FINAL REPORT

UGC-Minor Research Project

[File No: 47-561/08 (WRO)]

Title: Influence of substitution of Cu and Mn on

magnetic, electric and dielectric properties of

Ni0.6-xRxZn0.4Fe2O4 (R=Cu and Mn and x=0.0-0.6)

Name of the Principal Investigator: Dr. Umesh B. Gawas

Name of the Institution:

DM’s College of Arts,

Sou. Sheela Premanand Vaidya College of Science and

V.N.S. Bandekar College of Commerce,

P.V.S. Kushe Nagar, Assagao- Mapusa, Goa

UNIVERSITY GRANTS COMMISSION

BAHADUR SHAH ZAFAR MARG

NEW DELHI – 110 002

Annual Report of the work done on the Minor Research Project

1. Project report No. 1st/2

nd/3

rd / Final : Final

2. UGC Reference No. : File No. 47-561/08(WRO), dated 15/1/2009

3. Period of report : March 2009 to March 2013

4. Title of research project :

Influence of substitution of Cu and Mn on magnetic, electric and dielectric properties of

Ni0.6-xRxZn0.4Fe2O4 (R=Cu and Mn and x=0.0-0.6)

5. (a) Name of the Principal Investigator : Dr. Umesh B. Gawas

(b) Dept. and University/College where work has progressed: Department of Chemistry,

DM‟s College of Arts, Sou. Sheela Premanand Vaidya College of Science and

V.N.S. Bandekar College of Commerce, P.V.S. Kushe Nagar, Assagao- Mapusa,Goa.

6. Effective date of starting of the project: 1st March 2009

7. Grant approved and expenditure incurred during the period of the report:

a. Total amount sanction : Rs. 1,85,000 /-

b. Total amount released : Rs. 1,45,000 /-

c. Total amount utilised : Rs. 1,44,743 /-

c. Report of the work done : Enclosure

i. Brief objective of the project:

The main objective of the project is in studying the effect of the substitution of Cu and

Mn on magnetic, electric and dielectric properties of Ni0.6-x RxZn0.4Fe2O4 (R= Cu, Mn

and x = 0.0- 0.6 ) and to develop an synthestic strategy which can be used to prepare

ceramic material with optimum properties suitable for the practical applications.

ii. Work done so far and results achieved and publications, if any, resulting from the work

(Give details of the papers and name s of the journals in which it has been published or

accepted for publication) : ---

iii. Has the progress been according to original plan of work and toward achieving the

objective. if not, state reasons : Yes

iv. Please indicate the difficulties, if any, experienced in implementing the

project : --

v. If project has not been completed, please indicate the approximate time by which it is

likely to be completed. A summary of the work done for the period (Annual basis) may

please be sent to the Commission on a separate sheet : Completed

vi. If the project has been completed, please enclose a summary of the findings of the study

Two bound copies of the final report of work done may also be sent to the Commission:

vii. Any other information which would help in evaluation of work done on the project. At

the completion of the project, the first report should indicate the output, such as (a)

Manpower trained (b) Ph. D. awarded (c) Publication of results (d) other impact,

if any : 03 Publication in Conference /symposium

SIGNATURE OF MINOR PROJECT SIGNATURE OF PRINCIPAL

RESEARCHER

(Dr. Umesh B. Gawas) (Dr. D. B. Arolkar)

Project Summary

1. Introduction

Ferrites are ceramic, homogeneous magnetic materials composed of various oxides

with iron oxide as their main constituent. The technology of ferrites and magnetic ceramics

has assumed a new importance during the last several decades especially in the last few years

because of their interesting electrical and magnetic properties which are useful in

applications such as information storage systems, magnetic bulk cores, magnetic fluids,

microwave absorbers, medical diagnostics etc [1]. The most recent reason for upsurge in

ferrite interest has been the development of the new, small, efficient power supplies using

solid state switching called switch mode power supplies (SMPs). These SMP‟s are the

integral components of modern day electronic equipments such as computers, laptops and

entertainment applications. Besides the ease of preparation and low manufacturing cost, the

advantage of spinel ferrites is their high magnetic permeability and high electrical resistivity.

Also these materials can be shaped in a variety of different geometries meant for specific

applications.

Nickel zinc ferrites are characterized by high material resistivity suitable for high

frequency applications from 1MHz to several hundred megahertz‟s. Hence, these ferrites find

use in microwave devices, power transformer, rod antennas, read/write heads for high speed

digital tapes [2]. Use of nickel zinc ferrites is limited due to their low permeability at higher

frequencies and increasing cost. Manganese zinc ferrite on the other hand posses high

permeability and saturation magnetization with nearly zero magneto-crystalline anisotropy

and magneto-restriction. Hence these ferrites find use in transformer cores, noise filters,

recording heads etc [3]. Manganese zinc ferrites also have certain limitations for magnetic

applications at high frequencies, because of their low resistivity and hence high eddy current

losses. For high frequency magnetic applications, ferrite materials with high permeability as

well as high resistivity are more suitable which can reduce eddy current losses. Therefore, an

appropriate combination of these two ferrites can result in the material with enhanced

properties, more suitable for high frequency applications [4]. It is well established fact that,

the magnetic and electrical properties of ferrites are sensitive to the cation distributions,

which in turn depend on the method of synthesis. Hence, there is growing interest in the

newer and newer synthetic strategies to improve on the properties of ferrite materials

Recently surface mounting device (SMD) has been developed rapidly with

development of ceramic electronics and information technology. As one of the most

important SMD, Multiplier Chip Indicator (MLCI) made from soft ferrite become more and

more miniaturized and integrated and this required that the soft ferrites be co-fired with

internal contact material layer by layer considering the conductivity and cost. Pure silver is

the most suitable contact material. Chip inductors are one of the passive surface mount

devices, which are important components for the latest electronic product such as cellular

phones, video cameras, notebook computers, hard and floppy drives etc. which require small

dimension, lightweight and better functions [5]. Nickel copper zinc ferrites are well

established magnetic materials for multilayer chip inductor applications because of their

relatively low sintering temperature, high permeability in the RF region and high electrical

resistivity [6,7]. For multilayer chip inductor application, the ferrite needs to be sintered at <

950oC in order to bond with the internal silver electrode during the manufacturing of

multilayer chip inductor applications. To decrease the sintering temperature fine ferrite

powder with the non-stoichiometry ratio of Ni-Cu-Zn in the starting chemical composition

must be used. It is known that the magnetic properties of spinel ferrites are strongly

dependent on the microstructures. Small amount of additives are often used for

microstructure refinement. The rare earth oxides are becoming the more promising additives

for the improvement of the ferrites properties.

It is well established fact that, the magnetic and electrical properties of ferrites are

sensitive to the cation distributions, which in turn depend on the method of synthesis. Hence,

there is growing interest in the newer and newer synthetic strategies to improve on the

properties of ferrite materials.

2. Experimental details: Synthesis of Ni0.6-xRxZn0.4Fe2O4 [(R=Cu, Mn) (x = 0.0-0.6)]

2.1. Combustion synthesis: Principle

The combustion process is an exothermic reaction between an oxidizer and a fuel.

When the heat evolved is more than the heat requires for the reaction, the system becomes

self-sustained. Also, the exothermicity of such reactions takes the system to a high

temperature. Hence, this process, popularly known as self- propogating high temperature

synthesis (SHS) is also called furnaceless or fire synthesis. The interesting feature of this

process is that the sample once ignited continues to burn to consume itself. In any combustion

process, the reactant mixture (fuel and oxidiser) can be hypergolic (ignite by contact) or is

ignited in a controlled way by an external source. The residue or the ash that emerges after

complete combustion is the oxide material. Of late, this ash has been recognized to be of

great technological interest. By use of the combustion method, a number of useful oxide

materials for various applications such as refractory, magnetic, dielectric, semiconducting,

insulators, catalysts, sensor, phosphor etc. have been synthesized. Recently, attempts have

been made to modify SHS so as to eliminate and overcome some of the ensuring problems by

using various innovative synthetic strategies, with a similar view, carrying out the reaction in

solution form has made a different approach to SHS. This approach called the solution

combustion method and it uses a solution of the redox mixture.

The combustion process by the mixture method is carried out as follows:

Redox mixture: The redox mixture is made up by mixing a stoichiometric amount of the

metal nitrate or perchlorate with urea or hydrazide derivative (fuel) and igniting at

temperature between 300°C and 350°C. The mass left after complete combustion is the oxide

material. This method uses the experiences of propellant chemistry in making the redox

mixture. The stoichiometry or the equivalence ratio ( ɸe , O/F ) at which the total combustion

reaction takes place is very important and crucial. Combustion may not take place at all, if the

stoichiometry is not maintained. The calculation of the equivalence ratio is based on

balancing the oxidising (O) and reducing valency (F) of the reactants. The energy released by

the combustion of the redox mixtures will be maximum when the equivalence ratio (ɸe, O/F)

is unity (O is the total oxidising and F is reducing valency of the components.) In propellant

chemistry, the elements C, H and metal ions are considered as reducing species. E.g., C= +4,

H = +1, M+2

= +2 , M+3

= +3 , M+4

= +4 , etc. O is considered as oxidiser with a valency of -2

and N is considered to have zero valency.

2.2. Materials

All reagents used were of analytical grade and are used without further purification.

2.3. Experimental

The hexamine to nitrate ratio was calculated by using the oxidizing and reducing

valences of the metal nitrates and hexamine. Stoichiometirc quantities of metal nitrate (Mn,

Ni, Cu, Zn, Fe) were melted by heating on the hot plate. To the hot melted stoichiometric

quantity of finely powdered hexamine was added and stirred till the slurry was obtained. The

slurry was kept in preheated furnace maintained at 300oC for 20 minutes. The hexamine

nitrite mixtures froths and finally ignites leaving brown coloured residue of the mixed metal

oxides and the residue was found to be magnetic.

9[R(NO3)2 + Ni(NO3)2 + Zn(NO3)2 ]+ 18 Fe(NO3)3 + 10(CH2)6N4

9R-Ni-Zn-Fe2O4 (s) + 60CO2 (g) + 56N2 (g) + 60H2O(g) [R=Mn,Cu]

3. Characterization of Ni0.6-xMnxZn0.4Fe2O4 (x=0.0-0.6) ferrites

3.1. XRD pattern of Ni0.6-xMnxZn0.4Fe2O4 (x=0.0-0.6) ferrites

The XRD patterns of all the Ni0.6-xMnxZn0.4Fe2O4 (x = 0.0, 0.1, 0.2, 0.3, 0.4, 0.5, 0.6)

ferrites are presented in Fig. 2.1a. The X-ray diffractograms displays all the peaks

characteristics of the cubic spinel ferrites which confirm the formation of these ferrite

samples. The broadening of XRD peaks is indicative of the ultrafine (nanocrystalline) nature

of all Ni0.6-xMnxZn0.4Fe2O4 (x = 0.0-0.6) ferrites

Fig.1. (a) XRD pattern of Ni0.6-xMnxZn0.4Fe2O4 (x=0.0-0.6) ferrites, (b) Variation of lattice

parameter with composition of Ni0.6-xMnxZn0.4Fe2O4 (x=0.0-0.6) ferrites

The interplanar distance for each diffraction „hkl‟ planes were calculated using

Bragg‟s equation. The observed and calculated values of interplanar distances show good

agreement. The lattice parameter „a‟ was calculated for each plane from interplanar distance.

The Fig. 1b represents the variation of lattice parameter „a‟ with Mn substitution „x‟. It was

observed that the lattice parameter increases linearly with increasing Mn substitution in

accordance with the Vegard‟s law [8]. This behavior has been attributed to the replacement

of smaller Ni2+

ions (0.70 A

o) by larger Mn

2+ ions (0.81 A

o) in the crystal lattice. Thus, the

introduction of Mn2+

ions in lattice causes the expansion of unit cell while preserving the

overall cubic symmetry. The lattice parameters of ferrite samples calculated from their XRD

patterns were found to be in the range 8.3821 Ao to 8.4632 A

o which are in agreement with

the reported values for Ni0.6-xMnxZn0.4Fe2O4 (x = 0.0-0.6) ferrites system [9]. The small

impurity peaks observed in the diffractograms are assigned to the α-Fe2O3 secondary phase.

The X-ray density decreases with increasing Mn substitution. This density behaviour is

attributed to the replacement of heavier NiO (6.72 g / cc) by the lighter MnO (5.37 g / cc) in

the spinel lattice. The average crystallite size was calculated from most intense XRD peaks

using Debye-Scherrer formula. The average crystallite size was observed in the range 22 nm

to 33 nm suggesting the nanocrystalline nature of these ferrites.

3.2. FTIR spectra of Ni0.6-xMnxZn0.4Fe2O4 (x=0.0-0.6) ferrites

The infrared spectroscopy is a very important technique to derive information about

the positions of ions in the crystal lattice through the crystal‟s vibration modes. The IR bands

in the region 700 cm-1

to 300 cm-1

are assigned to the fundamental vibrations of the ions of

the crystal lattice. The FTIR spectra of all Ni0.6-xMnxZn0.4Fe2O4 (x = 0.0-0.6) ferrites were

represented in the Fig. 2. All the ferrite samples display two principal absorption bands in the

frequency region from 4000 cm-1

to 400 cm-1

. The high frequency band (ν1) in the region

593 cm-1

to 563 cm-1

results from stretching vibration of the tetrahedral Fe3+

--O2-

bond, while

low frequency band (ν2) in the region 420 cm-1

to 400 cm-1

arises due to Fe3+

--O2-

stretching

vibration in octahedral sites [10]. The difference in the positions and intensities of ν1 and ν2

band are due to the different Fe3+

--O2−

distances for the tetrahedral and octahedral sites, since

the vibrational frequencies depend on cation mass, Mn+

--O2-

distance and the bonding force

[11].

Fig.2. FTIR spectra of Ni0.6-xMnxZn0.4Fe2O4 (x=0.0-0.6) ferrites

3.3. SEM of Ni0.6-xMnxZn0.4Fe2O4 (x=0.0-0.6) ferrites

The SEM was used to investigate into the size and shape and to confirm the

nanocrystalline nature of the Ni0.6-xMnxZn0.4Fe2O4 (x = 0.0-0.6) ferrites. The Fig. 3 represents

the SEM micrographs of the samples of above mentioned ferrite compositions. The ferrite

nanoparticles were polydispersed. These nanoparticles display low tendency towards

agglomeration and hence occur as loose agglomerates. The crystallite size calculated using

Scherrer method from XRD measurements was found to be in the range 22-33 nm.

Fig.3. SEM of Ni0.6-xMnxZn0.4Fe2O4 (x=0.0-0.6) ferrites

4. Studies on solid state properties of Ni0.6-xMnxZn0.4Fe2O4 (x=0.0-0.6) ferrites

4.1. DC resistivity measurements

Spinel ferrites are known to exhibit semiconducting behaviour, though the mechanism

of conduction is different. The mechanism of electrical conductivity in ferrites involves

hopping of electrons between cations of same metal present in different oxidation states as

explained by the Verwey model [12]. According to this model, in close-packed lattice formed

by oxygen (anions), the metal ions occupy tetrahedral (A) sites and the octahedral [B] sites.

The cations at these A and B sites can be treated as isolated from each other. The electron

hopping at between two tetrahedral sites (A-A) does not take place since distance between

two tetrahedral sites is larger than the distance between two octahedral sites [B-B], hence the

hopping between the Fe2+

and Fe3+

ions occupying the octahedral [B] sites is primararily

responsible for conduction [13]. Besides electron hopping, other factors such as particle size,

grain boundaries, nature and concentration of other substituents present are known to affect

the conductivities of ferrites [14].

Fig.4. Plot of log resistivity against 103 / T of Ni0.6-xMnxZn0.4Fe2 O4 (x = 0.0-0.6) ferrites

In case of nanocrystalline ferrite materials, their resistivity was found to be affected

by moisture content which results from their high porosity and low green density [15]. The

temperature dependence of the dc resistivity (log ρ) for the Ni0.6-xMnxZn0.4Fe2 O4 (x = 0.0-0.6)

ferrite nanoparticles is shown in Fig.4. The plot displays two distinct regions of conductivity.

In the first region from room temperature to 393 K to 403 K resistivities of the order of 105

Ωcm to 107

Ωcm were observed depending upon the composition of the nanosize ferrites.

With increase in temperature in this region, the resistivity increases and reaches maximum in

the temperature range 373 K to 388 K. This behaviour is attributed to the presence of open

porosity, loose agglomeration and entrapped moisture inside the pores of the powders [16].

The heating from room temperature upto ~383K causes total evaporation of moisture from

the samples and therefore, maximum resistivities (ρ = 108 Ωcm to 10

9 Ωcm) were observed in

the temperature region 373 K to 388 K. The low resistivity at room temperature is resultant of

protonic conductivity due to entrapped moisture [17]. In the second region above 393 K, the

samples exhibits typical negative temperature coefficient of resistance (NTCR) behaviour of

ferrites [18] and linear plots were obtained.

4.2. AC susceptibility studies of Ni0.6-xMnxZn0.4Fe2O4 (x=0.0-0.6) ferrites

The magnetic properties of materials are determined by the types of particles which

includes, single domain (SD), multidomain (MD) and superparamagnetic (SP) particles. The

ac susceptibility measurements can be used to find out the types of particles responsible for

magnetic properties. The variation of normalized ac susceptibility against temperature of

Ni0.6-xMnxZn0.4Fe2O4 (x = 0.0-0.6) ferrites is shown in Fig. 5a.

Fig.5. (a) Plot of normalised ac susceptibility against temperature of Ni0.6-xMnxZn0.4Fe2O4

(x=0.0-0.6) ferrites, (b) Variation of Curie temperature with composition of Ni0.6-

xMnxZn0.4Fe2O4 (x=0.0-0.6) ferrites

These graphs show normal ferrimagnetic behaviour and the susceptibility suddenly

drops to zero at certain temperature, this temperature is called Curie temperature (Tc). The

nature of plots indicates that, the sample contains clusters of both single domain and

superparamagnetic particles. The variation of Curie temperature with composition of the

ferrite series under study is shown in the Fig.5b which can be correlated with the Mn2+

ions

concentration and A-B interactions. The decrease in Curie temperature with increasing

temperature suggests decrease of A-B interactions.

4.3. Dielectric studies of Ni0.6-xMnxZn0.4Fe2O4 (x=0.0-0.6) ferrites

The dielectric properties of Ni0.6-xMnxZn0.4Fe2O4 (x = 0.0-0.6) ferrites were studied in

a frequency range from 100 Hz to 10 MHz at room temperature.

The frequency variation of dielectric constant of Ni0.6-xMnxZn0.4Fe2O4 (x = 0.0-0.6)

ferrites at room temperature in the frequency range of 100 Hz to 10 MHz is presented in

Fig.6a. The dielectric constant (ε‟) shows sharp decrease upto 1 kHz, followed by a gradual

decrease from 1 kHz to 10 kHz, and is nearly independent of frequency from 10 kHz to 10

MHz.

Fig.6. (a) Frequency variation of dielectric constant of Ni0.6-xMnxZn0.4Fe2O4 (x = 0.0-0.6)

ferrites at room temperature, (b) Frequency variation of dielectric loss of Ni0.6-

xMnxZn0.4Fe2O4 (x = 0.0-0.6) ferrites

The decrease in dielectric constant with increasing frequency is a normal behaviour

observed in most of the ferromagnetic materials. The dielectric constant of any material, in

general, is due to dipolar, electronic, ionic and interfacial polarizations [19]. In a lower

frequency region, surface polarization contributes predominantly than electronic or ionic

polarization in determining the dielectric properties of ferrite materials [20]. The dispersion

in dielectric constant observed in lower frequency region is due to Maxwell-Wagner

interfacial type of polarization [21] which is well in agreement with Koop‟s

phenomenological theory of dielectrics [22]. According to this model, the dielectric structure

is assumed to be composed of two layers; the first layer is composed of well conducting

grains separated by thin layer which is composed of relatively poor conducting grain

boundaries. This creates inhomogeniety in the dielectric material which results in local

accumulation of charge under the influence of an electric field. The electrons reach the grain

boundary through hopping and if the grain boundary resistance is high enough, the electrons

pile up at the grain boundaries and produce polarization. However, as the frequency of the

applied field is increased, the electrons reverse their direction of motion more often. This

decreases the probability of electrons reaching the grain boundary and as a result the

polarization decreases. Therefore, the dielectric constant decreases with increasing frequency

of the applied field. The dielectric constant values are quite low and are in the range of 30 to

450 at room temperature. These low dielectric constant values are attributed to homogeneity,

better symmetry and small grain size [23]. Small grains have large surface boundaries which

act as scattering centres for the flow of electrons thus reducing the interfacial polarization

[24]. The variation of dielectric loss (tan δ) with frequency at room temperature is depicted in

the Fig. 6b. It was observed that dielectric loss decreases initially with frequency. The

dielectric loss in ferrite materials depend on a number of factors such as stoichiometry, Fe2+

concentration and structural homogeneity which in turn depend on the composition and

method of preparation. The dielectric loss gives the loss of energy from the applied field into

the sample. This is caused by domain wall resonance. At higher frequencies, the losses are

found to be low, since domain wall motion is inhibited and magnetization is forced to change

rotation. The initial decreased can be understood from Koop‟s phenomenological model [22].

The dielectric loss of Ni0.6-xMnxZn0.4Fe2O4 (x = 0.0-0.6) ferrites was found to very low in the

higher frequency region.

4.4. Summary

The work incorporated in project involves synthesis, characterization and studies on

the electrical, magnetic and dielectric properties Ni0.6-xMnxZn0.4Fe2O4 (x = 0.0-0.6) ferrites.

The ferrite materials were synthesized using combustion technique involving hexamine and

metal nitrate mixture. The single phase formation of Ni0.6-xMnxZn0.4Fe2O4 (x = 0.0-0.6)

ferrites was confirmed by the XRD measurements wherein all the XRD peaks characteristics

of cubic spinel ferrite were observed. The small impurity peaks observed in the

diffractograms are assigned to the α-Fe2O3 secondary phase. The lattice parameters of all

ferrites were found to increase gradually with increasing Mn substitution. The peak

broadening observed for these ferrites indicates their nanocrystalline nature. The FTIR

spectra of ferrites display two absorption bands which are chracteristics of M---O stretching

in tetrahedral and octahedral sites in the spinel lattice. The SEM observations shows that the

consists of loose agglomerates of primary particles.

The Mn substitution has sigificant influence on the electromagnetic properties such as

dc resistivity, dielectric constant, dielectric loss tangent etc. The lower dc resistivity values in

the range 106 Ωcm to 10

7 Ωcm observed for Ni0.6-xMnxZn0.4Fe2O4 (x = 0.0-0.6) ferrites at

room temperature suggest the protonic conductivity due to moisture trapped inside the pores.

The ac susceptibility studies reveal the presence of clusters of both superparamagnetic and

single domain particles in ferrites. The Curie temperature was found to decrease with

increasing Mn concentration which is due to the decrease A-B sublattice interaction. The

decrease A-B sublattice interaction suggests the transfer of Fe3+

from A-site to B-site with

increasing Mn content. The dielectric constant values are quite low and are in the range of 30

to 450 at room temperature. These low dielectric constant values are attributed to

homogeneity, better symmetry and small grain size. Small grains have large surface

boundaries which act as scattering centres for the flow of electrons thus reducing the

interfacial polarization. The dielectric loss of ferrites was found to very low in the higher

frequency region.

4.5. Conclusions

The present investigation was focused on synthesis, characterization and solid state

properties of Ni0.6-xMnxZn0.4Fe2O4 (x = 0.0-0.6) ferrites. The significant findings of this

investigation are as follows:

1. Nanocrystalline Ni0.6-xMnxZn0.4Fe2O4 (x = 0.0-0.6) ferrites were successfully synthesized

using combustion technique involving hexamine and metal nitrate mixture. The

temperature and time of preparation were reduced as compared to the conventional solid

state process.

2. The XRD studies indicate formation of cubic spinel ferrites with lattice parameters in the

range 8.3821 Ao to 8.4632 A

o.

3. The FTIR spectra displays two principal absorption bands in the region 593 cm-1

to 563

cm-1

and 420 cm-1

to 400 cm-1

which arises due to Fe3+

--O2-

stretching vibration in

tetrahedral and octahedral sites respectively.

4. The low values dc resistivity observed at room temperature are attributed to the protonic

conductivities due to entrapped moisture in the porous structure of the ferrites. The

resistivity decreases with increasing Mn substitution at higher temperatures.

5. The Curie temperature was found decrease with increasing Mn substitution indicating the

decrease in the A-B sublattice interactions.

6. The dielectric constant and dielectric loss tangent values are appreciably lower than those

reported for samples prepared by solid state processes.

5. Characterization of Ni0.6-xCuxZn0.4Fe2O4 (x=0.0-0.6) ferrites

5.1. XRD pattern of Ni0.6-xCuxZn0.4Fe2O4 (x=0.0-0.6) ferrites

The XRD patterns of all the Ni0.6-xCuxZn0.4Fe2O4 (x = 0.0, 0.1, 0.2, 0.3, 0.4, 0.5, 0.6)

ferrites are presented in Fig.7a. The X-ray diffractograms displays all the peaks

characteristics of the cubic spinel ferrites with no detectable secondary phases which confirm

the formation and purity of these ferrite samples. This reveals that the Cu substituted Ni-Zn

ferrites can be directly synthesized from the auto-combustion of hexamine-nitrate mixture.

Fig.7 (a) XRD pattern of Ni0.6-xCuxZn0.4Fe2O4 (x=0.0-0.6) ferrites, (b) Variation of lattice

parameter and density with composition of Ni0.6-xCuxZn0.4Fe2O4 (x = 0.0-0.6) ferrites

The XRD peaks show broadening which is indicative of the ultrafine

(nanocrystalline) nature of all Ni0.6-xCuxZn0.4Fe2O4 (x = 0.0-0.6) ferrites. The interplanar

distances for each diffraction „hkl‟ planes were calculated using Bragg‟s equation. The

observed and calculated values of interplanar distances show good agreement. The lattice

parameter „a‟ was calculated for each plane from interplanar distance. The Fig. 7b represents

the variation of lattice parameter „a‟ with Cu substitution „x‟. It was observed that the lattice

parameter increases linearly with increasing Cu substitution in accordance with the Vegard‟s

law [8]. This behavior has been attributed to the replacement of smaller Ni2+

ions (0.70 A

o)

by larger Cu2+

ions (0.73 Ao) in the crystal lattice. Thus, the introduction of Cu

2+ ions in

lattice causes the expansion of unit cell while preserving the overall cubic symmetry. The

lattice parameters of ferrite samples calculated from their XRD patterns were found to be in

the range 8.3821 Ao to 8.4046 A

o. The X-ray density increases with increasing Cu

substitution. This density behaviour is attributed to the replacement of lighter NiO by the

heavier CuO in the spinel lattice. The average crystallite size was calculated from most

intense XRD peaks using Debye-Scherrer formula . The average crystallite size was observed

in the range 22 nm to 30 nm suggesting the nanocrystalline nature of these ferrites.

5.2. FTIR spectra of Ni0.6-xCuxZn0.4Fe2O4 (x=0.0-0.6) ferrites

The infrared spectroscopy is a very important technique to derive information about

the positions of ions in the crystal lattice through the crystal‟s vibration modes. The IR bands

in the region 700 cm-1

to 300 cm-1

are assigned to the fundamental vibrations of the ions of

the crystal lattice. The FTIR spectra of all Ni0.6-xCuxZn0.4Fe2O4 (x = 0.0-0.6) ferrites were

represented in the Fig. 8. All the ferrite samples display two principal absorption bands in the

frequency region from 4000 cm-1

to 400 cm-1

. The high frequency band (ν1) in the region

593 cm-1

to 563 cm-1

results from stretching vibration of the tetrahedral Fe3+

--O2-

bond, while

low frequency band (ν2) in the region 420 cm-1

to 400 cm-1

arises due to Fe3+

--O2-

stretching

vibration in octahedral sites [10]. The difference in the positions and intensities of ν1 and ν2

band are due to the different Fe3+

--O2−

distances for the tetrahedral and octahedral sites, since

the vibrational frequencies depend on cation mass, Mn+

--O2-

distance and the bonding force

[11].

Fig.8. FTIR spectra of Ni0.6-xCuxZn0.4Fe2O4 (x=0.0-0.6) ferrites

5.3. SEM of Ni0.6-xCuxZn0.4Fe2O4 (x=0.0-0.6) ferrites

The SEM was used to investigate into the size and shape and to confirm the

nanocrystalline nature of the Ni0.6-xCuxZn0.4Fe2O4 (x = 0.0-0.6) ferrites. The Fig. 9 represents

the SEM micrographs of the samples of above mentioned ferrite compositions. The ferrite

nanoparticles were polydispersed. These nanoparticles display low tendency towards

agglomeration and hence occur as loose agglomerates. The crystallite size calculated using

Scherrer method from XRD measurements was found to be in the range 22-28 nm.

Fig.9. SEM of Ni0.6-xCuxZn0.4Fe2O4 (x=0.0-0.6) ferrites

6. Studies on solid state properties of Ni0.6-xCuxZn0.4Fe2O4 (x=0.0-0.6) ferrites

6.1. Dc resistivity measurements

Spinel ferrites are known to exhibit semiconducting behaviour, though the mechanism

of conduction is different. The mechanism of electrical conductivity in ferrites involves

hopping of electrons between cations of same metal present in different oxidation states as

explained by the Verwey model [12]. According to this model, in close-packed lattice formed

by oxygen (anions), the metal ions occupy tetrahedral (A) sites and the octahedral [B] sites.

The cations at these A and B sites can be treated as isolated from each other. The electron

hopping at between two tetrahedral sites (A-A) does not take place since distance between

two tetrahedral sites is larger than the distance between two octahedral sites [B-B], hence the

hopping between the Fe2+

and Fe3+

ions occupying the octahedral [B] sites is primararily

responsible for conduction [13]. Besides electron hopping, other factors such as particle size,

grain boundaries, nature and concentration of other substituents present are known to affect

the conductivities of ferrites [14]. In case of nanocrystalline ferrite materials, their resistivity

was found to be affected by moisture content which results from their high porosity and low

green density [15]. The variation of DC resistivity ( log ) versus 103/ temperature shown in

Fig.10. Linear decrease in the resistivity of ferrites with temperature shows their

semiconducting nature. The resistivity of ferrites is known to depend upon the purity starting

materials, sintering temperature and sintering time, which influence the microstructure and

composition of the samples. The plot displays two distinct regions of conductivity. In the first

region from room temperature to 393 K to 403 K resistivities of the order of 105 Ωcm to 10

7

Ωcm were observed depending upon the composition of the nanosize ferrites. With increase

in temperature in this region, the resistivity increases and reaches maximum in the

temperature range 373 K to 388 K. This behaviour is attributed to the presence of open

porosity, loose agglomeration and entrapped moisture inside the pores of the powders [16].

The heating from room temperature upto ~383K causes total evaporation of moisture from

the samples and therefore, maximum resistivities (ρ = 108 Ωcm to 10

9 Ωcm) were observed in

the temperature region 373 K to 388 K. The low resistivity at room temperature is resultant of

protonic conductivity due to entrapped moisture [17]. In the second region above 393 K

(Fig.10), the samples exhibits typical negative temperature coefficient of resistance (NTCR)

behaviour of ferrites [18] and linear plots were obtained. The variation of room-temperature

resistivity with Cu content indicates that resistivity decreases with increase in Cu content

(except x=0.4). The decrease in resistivity is attributed to the increase in grain size. Smaller

grains also imply smaller grain to grain contacts, which reduces the electron flow. As the

crystallite size increases (with increase in Cu content) the resistivity is found to decrease.

Fig.10. Plot of log resistivity against 103 / T of Ni0.6-xCuxZn0.4Fe2 O4 (x = 0.0-0.6) ferrites

6.2. AC susceptibility studies of Ni0.6-xCuxZn0.4Fe2O4 (x=0.0-0.6) ferrites

The magnetic properties of materials are determined by the types of particles which

includes, single domain (SD), multidomain (MD) and superparamagnetic (SP) particles. The

ac susceptibility measurements can be used to find out the types of particles responsible for

magnetic properties. The variation of normalized ac susceptibility against temperature of

Ni0.6-xCuxZn0.4Fe2O4 (x = 0.0-0.6) ferrites is shown in Fig. 11a. These graphs show normal

ferrimagnetic behaviour and the susceptibility suddenly drops to zero at certain temperature,

this temperature is called Curie temperature (Tc). The nature of plots indicates that, the

sample contains clusters of both single domain and superparamagnetic particles. The

variation of Curie temperature with composition of the ferrite series under study is shown in

the Fig. 11b which can be correlated with the Cu2+

ions concentration and A-B interactions.

The decrease in Curie temperature with increasing temperature suggests decrease of A-B

interactions.

Fig. 11a. Plot of normalized ac susceptibility plot of Ni0.6-xCuxZn0.4Fe2O4 (x=0.0-0.6) ferrites

(b) Variation of Curie temperature with composition of Ni0.6-xCuxZn0.4Fe2O4 (x = 0.0-0.6)

6.3. Dielectric studies of Ni0.6-xCuxZn0.4Fe2O4 (x=0.0-0.6) ferrites

The dielectric properties of Ni0.6-xCuxZn0.4Fe2O4 (x = 0.0-0.6) ferrites were studied in

a frequency range from 100 Hz to 10 MHz at room temperature.

The frequency variation of dielectric constant of Ni0.6-xCuxZn0.4Fe2O4 (x = 0.0-0.6)

ferrites at room temperature in the frequency range of 100 Hz to 10 MHz is presented in

Fig.12a. The dielectric constant (ε‟) shows sharp decrease upto 1 kHz, followed by a gradual

decrease from 1 kHz to 10 kHz, and is nearly independent of frequency from 10 kHz to 10

MHz. The dielectric constant of any material, in general, is due to dipolar, electronic, ionic

and interfacial polarizations [19]. In a lower frequency region, surface polarization

contributes predominantly than electronic or ionic polarization in determining the dielectric

properties of ferrite materials [20]. The decrease in dielectric constant with increasing

frequency is a normal behaviour observed in most of the ferromagnetic materials. The high

value of dielectric constant observed at lower frequencies is explained on the basis of space

charge polarization due to in homogeneous dielectric structure. The inhomogeneities in

ferrites are impurities, porosity and grain size [21]. Also, the polarization in the ferrites is

through a mechanism similar to the conduction process. The presence of Fe3+

and Fe2+

ions

render ferrite materials dipolar. The rotational displacement of dipoles results in orientational

polarization. In ferrites, rotation of Fe2+

to Fe3+

can be visualized as the exchange of electrons

between two ions, so that the dipoles align themselves in response to alternating electric field.

The polarization at lower frequencies may result from electron hopping between Fe3+

and

Fe2+

ions in ferrite lattice. The polarization decreases with increase in frequency and reaches

a constant value due to the fact that beyond a certain frequency of external field the electron

exchange Fe3+

and Fe2+

cannot follow the alternating field. Also, the presence of Ni3+

/ Ni2+

ions, which gives rise to p-type carriers, contributes to net polarization, though it is small.

The net polarization increases initially and then decreases with decrease in frequency [22].

Fig.12(a). Frequency variation of dielectric constant of Ni0.6-xCuxZn0.4Fe2O4 (x = 0.0-0.6)

ferrites at room temperature (b) Frequency variation of dielectric loss at room temperature of

Ni0.6-xCuxZn0.4Fe2O4 (x = 0.0-0.6) ferrites

The dielectric constant values in the higher frequency region are quite low and are in

the range of 14 to 54 at room temperature. These low dielectric constant values are attributed

to homogeneity, better symmetry and small grain size [23]. The mechanism of conduction in

polycrystalline ferrites is mainly reported to be hopping of electrons between ions of the same

element having different oxidation states. As these ferrites are not sintered, the probability of

ion existing in different valance states is rather low, reducing the possibility of electron

hopping and hence, the polarization which results in low dielectric constant. It is also affected

by stoichiometry, density, grain size and homogeneity of the ferrites [24]. As Cu2+

ions are

substituted for Ni2+

ions, the change in structural homogeneity results in the increase of

polarization which results in the increase of dielectric constant. The variation of dielectric

loss (tan δ) with frequency at room temperature is depicted in the Fig. 12b. It was observed

that dielectric loss decreases initially with frequency. The dielectric loss in ferrite materials

depend on a number of factors such as stoichiometry, Fe2+

concentration and structural

homogeneity which in turn depend on the composition and method of preparation. The

dielectric loss gives the loss of energy from the applied field into the sample. This is caused

by domain wall resonance. At higher frequencies, the losses are found to be low, since

domain wall motion is inhibited and magnetization is forced to change rotation. The initial

decreased can be understood from Koop‟s phenomenological model [22]. The dielectric loss

of Ni0.6-xCuxZn0.4Fe2O4 (x = 0.0-0.6) ferrites was found to very low in the higher frequency

region.

6.4. Summary

The experimental studies reported in this project involves synthesis, characterization

and studies on the electrical, magnetic and dielectric properties Ni0.6-xCuxZn0.4Fe2O4 (x = 0.0-

0.6) ferrites. The Cu-substituted Ni-Zn ferrites were synthesized using combustion technique

involving hexamine and metal nitrate mixture. The single phase formation of Ni0.6-

xCuxZn0.4Fe2O4 (x = 0.0-0.6) ferrites was confirmed by the XRD measurements wherein all

the XRD peaks characteristics of cubic spinel ferrite were observed. The lattice parameters

shows gradual increase with increasing Cu concentration. The peak broadening of XRD

peaks is indicative of their nanocrystalline nature. The FTIR spectra of ferrites display two

absorption bands which are chracteristics of M---O stretching in tetrahedral and octahedral

sites in the spinel lattice. The SEM observations shows that the particle occurs as loose

agglomerates of primary particles.

The substitution of Cu has sigificant influence on the electromagnetic properties of

Ni-Zn ferrites. The lower dc resistivity values in the range 106 Ωcm to 10

7 Ωcm observed for

Ni0.6-xCuxZn0.4Fe2O4 (x=0.0-0.6) ferrites at room temperature suggest the protonic

conductivity due to moisture trapped inside the pores. The ac susceptibility studies reveal the

presence of clusters of both superparamagnetic and single domain particles in ferrites. The

Curie temperature was found to decrease with increasing Cu concentration which is due to

the decrease A-B sublattice interaction. The decrease A-B sublattice interaction suggests the

transfer of Fe3+

from A-site to B-site with increasing Cu content. The dielectric constant

values are quite low and are in the range of 14 to 54 at room temperature. These low

dielectric constant values are attributed to homogeneity, better symmetry and small grain

size. Small grains have large surface boundaries which act as scattering centres for the flow

of electrons thus reducing the interfacial polarization. The dielectric loss of all ferrite samples

was found to very low in the higher frequency region.

6.5. Conclusions

The present investigation was focused on synthesis, characterization and solid state

properties of Ni0.6-xCuxZn0.4Fe2O4 (x=0.0-0.6) ferrites. The significant findings of this

investigation are as follows:

1. Nanocrystalline Ni0.6-xCuxZn0.4Fe2O4 (x = 0.0-0.6) ferrites were successfully prepared

using combustion technique involving hexamine and metal nitrate mixture. The

temperature and time of preparation were reduced as compared to the conventional solid

state process.

2. The XRD studies indicate formation of cubic spinel ferrites with lattice parameters in the

range 8.3821 Ao to 8.4046 A

o.

3. The FTIR spectra displays two principal absorption bands in the region 593 cm-1

to 563

cm-1

and 420 cm-1

to 400 cm-1

which arises due to Fe3+

--O2-

stretching vibration in

tetrahedral and octahedral sites respectively.

4. The low values dc resistivity observed at room temperature are attributed to the protonic

conductivities due to entrapped moisture in the porous structure of the ferrites. The

resistivity decreases with increasing Cu substitution at higher temperatures.

5. The Curie temperature was found decrease with increasing Cu substitution indicating the

decrease in the A-B sublattice interactions.

6. The dielectric constant and dielectric loss tangent values are appreciably lower than those

reported for samples prepared by solid state processes.

Overall summary

Effect of

substitution

of R on Ni-

Zn ferrite

(R=Cu,Mn)

Parameter Cu Mn

Lattice constant increases increases

X-ray density increases decreases

DC resistivity decreases decreases

Curie temperature decreases decreases

Dielectric constant decreases (except x=0.4) decreases

Dielectric loss decreases decreases

Finally, from the overall results observed in the present study it can be concluded that

with combustion technique involving hexamine and metal nitrate mixture, it is possible to

prepare the nanocrystalline ferrites at relatively lower temperature and in much shorter time

duration than that require in the conventional solid state technique.

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Publications in Conferences/symposium

1. Vasudev Gawade, Saju Konadkar, Pooja Halarnkar, Mandali Borkar, Aarti

Chaudhary and U.B. Gawas, “Effect of substitution of Cu and Mn on structural and

solid state properties of Ni-Zn ferrite nanoparticles” in 1-day symposium, organized

by Department of Chemistry, Goa-University on 15th

March 2014. (Oral presentation)

2. M.M. Kothawale, R.M. Pednekar and U.B. Gawas „Structural, magnetic and dielectric

characteristics of nano crystalline Mn-Ni-Zn ferrites synthesized by combustion route‟ in 1-

day National Conference on Emerging Trend in Chemistry and Material Science,

organized by Department of chemistry, KLS Gogte institute of Technology, Belgaum,

Karnataka, 13th

Oct. 2014. (Oral presentation)

3. U.B. Gawas, S.G. Gawas and V.M.S. Verenkar, „Effect of Cu-substitution on structural,

dielectric and magnetic properties of Ni-Zn ferrite nanoparticles‟ in 1-day National

Conference on Emerging Trend in Chemistry and Material Science, organized by

Department of Chemistry, KLS Gogte institute of Technology, Belgaum, Karnataka,

13th

Oct. 2014. (Oral presentation)