Microwave synthesis and electrical properties of indium-substituted

Mn�Zn ferrites

Surender Kumara*, Tukaram J. Shindeb and Pramod N. Vasambekarc

aDepartment of Physics, Pandit Anant Ram Sanatan Dharam College, Baroh, India; bDepartment ofPhysics, KRP Kanya Mahavidyalaya, Islampur, India; cDepartment of Electronics, ShivajiUniversity, Kolhapur, India

(Received 9 June 2014; revised 20 July 2014; accepted 31 July 2014)

Nanocrystalline ferrites Mn0.6Zn0.4InyFe2-yO4 (y D 0, 0.035, 0.070, and 0.100) wereprepared by the oxalate co-precipitation technique followed by microwave heating.The X-ray diffraction technique confirmed formation of single-phase spinel ferriteswith crystallite size in the range of 11�16 nm. Vibrations of metal�oxygen bonds atoctahedral and tetrahedral sites in the crystal lattice were confirmed by Fouriertransform infrared spectra. Morphology was verified by scanning electron microscopy.The effect of composition on room temperature dielectric permittivity, dielectric loss,and AC conductivity of samples was studied in the frequency range from 20 Hz to1 MHz. Decrease in dielectric constant and dielectric loss were observed withincreasing frequency. AC conductivity peaked in the narrow frequency band centeredaround 500 KHz was observed. Conductivity at resonance increases with increase inindium concentration. The behavior of nanopowders under the influence of time-varying signal was analyzed on the basis of Maxwell�Wagner theory.

Keywords: ferrites; co-precipitation; electrical properties

1. Introduction

Nanocrystalline materials have attracted considerable attention due to the presence of a

large number of atoms at the grain surface compared to coarse-grained polycrystalline

counterparts. The fraction of atoms on the surface of grains increases with decreasing

grain size. Hence, the interface structure plays an important role in determining the elec-

trical properties of nanocrystalline materials.[1] Spinel ferrites are magnetic semiconduc-

tors with the general formula AB2O4, where A is a divalent metal ion and B is iron;

however, iron can be partially substituted by other trivalent metal ions. Ferrites crystallize

in the face-centered cubic crystal system and belong to the space group Fd3m(O7h).[2]

Oxygen anions arranged in a cubic close-packed lattice form 64 tetrahedrons and 32 octa-

hedrons in a unit cell. The cations occupy 8 of tetrahedral sites (A sites) and 16 of octahe-

dral sites (B sites); the conventional unit cell then contains eight of such AB2O4 formula

units.[3] The tetrahedral and octahedral sites accommodate various metal cations in order

to yield a stable spinel structure. Variation in cation distribution is used to tailor physical

properties of spinel ferrites.[4]

Polycrystalline spinel ferrites have a wide range of applications due to high electrical

resistivity, dielectric constant and permeability, chemical stability, and reasonable cost.

[5] They are extensively used in radiofrequency circuits, high-quality filters, rod

*Corresponding author. Email: surender.postbox@gmail.com

� 2014 Chinese Advanced Materials Society

Journal of the Chinese Advanced Materials Society, 2014

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antennas, transformer cores, read/write heads for high-speed digital tapes, and magneto-

caloric refrigeration.[6,7] The mechanism of dielectric polarization in ferrites is similar

to the electrical conduction mechanism. In the case of manganese zinc ferrites, localized

charge hopping between Fe3C$Fe2C ions and Mn2C$Mn3C ions is responsible for elec-

trical conduction. In the nanoregime, the presence of Fe2C/ Fe3C and Mn3C/ Mn2C will be

dependent on the conditions of preparation. The cation distribution of ferrites in the nano-

regime is different from their bulk counterparts .These factors are likely to influence elec-

trical properties of ferrites.[5] The study of the dielectric constant, dielectric loss, and AC

conductivity allows greater insight and understanding of the behavior of localized charge

carriers under the influence of AC field. Indium-substituted ferrites sintered at high tem-

peratures have been systematically studied by Laxman et al.[8] However, at nanoscale,

materials exhibit novel properties such as superparamagnetism and quantum tunneling of

magnetization; this is due to quantum size effects and the large surface area of nanopar-

ticles.[9] Therefore, it is important to investigate the behavior of indium-substituted

Mn�Zn ferrites at nanoscale. Since no systematic study has been conducted on electrical

properties of In3C-substituted Mn�Zn nanoferrites, this work aims at exploring new tech-

nological possibilities at nanoscale and the underlying dynamics of the localized charge

carrier.

The dielectric properties and electrical conduction of ferrites depend on the composi-

tion, method of preparation, types, and quantity of additives. These factors regulate the

frequency variation of dielectric properties, thereby providing us a good understanding of

electrical conduction and underlying processes.

The properties of ferrites are substantially altered with decrease in grain size

from micro to nano range. Reduction in grain size improves resistivity due to increase

in grain boundary volume; this obstructs intergrain movement of localized charge

carriers. The dielectric constant increases with increase in resistivity of ferrites.

The dielectric properties of spinel ferrites are based on a heterogeneous structure of

ferrites which consists of grains and grain boundaries. E.V. Gopalan et al. [5] have

discussed dielectric relaxation in micron-sized particles in the frequency range

1�100 KHz. Dielectric relaxation can change with grain size. In the case of nano-

crystalline ferrites, the existence of a large number of interface dipoles can give rise

to different relaxation processes, resulting in a broader relaxation. Sharp dispersion

and a relaxation effect with multiple relaxation times are reported in ferrites in the

nanoregime.

The co-precipitation technique is well known for producing nanocrystalline materi-

als [10, 11]; however, this process requires hours of heating in a furnace to produce fer-

rites. This is an energy-intensive process. Microwave heating has emerged as an

energy-efficient method for the synthesis of novel materials.[12�14] This work aims to

investigate the application of microwave heating to microwave transparent precursors

obtained by the oxalate co-precipitation technique. SiC is the preferred susceptor used

for microwave synthesis of ceramics for microwave transparent precursors. However,

SiC is a strong absorber of microwaves and hence leads to rapid rise of temperature,

which is difficult to control.[15] For nanoscale synthesis, a susceptor with moderate

microwave absorption capacity is required so that the rate of heating could be easily

controlled. The nanocrystalline ferrites are synthesized at low temperatures, and a mod-

erate absorber of microwave radiation is desirable. The aluminum metal powder was

used as the microwave susceptor for heating microwave transparent precursors in this

work and the effect of composition on microstructural and electrical properties was

studied.

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2. Experimental

The synthesis of nanocrystalline ferrites Mn0.6Zn0.4InyFe2-yO4 (y D 0.000, 0.035, 0.070,

0.100) was carried out by heating oxalate precursors in a commercial microwave oven.

Manganese sulfate monohydrate (MnSO4¢H2O � 99%, Merck), iron(II) sulfate heptahy-

drate (FeSO4¢7H2O � 99%, Merck), zinc sulfate heptahydrate (ZnSO4¢7H2O � 99%,

Merck), indium sulfate (In2(SO4)3 > 99%, Hi-Media), and di-ammonium oxalate mono-

hydrate ((NH4)2C2O4¢H2O, Guaranteed reagent, Merck) were used as starting materials.

The metal sulfates were dissolved in de-ionized and double-distilled water at room tem-

perature and then heated to 45 �C. The aqueous solution of di-ammonium oxalate was

rapidly added to the solution of metal sulfates under continuous stirring. Addition of a

precipitating agent was stopped when the pH of the metal ion solution undergoing precip-

itation reached 3.5. Bright yellow precipitates were allowed to settle in the beaker and the

supernatant liquid was poured off. Excess of sulfate ions in the precipitate were washed

with distilled water, allowing the precipitate to settle, and then pouring off the excess

water. The barium chloride test was used to confirm the absence of sulfate ions in the pre-

cipitates. The precipitates were dried in an oven at 100 �C for 8 hours.

The heating setup used in the microwave oven was built in-house. A cylindrical cavity

(diameter D 4 cm, height D 5 cm) was created in a high alumina brick (11.5 cm £11.5 cm £ 10 cm). A quartz crucible was placed in it. The gap between the outer wall of

the crucible and the wall of the cylindrical cavity was filled with aluminum powder. The

precipitate was poured in the crucible. The setup was now placed on the turn table of the

commercial microwave oven (Onida, 25XL Power Convection) operated at a frequency

of 2.45 GHz with maximum output power of 900 watts. The oven was operated at 60% of

optimum power for 630 seconds to raise the temperature of the precursors to 450 �C. Thebrick was immediately brought out of the oven and allowed to cool.

The phase purity of ferrite powder was characterized using a Bruker AXS D8

Advance X-ray diffractometer with Cu Ka radiations(λ D 1.5406 A�), operated at 40kV,

30mA. The X-ray diffraction patterns were recorded in the scattering range (2u) of

10��70� at the scan rate of 0.05� per second. Fourier transform infrared (FTIR) spectra

were recorded in the range of 350�800 cm¡1 on a Perkin Elmer, spectrum one spectro-

photometer using a KBr pellet (1 mm thickness, 1:100 ratio of KBr to the specimen)

method. The microstructure and morphology of nanocrystalline ferrites were investigated

by scanning electron microphotographs under a JEOL JSM-6610LV scanning electron

microscope.

The ferrite powder was added with polyvinyl alcohol in an agate mortar and thor-

oughly mixed. The powder was pressed in a pellet die under a force of 6 tons for three

minutes under a hydraulic press. The pellets (thickness 2 mm and diameter 10 mm) were

sintered at 400 �C for four hours. The ferrite pellets were firmly held between circular sil-

ver foils in a conductivity cell to study electrical properties. Dielectric properties were

studied in the frequency range of 50 Hz�1 MHz at room temperature using an LCR meter

(Agilent 4284A).

3. Results and discussion

3.1. Characterization

The X-ray diffraction patterns in Figure 1 contain peaks corresponding to the (111),

(220), (311), (222), (400), (422), (511), and (440) planes. The average crystallite size of

samples was determined from the line broadening of the (311) peak using the

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Debye�Scherrer equation [16]

D ¼ 0:94 λb cosu

: (1)

Here D is the crystallite size, λ is the wavelength of incident X-rays, and b is the full

width at half-maximum of the most intense peak. Crystallite size increases with increase

in indium content. This is due to the larger bond length of In3C¡O2¡ as compared to

Fe3C¡O2-.[17] Diffraction peak intensities increase with increasing indium concentra-

tion, thereby leading to substantial improvement in crystalline character.

Figure 1. XRD patterns of Mn0.6Zn0.4InyFe2-yO4.

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X-ray density (rx) of prepared ferrites was calculated using the expression

rx ¼ ZM=ðNa3Þ, where M is the weight of the corresponding composition, N is

Avogadro’s number, a is a lattice constant, and Z is the number of molecules per unit cell

(Z D 8 for ferrites). The physical parameters like crystallite size, lattice constant, and X-

ray density with composition are enlisted in Table 1. There is a correlation between ionic

radius and lattice constant. The lattice constant is found to increase with increase in

indium concentration in Mn�Zn ferrites. This is because In3C with a larger ionic radius

(0.92 A�) substitutes Fe3C having ionic radius 0.67 A

�. The X-ray density also increases

with increase in indium content. It is attributed to higher atomic weight of In3C ion com-

pared to Fe3C ion.[17]

FTIR spectra are explained on the basis of the group theory representation of spinel

ferrites which is written as

Gred ¼ A1g þ Eg þ T1g þ 3T2G þ 2A2u þ 2Eu þ 5T1u þ 2T2u: (2)

For an ideal spinel system, only T1u vibrations are IR active, whereas T1g, A2u, Eu, and

T2u symmetry vibrations are silent. A1g, Eg, and T2g symmetry modes are associated with

Raman spectroscopy and hence will not be discussed hereafter. FTIR spectra of spinel fer-

rites correspond only to T1u and experimentally reflected in the form of four absorption

bands in the range of 250�800 cm¡1.[18,19] FTIR spectra were recorded from 350 to

800 cm¡1 (Figure 2). The spectra indicate presence of two primary absorption bands, v1and v2. The absorption band v1 (»600 cm¡1) is caused by metal�oxygen vibrations in tet-

rahedral sites and v2 (»400 cm¡1) is caused by stretching vibrations of the octahedral

metal�oxygen bond.[20] In addition, v1sp and v2sp represent additional bands/splitting of

primary bands v1 and v2 (Table 2). Vibrational spectra of ionic materials can be substan-

tially different from those in gas or liquid phases. The constrained and complex environ-

ment surrounding a metal ion in crystalline materials can alter the fundamental

absorption bands in ideal IR spectra. The alteration could be (1) shifting to higher or

Table 1. Composition-dependent structural parameters of the Mn0.6Zn0.4InyFe2-yO4 system.

Composition y Lattice constant (A�) Crystallite size (nm) rx (g/cm

3)

0.000 8.3601 11.6 5.304

0.035 8.3692 12.1 5.364

0.070 8.3950 14.0 5.366

0.100 8.4130 16.1 5.369

Table 2. The IR absorption bands (wavenumber in cm¡1) for Mn0.6Zn0.4InyFe2-yO4.

Tetrahedral bands Octahedral bands

y n1sp n1 n2 n2sp

0.000 654 563 440 399

0.035 651 567 455 �0.070 652 564 450 399

0.100 650 563 449 401

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lower wavelength; (2) increasing bandwidth; (3) splitting into two or more components;

and (4) emergence of additional bands. It is also well known that the number of molecular

modes increases on going from gaseous or liquid phase to solid phase. This occurs due to

lower symmetry at the lattice site compared to that of the free molecule. Reduction in site

symmetry of molecular units in an ideal lattice lifts the degeneracy of degenerate normal

modes and allows expression of silent modes in IR spectra.[21]

The conventional view projects that the cation distribution in a spinel structure does

not change its space group; hence, the number of bands in IR spectra should remain

largely unaffected. This view ignores the capacity of variable cation mass or cation�oxy-

gen bond strength to change vibrational spectra. Ordering of cations at tetrahedral and

octahedral sites can lower crystal symmetry and hence bring out more IR active bands.

Additionally, octahedral and tetrahedral sites in a spinel structure are not isolated in a

Figure 2. FTIR spectra of Mn0.6Zn0.4InyFe2-yO4.

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spinel lattice. Four parameters which have a primary influence on IR spectra of ferrites

are cation mass, metal�oxygen bond strength, metal�oxygen bond length, and the

dimensions of the unit cell.[19] These four parameters do not represent a complete

description of FTIR spectra. The spinel ferrites are very complex crystalline oxides.

Keeping in view the fact that a spinel unit cell contains 32 anions (O2¡), entrapping 32

octahedral and 64 tetrahedral sites, the independence of individual octahedral and tetrahe-

dral complexes is seriously compromised by the surrounding ions. This is so because oxy-

gen atoms in spinel ferrites have to move from their ideal positions in order to

accommodate cations of variable sizes. The anion sub-lattice expands or contracts by

changing u (oxygen position parameter) until the volume of tetrahedral and octahedral

complexes matches the radii of constituent atoms.[22] This causes deviation from ideal

spinel structure. Further covalent bonding in spinel ferrites influences lattice distortion.

So, the presence of the bands v1sp and v2sp reflects this complex interplay of the various

factors discussed above and activation of silent spectral modes/splitting of absorption

bands. In a comprehensive work reported by Bujakiewicz-Koronska et al.,[18] silent

modes are found to be activated in spinel ferrites. The additional absorption bands are

due to lowering of symmetry and they correspond to A2u, Eu, and T2u modes. The pure

Mn�Zn ferrite has lowest band position of v1 at 563 cm¡1. The v1 and v2 peak positions

rise sharply for y D 0.35 and then decline with further increase in indium concentration.

Presence of larger atoms in the interstitial sites can increase the unit cell dimension. This

is also corroborated from Table 1. The low values of v1 and v2 for y D 0.000 are attributed

to covalent character Zn2C ions at tetrahedral sites which hinders lattice distortion.[23]

The shifting of primary absorption bands towards lower frequency with increasing In3C

ions is attributed to increase in unit cell dimensions and substitution of larger In3C for

Fe3C in the system.[24] Further, the redistribution of cations on account of nanoscale

nature of particles and the increased covalent character of the spinel ferrites due to

increased incorporation of In3C ions caused the observed variation in the intensity and the

peak positions of absorption bands. The position of absorption bands and their shift with

variation in composition reveals distortion of bonds due to the behavior of neighboring

ions of a particular site.[25] The presence of Fe2C ions in ferrites can cause a shoulder or

splitting of the absorption band Jahn�Teller effect which can lead to the formation of a

non-cubic component of crystal field component and hence to splitting of the band v1.

[26] This is further supported by the absence of fine structure in the FTIR spectra. Fine

structure in primary absorption bands represents long range order. Long range order cor-

responds to higher symmetry in crystalline structure; the crystal symmetry increases with

growing strength of fine structure. As the fine structure in absorption bands vanishes,

long-range order is transformed into short-range order.[27,28] Absence of fine structure

in the FTIR spectra in Figure 3 indicates lowering of crystal symmetry from ideal spinel

structure and only short-range order exists. Intensity ratio of absorption bands is a func-

tion of the change in dipole moment with inter-nuclear distance.[29] This value corre-

sponds to the contribution of the ionic bond Fe�O in the lattice. Therefore, the

compositional dependence of FTIR spectra is indicative of the distortion in the fcc cubic

spinel structure of the ferrites due to the perturbation exerted on Fe�O bonds [17] by

introducing In3C ion.

The scanning electron micrograph in Figure 3 presents a representative sample with

composition Mn0.6Zn0.4In0.100Fe1.900O4. Morphology of the ferrites shows that the ferrite

nanoparticles as well as agglomerates are present in the sample. Similar results were

reported by Hankare et al. [7] in a systematic study of nanoscale ferrites. Agglomeration

of nanoparticles has been caused by attractive Van der Waal forces. Since a surfactant

7

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was not used during synthesis, particles have hydrophobic surfaces with a large surface

area to volume ratio. Hydrophobic interactions between the particles lead to the formation

of some particle agglomerates, resulting in increased particle size.[9]

Successful synthesis of indium-substituted ferrites by applying microwave heating to

the microwave-transparent materials has proven the effectiveness of this method as a

novel technique. The ceramic technique for synthesis of ferrites is energy intensive and

time consuming; as compared to this, microwave synthesis has emerged as an efficient

technique for synthesis of new materials in recent times.[30�32] Microwave heating is

low on energy consumption as compared to a conventional furnace. This is because

volumetric heating converts precursors into desired material within minutes; therefore,

microwave heating consumes low energy as compared to a conventional furnace. It is a

cost-effective, time-saving and environment-friendly technique. These advantages are

associated with microwave heating due to the way the microwaves interact with the mate-

rials. Materials interact in three different ways with microwave radiations; they undergo

reflection, absorption or transmission while passing through materials. Reflection or

transmission of microwaves does not cause thermal changes; however, absorption of

microwave radiation causes rise in temperature of materials due to dielectric heating.[33]

This work has been successful in developing a new class of microwave synthesis, where

moderate temperatures and controlled temperature growth are ensured by using aluminum

metal powder as the microwave susceptor.

3.2. Dielectric constant and loss

Electrical conduction in spinel ferrites is due to small polaron hopping between ions of

the same element existing in more than one valence state and distributed randomly over

crystallographically equivalent lattice sites.[8] The resistance to flow of charge careers is

of two types, inter-granular and intra-granular resistivity. In nanocrystalline ferrites,

Figure 3. Scanning electron micrograph of Mn0.6Zn0.4In0.100Fe1.900O4.

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intra-granular resistivity does not exist since the grains are simply the crystallites of spinel

ferrites. The real part of dielectric constant was calculated using the expression

e 0 ¼ Cd

e0A; (3)

where C is the capacitance of the pellet, d is the thickness of the pellet, A is the cross-sec-

tional area of the flat surface of the pellet and e0 is the permittivity of free space. The AC

conductivity (sac) was obtained from the dielectric constant (e0 ) and dielectric loss tan-

gent (tan d) using the expression [6]

sac ¼ 2pf e0e 0 tand (4)

where f is the frequency of applied signal. Figures 4 and 5 display the frequency depen-

dence of dielectric constant and loss, respectively.

Ferrites are heterogeneous systems with grains and grain boundaries. Grains are well

conducting, whereas grain boundaries are poorly conducting. Dielectric dispersion is

explained by Maxwell�Wagner theory [34,35] of interfacial polarization in consonance

with Koops phenomenological theory.[36] The electrons approach the grain boundary

under the influence of applied electric field. Since the resistance of grain boundaries is

large, electrons pile up at the grain boundary and cause polarization.[6,37] So, the dielec-

tric constant is dependent on the number of grains. The number of grains and grain bound-

aries is much higher in the studied samples compared to those reported by Gopalan et al.

[6] This is attributed to lower crystallite size (�16 nm) that increases the grain boundary

contribution per unit volume of the sample and hence leads to substantially higher dielec-

tric permittivity of indium-substituted Mn�Zn ferrites. At low frequencies of applied

Figure 4. Dielectric permittivity of Mn0.6Zn0.4InyFe2-yO4.

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signal, electrons have sufficient time to reach the grain boundary before reversal of field

direction. This causes large polarization. However, at high frequencies, low mobility of

charge carriers in ferrites prohibits quick movement to the grain boundary and hence

interfacial polarization reduces substantially at high frequencies.

It is observed that dielectric constant decreases with increasing frequency of applied

signal. This is attributed to interfacial polarization because electronic and atomic polariza-

tions are largely unaffected by frequency variation. For frequencies greater than 104 Hz,

dielectric permittivity becomes independent of frequency of applied signal due to reduced

polarization.[6] This reduced polarization corresponds to the diminishing role of the grain

boundaries. Dielectric constant of unsubstituted ferrites is substantially higher than In3C-substituted ferrites at low frequencies. This is due to higher mobility of charge carriers in

Mn�Zn ferrites. Higher mobility allows charge carriers to quickly accumulate at the grain

boundaries. This creates higher interfacial polarization for pure Mn�Zn ferrites. It is

known that In3C ions occupy tetrahedral sites and push Mn2C ions into octahedral sites.

Increasing In3C concentration in tetrahedral sites progressively reduces the number of avail-

able sites for hopping of localized charge carriers.[8] This reduces the probability of hop-

ping between Fe3C and Fe2C ions. This reduces mobility and hence interfacial polarization.

The real and imaginary parts of dielectric permittivity, and the dielectric loss are

given by the Debye equations [38]

e 0 ¼ e1 þ e0¡ e11þ v2t2

; (5)

e00 ¼ ðe0 ¡ e1 Þvt1þ v2t2

; (6)

tand ¼ e00

e 0¼ ðe0 ¡ e1 Þ

ðe0 ¡ e1v2t2Þ ; (7)

Figure 5. Dielectric loss of Mn0.6Zn0.4InyFe2-yO4.

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where t is relaxation time, e0 is dielectric permittivity at very low frequencies and e1 is

dielectric permittivity at very high frequencies. From Figure 6 it can be observed that at

low frequencies dielectric loss increases with increasing indium concentration. However,

as the frequency increases above 1000 Hz, the distinction between various samples

reduces substantially. The variation of dielectric permittivity and dielectric loss with fre-

quency obeys the relationship predicted by Equations (5) and (7).

3.3. AC conductivity

The conduction mechanism in ferrites is explained on the basis of the Verwey and De

Boer [39] mechanism that involves exchange of electrons between the ions of the same

element present in more than one valence state and distributed randomly over equivalent

crystallographic lattice sites. Cations are surrounded by close-packed oxygen anions in

spinel ferrites and hence cations are well isolated from each other. In other words, charge

carriers associated with a particular ion are localized in a region. So, AC conductivity

reflects the response of localized charge careers to the frequency of applied electric field.

The conduction process in ferrites is attributed to hopping from Fe3C to Fe2C ions. The

increase in frequency of applied signal increases the hopping frequency and hence con-

duction. Ferrites are low-mobility materials and the increase in conductivity is due to

increase in mobility. There is no increase in the number of charge carriers.[40]

The AC conductivity (sac) as a function of frequency (20�106 Hz) at room tempera-

ture for the investigated compositions is presented in Figure 6. It is noted that sac at reso-

nance (»500 kHz) increases with increase in indium concentration. The dispersion in sac

with frequency has been explained by Koop’s theorem which assumes that the ferrite

compact acts as a multilayer capacitor (ferrite grains C grain boundaries). Beyond 104

Hz, the effect of the multilayer capacitor rises with frequency; hence, the conductivity

Figure 6. AC conductivity of Mn0.6Zn0.4InyFe2-yO4.

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increases. The conductivity is a complex quantity (sac ¼ s 0 þ is00), whose real and imag-

inary components are as follows [41]:

s 0 ¼ s0

1þ v2t2; s00 ¼ s0vt

1þ v2t2: (8)

At low frequencies grain boundaries have a dominant contribution to AC conductiv-

ity. Scanning electron microscopy confirms that grains of pure Mn�Zn ferrites as well as

In3C-substituted ferrites have similar morphology; hence, a low-frequency contribution

of grain boundaries is indistinguishable. This is also verified mathematically from Equa-

tion (7). In the low-frequency region, vt � 1, s00 � s 0 , that is, samples exhibit resistive

nature due to grain boundaries. However, in the high-frequency region, vt � 1,

s00 � s 0 , the conduction process is primarily inductive in nature due to conducting

grains.[42] The applied frequency acts as a pumping force, pushing the localized charges

between the different conduction states. The conductivity maximum at high frequency

(� 105Hz) is due to the resonance effect at which the lattice vibration frequency is equal

to the frequency of applied signal. After the maximum, the decrease in conductivity is

due to generation of some phonons as a result of lattice vibrations. These phonons interact

with charge carriers, resulting in a decrease in conductivity.[42]

4. Conclusion

Indium-substituted manganese�zinc ferrites were successfully prepared by microwave

synthesis. Microwave radiations, consequent upon their interaction with a microwave sus-

ceptor, quickly convert oxalate precursors into nanocrystalline ferrites. X-ray diffraction,

FTIR, and scanning electron microscopy confirmed the formation of desired material.

Improvement in crystalline character and X-ray density was confirmed with increasing

substitution of indium in Mn�Zn ferrites. Lowering of symmetry from ideal-face-cen-

tered cubic spinel structure was confirmed. Long range order is absent. The dielectric

behavior can be explained in terms of the electron exchange between Fe2C and Fe3C.Conductivity maximum is obtained when the jumping frequency of electrons between

Fe2C and Fe3C equals that of the applied signal. Conductivity at resonance increases with

increase in indium concentration.

References

[1] Venkataraju C, Sathishkumar G, Sivakumar K. Effect of cation distribution on the structuraland magnetic properties of nickel substituted nanosized Mn�Zn ferrites prepared by copreci-pitation method. J Magnetism Magn Mater. 2010;322:230�233.

[2] Patil RP, Patil NM, Sasikala R, Hankare PP, Delekar SD. Effects of Ti substitution on struc-tural and magnetic properties of Zn�Mn ferrospinels. Mater Res Bull. 2013;48:1791�1795.

[3] Chlan V. Hyperfine interactions in ferrites with spinel structure [PhD thesis]. Prague: CharlesUniversity of Prague; 2010. p. 3.

[4] Rath C, Sahu KK, Anand S, Date SK, Mishra NC, Das RP. Preparation and characterization ofnanosize Mn�Zn ferrite. J Magnetism Magn Mater. 1999;202:77�84.

[5] Gopalan EV, Malini KA, Saravanan S, Kumar DS, Yoshida Y, Al-Omari IA, AnantharamanMR. On the dielectric dispersion and absorption in nanosized manganese zinc mixed ferrites.J Phys. 2009;21:146006.

[6] Gopalan EV, Malini KA, Saravanan S, Kumar DS, Yoshida Y, Anantharaman MR. Evidencefor polaron conduction in nanostructured manganese ferrite. J Phys D. 2008;41:18005.

12 S. Kumar et al.

J. Chin. Adv. Mater. Soc., 2014, 1�14

Dow

nloa

ded

by [

Sure

nder

Kum

ar]

at 0

8:01

23

Sept

embe

r 20

14

[7] Hankare PP, Patil RP, Garadkar KM, Sasikala R, Chougule BK. Synthesis, dielectric behaviorand impedance measurement studies of Cr-substituted Zn�Mn ferrites. Mater Res Bull.2011;46:447�452.

[8] Lakshman A, SubbaRao PSV, Rao BP, Rao KH. Electrical properties of In3C and Cr3C substi-tuted magnesium�manganese ferrites. J Phys D. 2005;38:673.

[9] Gupta AK, Gupta M. Synthesis and surface engineering of iron oxide nanoparticles for bio-medical applications. Biomaterials. 2005;26:3995.

[10] Gyergyek S, Drofenik M, Makovec D. Oleic-acid-coated CoFe2O4 nanoparticles synthesizedby coprecipitation and hydrothermal synthesis. Mater Chem Phys. 2012;133:515�522.

[11] Kumar N, Khurana G, Gaur A, Kotnala RK. Observation of superparamagnetism in ultra-fineZnxFe1¡xFe2O4 nanocrystals synthesized by coprecipitation method. Mater Chem Phys.2012;134:783�788.

[12] Seol S, Kim D, Jung S, Hwu Y. Microwave synthesis of gold nanoparticles: effect of appliedmicrowave power and solution Ph. Mater Chem Phys. 2011;131:331�335.

[13] Lim CS. Microwave-assisted synthesis of CdWO4 by solid-state metathetic reaction. MaterChem Phys. 2012;131:714�718.

[14] Sha L, Liu Y, Zhang Q, Hu M, Jiang Y. Microwave assisted coprecipitation synthesis of highpurity b-tricalcium phosphate crystalline powders. Mater Chem Phys. 2011;129:1138�1141.

[15] Roy R, Agrawal D, Cheng J, Gedevanishvili S. Full sintering of powdered-metal bodies in amicrowave field. Nature. 1999;399:668�670.

[16] Khorrami SA, Mahmoudzadeh G, Madani SS, Sepehr SS, Manie S, Moradi S, Gharib F, SharifAM, Azar PA. Determination of magnetic properties of nano-size CoFe2O4 particles synthe-sized by combination of sol-gel auto-combustion and ultrasonic irradiation techniques. JTheor Appl Phys. 2010;4:1�4.

[17] Kaiser M. Structure, electric and dielectric studies of Indium substituted magnesium coppermanganese ferrites. Physica B. 2011;406:899�905.

[18] Bujakiewicz-Koronska R, Hetmanczyk L, Garbarz-Glos B, Budziak A, Kalvane A, BormanisK, Druzbicki K. Low temperature measurements by infrared spectroscopy in CoFe2O4

ceramic. Cent Eur J Phys. 2012;10:1137�1143.[19] Allen GC, Paul M. Chemical characterization of transition metal spinel-type oxides by infra-

red spectroscopy. Appl Spectrosc. 1994;49:451�458.[20] Raju VSR, Murthy SR, Gao F, Lu Q, Komarneni S. Microwave hydrothermal synthesis of

nanosize PbO added Mg-Cu-Zn ferrites. J Mater Sci. 2006;41:1475�1479.[21] Lutz HD, Haeuseler H. Infrared and Raman spectroscopy in inorganic solids research. J Mol

Struct. 1999;511�512:69�75.[22] Popovic J, Tkalcec E, Grzeta B, Kurajica S, Rakvin B. Inverse spinel structure of Co-doped

gahnite. Am Mineralogist. 2009;94:771�776.[23] Goodenough JB, Loeb AL. Theory of Ionic ordering, crystal distortion, and magnetic

exchange due to covalent forces in spinels. Phys Rev. 1955;98:391�408.[24] Modi KB, Gajera JD, Joshi HH, Pandya MP, Vora HG. Far infrared spectra studies of magne-

sium and aluminium co-substituted lithium ferrites. Pramana J Phys. 2004;62:1173�1180.[25] Chandrasekaran G, Selvanandan S, Manivannane K. Electrical and FTIR studies on Al substi-

tuted Mn�Zn mixed ferrites. J Mater Sci. 2004;15:1475�1479.[26] Josyulu OS, Sobhanadri J. The far-infrared spectra of some mixed cobalt zinc and magnesium

zinc ferrites. Physica Status Solidi A. 1981;65:479�483.[27] Mazen SA, Metawe F, Mansour SF. IR absorption and dielectric properties of Li-Ti ferrites.

J Phys D. 1997;30:1799�1808.[28] Iyer R, Desai R, Upadhyay RV. Low temperature synthesis of nanosized Mn1-xCdxFe2O4 fer-

rites. Indian J Pure Appl Phys. 2009;47:180�185.[29] Decius JC, Malan OG, Thompson HW. The effect of intermolecular forces upon the vibrations

of molecules in the crystalline State. I. The out-of-plane bending of the carbonate ion in arago-nite minerals. Proc Royal Soc A. 1963;275:295�309.

[30] Kashyap SC. Microwave synthesis � a new dimension in processing of materials. Appl Elec-tromagn Conf. 2009;1�4.

[31] Panapoy M, Supattanapalapol S, Ksapabutr B. Preparation and electrical conductivity ofNi/NiO composites using microwave radiation. Paper presented at: 2nd IEEE InternationalNanoelectric Conference; 2008 Mar 24�27; Shanghai.

13

J. Chin. Adv. Mater. Soc., 2014, 1�14

Dow

nloa

ded

by [

Sure

nder

Kum

ar]

at 0

8:01

23

Sept

embe

r 20

14

[32] Joshi UA, Jang JS, Borse PH, Lee JS. Microwave synthesis of single-crystalline perovskiteBiFeO nanocubes for photoelectrode and photocatalytic applications. Appl Phys Lett.2008;92:242106.

[33] Kumar S, Shinde TJ, Vasambekar PN. Microwave synthesis and characterization of Mn-Znferrites. Adv Mat Lett. 2013;4:373�377.

[34] Maxwell JC. Electricity and magnetism. London: Oxford University Press; 1973.[35] Wagner KW. Erklarung der Dielektrischen Nachwirkungsvorgange auf Grund Maxwellscher

Vorstellungen. Arch Elektrotech. 1914;2:371�387.[36] Koops CG. On the dispersion of resistivity and dielectric constant of some semiconductors at

audio frequencies. Phys Rev. 1951;83:121�124.[37] Ajmal M, Maqsood A. AC conductivity, density related and magnetic properties of

Ni1-xZnxFe2O4 ferrites with the variation of zinc concentration. Mater Lett. 2008;62:2077�2080.[38] Haque M, Huq M, Hakim MA. Thermal hysteresis of permeability and transport properties of

Mn substituted Mg�Cu�Zn ferrites. J Phys D. 2008;41:055007.[39] Verwey EJW, DeBoer JH. Cation arrangement in a few oxides with crystal structures of the

spinel type. Recueil des Travaux Chimiques des Pays-Bas. 1936;55:531�540.[40] Batoo KM, Abd El-Sadek MS. Electrical and magnetic transport properties of Ni�Cu�Mg

ferrite nanoparticles prepared by sol�gel method. J Alloys Compounds. 2013;566:112�119.[41] Mazen SA, Mansour SF, Elmosalami TA, Zaki HM. The conduction mechanism of Li�Ga

ferrite. J Alloys Compounds. 2009;472:307�313.[42] Radwan FA, Ahmed MA, Abdelatif G. Screening effect of Ti4C ions on the electrical conduc-

tivity and thermoelectric power of Mg ferrite. J Phys Chem Solids. 2003;64:2465�2477.

14 S. Kumar et al.

J. Chin. Adv. Mater. Soc., 2014, 1�14

Dow

nloa

ded

by [

Sure

nder

Kum

ar]

at 0

8:01

23

Sept

embe

r 20

14