SureshKumar,Mater.Lett.61(2007) 2616.

[2] M.I. Godinho, M.A. Catarino, M.I. da Silva Pereira, M.H. Mendonca, F.M. Costa,Electro. Acta 47 (2002) 4307.

[3] A.M. Abo El Ata, M.A. Ahmed, J. Magn. Magn. Mater. 208 (2000) 27.[4] A.M. AboEl Ata, M.K. ElNimr,D. ElKony, A.H. AL-Hammadi, J.Magn.Magn.Mater.

204 (1999) 36.[5] T.M. Meaz, S.M. Attia, A.M. Abo El Ata, J. Magn. Magn. Mater. 257 (2003) 296.[6] L. Gunther, Phys. World 3 (1990) 28.[7] R.F. Zolio, US Patent 4,474,866 (1984).[8] L. Nixon, C.A. Koval, R.D. Noble, G.S. Slaff, Chem. Mater. 4 (1992) 117.[9] I. Anton, J. Magn. Magn. Mater. 85 (1990) 219.[10] R.D. MacMichael, R.D. Shull, L.J. Swartzendruber, R.E. Watson, J. Magn. Magn.

Mater. 111 (1992) 29.[11] J.W.M. Bulte, J. Magn. Reson. Imaging 4 (1994) 497.[12] F. Wei, L. Baoshum, Y. Jizhong, L. Xi, Z. Muyu, J. Magn. Soc. Jpn. 22 (1998) 366.[13] J.T. Irvine, A. Huanosta,R. Valenzuela, A.R.West, J. Am. Ceram. Soc. 73 (1990)729.[14] P.S. Anil Kumar,J.J. Shrotri,S.D. Kulkami, C.E. Deshpande, S.K. Date,Mater.Lett.27

(1996) 293.[15] P.K. Roy, J. Bera, J. Mater. Process. Technol. 197 (2008) 279.[16] J. Jianga, Y. Yangb, L. Li, Physica B 399 (2007) 105.[17] O. Suwalka, R.K. Sharma, V. Sebastian, N. Lakshmi_, K. Venugopalan, J. Magn.

Magn. Mater. 313 (2007) 198.

[18] JCPDS Card No. 10-325.[19] S. Singhal,J. Singh,S.K.Barthwal,K. Chandra, J.SolidStateChem. 178(2005)3183.[20] M.P. Pandya, K.P. Modi, H.H. Joshi, J. Mater. Sci. 40 (2005) 5223.[21] M.A. Ahmeda, N. Okasha, M. Gabal, Mater. Chem. Phys. 83 (2004) 107.[22] A.M. El-Sayed, Mater. Chem. Phys. 82 (2003) 583.[23] B. Viswanathan,V.R.K.Murthy, FerriteMaterials:Science andTechnology, Narosa

Publishing House, New Delhi, India, 1990, p. 11.[24] A.D. Shiekh, V.L. Mathe, J. Mater. Sci. 43 (2008) 2018.[25] C.S. Kim, S.W. Lee, S.L. Park, J. Appl. Phys. 79 (1996) 5428.[26] L. Radhapiyari, S. Phanjoubam,H.N.K. Sarma,C. Prakash,Mater. Lett.44 (2000) 65.[27] C.G. Koops, Physiol. Rev. 83 (1951) 121.[28] L.L. Hench, J.K. West, Principles of Electronic Ceramics,Wiley,NewYork, 1990, p.

189.[29] M.A.Ahmed,E. Ateia, L.M.Salah,A.A.El-Gamal,Mater.Chem.Phys. 92 (2005) 310.[30] W.C. Kim, S.J. Kim, S.W. Lee, S.H. Ji, C.S. Kim, IEEE Trans. Magn. 36 (2000) 3399.[31] A. Thakur, P. Mathur, M. Singh, J. Phys. Chem. Solids 68 (2007) 378.[32] D. Ravinder, K. Vijaya Kumar, Bull. Mater. Sci. 24 (2001) 505.[33] A.K. Singh, T.C. Goel, R.G. Mendiratta, J. Appl. Phys. 91 (2002) 6626.

[34]

H.M. Zaki, J. Alloys Compd. 439 (2007) 1.

Fig. 4. (a) Variation of dielectric constant (e0) with temperature for Ni0.6Co0.4Fe2O4, at 1 kHz, 10 kHz, 100 kHz and 1 MHz as a representative plot. (b) Variation of dielectric

constant (e0) with temperature for NixCo1xFe2O4, for x = 1, 0.8, 0.6, 0.4, 0.2 and 0 AF at 1 kHz.

V.L. Mathe, R.B. Kamble /Materials Research Bulletin 48 (2013) 14151419 1419