- Synthesis and characterization of Nio.s,Zn,CuosFe204 mixed

spinel ferrite system

2.1 INTRODUCTION

Femtes are candidate substances which find wide applications in microwave

devices, computer memories and magnetic recording. They offer low eddy currents

and dielectric looses for the propagation of electromagnetic waves within the range

of frequencies up to few GHz. In view of their importance several researchers have

studied the electric and magnetic properties of ferrite materials in the past few years.

Smit and Wijn [l] have systematically and elaborately studied the electric and

magnetic properties of the femtes NiFe204 and CuFe204. They have reported that

the saturation magnetization value of NiFezO4 is 300 Gauss at O'K and its curie

temperature is 585'~. For the femte CuFe204 the values are 160 Gauss and 455 '~

respectively. Similarly h o t and Goldman [2] and Hartmann-Boutron and lmbert

[3] have studied ZnFe204 magnetically abd have found out ZnFe204 is

antiferromagnetic below 10%. Many attempts have been made to lower the curie

temperature of NiFe204 and CuFe204 and for increasing the transition point of

ZnFe204 thereby suitably tailoring the saturation magnetization, resitivity,

chemical stability and mechanical hardness. Such attempts have been made possible

by mixing the end membex ferrites namely NiFe204 and CuFezO4[4] NiFez04 and

znFe204 [5-21land ZnFe204 and CuFe204[22-281.

Continuing on the same lines, several reports of earlier researchers [29-3 I] on

mixed femtes of Ni-Zn-Cu have motivated to prepare ferrites oi4emary mixtures to

povide additional electnc and magnetic data. It is of interest to dope znZ' ions in

some of the sites of ~ i ' ' ions in Ni-Cu mixed ferrite system. It is known that the

magnetic behavior of Ni-Cu femte is largely governed by Fe-O-Fe interaction and

Ni-O-Fe interaction (the coupling of spins of the 3d electrons). lnooducing zn2'

ions in the spinel lattice is expected to bring in a substantial change in

magnetization , Curie temperature and electric properties.

It is also of interest to investigate the structural and magnetic properties of

ferrites depending upon the cooling rate employed during the synthesis of the

samples.

2.2 PREPARATION OF THE SAMPLE

To suit to the needs of technology different types of femtes are prepared by

varying the composition. In this chapter a system of ferrites namely N~os .

,ZnxCuo,5Fe204 is prepared and studied. The values of x are 0.0,O. 1,0.2,0.3,0.4 and

0 5. The systematic procedure of preparation falls into four categories. They are

1. Powder preparation

2. Compact formation

3. Heat treatment processes and

43

4. Machuung to final shape.

2.2.1 POWDER PREPARATlON

The composite oxides of N i ~ . s Z n F w sFe204 system are A.K grade NiO, ZnO,

CUO and Fe203. They are taken in stochiometric proportions as given below:

1. x = 0.0 3 O.S(NiO] +O.O[ZnOj +O.S[CuO] +I.O[Fq 0,j -+ N&zaO,oCu o.JFqO,

2, x = 0.1- 0.4INi01 +O.l[ZnOl +O.SICuO] +l.O[Fe, 0,1 + Nio,,Zno, Cu o.5 FQO,

3. x = 0.2 0.3[NiOl+0.2IZn01 +O.S[CuO] +l+O[Fh 011 + NioJZno.2 Cu F s 0 4

4, x = 0.3 3 O.Z[NiOj +O.3[Zn01 +0.51CuO] +l.O[Fe, OJ 1 + N ~ O . ~ Z ~ O , ~ Cu o 5 FRO,

5, x = 0.4 3 O.lINiOl+O.4[ZnO] +0.51Cu0] +l.OIFq 0 3 ] + Ni,, Znu4Cu us FelO,

6. x = 0.5 3 O.O[NiOl+0.5[ZnO] +0.S[CuO] +I.O[FQ 0, ] -t NiaoZno.5 Cu Fe104

The powders of reactants are mixed and ground well using an agate mortar.

The acetone is only used for cleaning the mortar after each concentration is ground.

Well ground powder is fired at 900 "C for 20 hours and furnace cooled

approximately at the rate of 100 "C per hour. This process is termed as pre-sintering.

2.2.2 COMPACT FORMATION

The pre-sintered powders are again ground well and are pelletized using

hydraulic press at a constant pressure of 70 ICg/cm2.

This powder pressing is an important and commonly used ceramic forming

technique that warrants a brief treatment. In essence, the process is the compaction

of a powdered mass into a desired shape. The degree of compaction is maximized

and fraction of void space is minimized by using coarse and fine particle mixed in

appropriate proportion.

2.2.3 HEATING SHEDULES

In the present study, the synthesis of polycrystalline mixed ferrite is under

taken through two heat treatment methods.

A. Sintering ( Double sintering) and

B. Quenchg (First sintering then quenching)

Accordingly the study of these femtes, prepared by differing heat treatments.

is presented in two sections. They are:

Section A: Characterization of the Sintered system and

Section B: Characterization of the Quenched system.

SECTION A

CHARACTERIZATION OF THE SINTERED

Ni~.l~,Zn,Cuo.sFqO~ SYSTEM.

Sintering is the heating process by whlch atomic mobility of the compact is

sufficient to pennit the decrease of the free energy assoc~dted with the grain

boundaries. Sintering is by far the most critical and an extensive step. If carefully

executed, it yields the required crystal structure, Oxidation State, microstructure,

and physical condition of the ferrite core.

High sintering temperature assures the more densification or less porosity. The

samples in the form of pellets (three specimens for each concentration ) are fired at

1100 "C for 20 hour and then furnace cooled at the rate of 100 "C per hour. This

procedure is called second sintering.

2.3A EXPERIMENTAL

2.3.1A X-RAY DIFFRACTION STUDY

An X-ray powder diffraction pattern is a set of lines or peaks, each of different

intensity and position [d-spacing or Bragg angle 0 1, on a length of chart paper. For

a given substance, the peak positions are essentially fixed and are characteristic of

that substance. The intensities may vary from sample to sample, depending on the

method of sample preparation and the instrumental conditions. For identification

Purpose, principal note is taken of line position together with a semi-quantitative

consideration of intensities. .

A finger print proof is provided for XRD in comparison with Powder

diffraction files of JCPDS (Joint Committee on Powder Diffraction Standards) for

similar fenites. Experimental determination of lattice constants, molecular densities,

particle size and oxygen parameter is taken up following the procedure highlighted

in chapter-1.

PROPOSAL FOR THE SITES OF CATIONS

A prior knowledge of the site preference of the constituent ions of a ferrite is

an important requirement for working out the spinel structure. Secondly a accurate

data of ionic size of the constituent ions is advantageous. In the present system of

ferrite C U ~ ' , N ~ ~ ' , Z ~ ~ ' and ~ e " a~ the constituent cations. Using knowledge of site

preference of the ions and the ionic size data of the respective ions viz Table- 2. IA

and substituting the ions in A and B sites of spinel sbucture. A systematic

calculation of radii of individual sites ra and rb is proceeded. Theoretical values of

lattice constants are estimated permuting the r, and rb values in the modified formula

for lattice constant calculation of Bhongale et.a1.[32]

a = {(r, 143 +2.0951r0) + [(ra/d3 +2.0951r,)~ -1.866 (1.3331; + 0.0675r: -

0.6rjb)]" } 10.933 . . ..(2.1)

An appropriate set of values of lattice constants is estimated according to the

criteria of Vegard's law[33]

1. Change of lattice constant with concentration

TABLE - 2.1

Ionic radii and magnetic moments of the cations used in the present study*

*CRC Hand book data.

Magnetic moment (PB)

5.92

4.90

2.84 I I

0.0

1.73

Ionic radius (AO )

0.64

0.74

0.69

0.74

0.72

r

Cations

Fe3'

Fe2+

~ i ~ +

zn2+

cu2+

2. A close agreement of lattice constant for each concentration with its

experimental value.

This procedure helps to propose a good fit set of cations and their respective

sites of spinel structure.

The theoretical and experimental lathce constants are tabulated in TABLE-

2.2A. The porosity, particle size, molecular density and u- parameter are also listed

in the same table. In order to appreciate the variation of lattice constant with

concentration a graph is drawn between them, which is given in Figure-2.2A. The

proposed cation distribution is given in TABLE-2.3A.

2.3.2 LOW FIELD MEASUREMENTS

Curie temperature is measured for the samples using the Low Field

Susceptibility Bridge as explained in Chapter-I. As per the procedure given in

Chapter-1 saniples of thickness 5 mm and diameter 15 mm are introduced in the

balancing coil of the bridge to measure a signal proportional to the actual moment of

the sample. The values of the moment corresponding to each temperature are noted

for different concentrations. A graph is plotted between susceptibility ratio i.e. XT /

xlrr versus temperature in OK for different concentrations and it is shown in the

Figure-2.3A. The temperature corresponding to the minimum value of XT I XKT gives

the curie temperature of the sample. The values of Curie temperature for different

concentrations are given in the TABLE 2.4A. A graph is drawn between

concentration and Curie temperature and it is given in the Figure-2.4A.

48

TABLE - 2.3A

Proposed Cations distribution for the,

Sintered Nio.s.lZn,Cuo.5Fe20 System

Concentration x

0.0

0.1

0.2

0.3

0.4

Tetrahedral site (A)

0.5

Octahedral site [B]

F ~ I o Znoo

Feo925 Zno 075

Feo 8s Zno I 5

Feo 775 Zno 225

F ~ O 7 Zno 3

Cuo 5 Nio sZno o Fel o

Cuo s Nio 4 Zno 025 Fe1075

CuosNio3 ZnoosFel I5

Cuo 5 Nio 2 Zno 075 Fel 225

Cua s Nio I Zno I F ~ I 3

Feo 625 Zno 375 CUO sNiooZno 12sFe1375

2.3.36 HIGH FIELD MEASUREMENTS

Hysteresis loops are traced using the A.C High Field Hysteresis Loop Tmca in

the field ranging up to 3600 Oe. A sample of approximately 3 mm thtchess and 15

mm diameter is placed in the upper part of the pick-up coil and pushed back into the

pole gap. When the current is increased, the hysteresis loop of the sample will be

seen on the oscilloscope screen. Vertical and horizontal sensitivities of the scope

be adjusted to get a suitable size of the hysteresis loop. The representative

loops for the present study is given in Figure-2.6A. The parameters calculated from

the hysteresis loop are given in TABLES-2.5A and 2.6A.

2.3.4A A.C CONDUCTIVITY MEASUREMENTS

A.C conductivity values are measured at different frequencies i.e. from 40 Hz

to 100 kHz with a desired interval of frequency steps at different temperatures for all

the concentrations of Nio.~.xZnxCu~,~Fe~04 system. The variation of conductivity

(4,) with frequency log (o), for all the concentrations, for one particular

temperature, is shown in the Figure-2.8A.

The dielectric loss factor tan6 and the dielectric constant of the materials are

measured at different temperatures and at different frequencies. The values of the

quantities measure by electrical studies are reported for 10 kHz frequency and at

150°c, for all the concentrations, which are given in the TABLE-2.7A. In order to

appreciate the behavior of dielectric constant and tan6 with frequency, curves are

drawn and are shown in the Figures-2.9A and 2.10A respectively.

49

The values of activation energy &) arc estimated, from the slopes of the set

of plots of log (0,) versus 1000R (Figure-2.11A) drawn for a particular frequency

of lOkHz, for all the concentrations. The values of activation energy are tabulated as

in TABLE-2.7A. A graph is drawn between concentration and activation energy

(I$,), at lOW which is shown in the Figure-2.12A. The site preference factor 'n'

and the grain boundary conductivity '8' are calculated from the log (a, - ad,) versus

log (o) plot for all the concentrations. The variation of '9' and 'n' with

concentration is shown in the Figures-2.13A and 2.14A respectively.

2.3.5A FTIR STUDY

The FTIR spectra are taken for all the samples of Nio 5,Zn,Cuo sFe204 system

using SHIMADZU-8700 FTIR Spectrometer. The spectra are shown in the Figure-

? 15A. The vibration frequencies are given in TABLE-2.9A.

2.4A RESULTS AND DISCUSSION

The observed XRD patterns of the samples of the present study are shown in

the Figure-2.1A. It is seen from Figure-2.1A that there are separated and

distinguished peaks with varying intensities. This observation easily ensures that the

samples prepared are in single phase and polyaystalline nature. The measured

values of diffraction angles vary between 34Oand 35' for the characteristic peak of

3 11 plane. The presence of 3 11 reflection plane for these fenites provides the first

evidence of the phase centered cubic structure (FCC). The values of experimental

lattice constants given in TABLE2.2A are found to range between 8 .535~ ' to

50

8,553~ ' for the spinel fcnites. The variation of experimental lattice constants is well

depicted fiom Figure-2.24. Figure-2.2A clearly indicates that there is an increase of

lattice constants with increase of concentration (x). According to the plan of

peparation of the spinel ferrite system ~ i ' ' ions are replaced by a proportionate

substitution of znZ' ions when concentration x is increased. It isryise to relate the

increase of znZ' ions at the expense of ~ i ? ' ions with the increase of lattice constant

as concentration increases. Actually the size of znZ' ions (0.74~') is larger than that

of the ~ i " ions (0.69~') which legitimately conforms the increase of lattice

constant as zn2+ ions increase.

The measwed values of particle size of the spinel ferrite lie in-between 2 8 6 ~ '

and 43 1 ~ ' . They are in good agreement with the values reported earlier for similar

femtes prepared by double sintering methods [7]. The values of % of porosity vary

from 17.31 to 21.32. The range of values of % of porosity of the present samples

reflects a good schedule of heat treatment of ceramics. Further the values of porosity

are found to be well with in the range of values reported by the earlier workers

16,101. 002728

The values of molecular density determined using experimental lattice constant

data are also given in the TABLE-2.2A. Molecular density of the spinel fenites is

found to vary between 5037 ~ g l m ~ and 5167Kg/m3 which show good agreement

with the molecular density of similar femtes. It may be concluded that the

polycrystaliine fenrite of the present study has a better compaction. The values of

Variation of Iafflco constant wlth cocantr8tlon for the sM~~wJ Ni,,=Zn.Cu,Fe,O, system ,

8 7x10.'~ -

E 8.6~10~"- - a - 5 85.10-~- e 8 .$ ~ 4 x 1 0 ' ~ -

!i - 8.3~10"~ -

Theoretical rY3 -L-t- *---- t----

-0 1 0.0 0 1 0 2 0.3 0 4 0 5 0.6

oxygen paranem change firom 0.398 to 0.400 which is clearly found to be close to

the ideal value of Oxygen parameter 0.375. It means that tbe spinel femtes of the

present study haw a better compaction with a small amount of loose packing. This

explanation is in agreement with the ones suggested by porosity, molecular density

and particle size measurements.

TABLE-2.3A provides a picture of distribution of cations in the A and B sites.

It is noticed form the TABLE-2.3A that zn2' ions show more preference for A sites

though they are supposed to occupy the B site where Ni2' ions might have created

vacancy positions. In order to explain the site preference of transition metal Ions two

theories have been proposed which differ in the concept of chemical bondings in

oxides. Dunitz and Orgel [34] have used crystal field theory, which is based on

purely ionic type of bonding, where as Blasse[35] has used a simplified molecular

orbital approach, taking into account, the covalent bonding between oxygen and

transition metal atoms. Normally large divalent ions like zn2' tend to occupy the

tetrahedral sites as this is favored by polarization effects. Daniels [36] and Matsui

et.a1.[37] have explained that Ni2' and cu2' ions have a tendency to occupy

octahedral sites. The proposed cations distribution of spinel femtes of the present

study is in vogue with the criteria reported earlier. However a small % of zn2' ions

is found to occur in the octahedral sites for compensating the condition of

covalancy. Theoretical value of lattice constant is also plotted with respect to

concentration and shown in the same Figure-2.2A for comparison. Both the curves

52

of theoretical and experimental lattice constants sbow a straight line behavior and an

increasing trend with concentration. Such an obsavation on theontical and

experimental lattice constants certifies that the proposed cation distribution is an

equivalent of actual distribution of the ions in this spinel fenites system. It is also

noticed in Figure-2.2A that there is an appreciable gap between k t w o straight line

curves of theoretical and experimental lattice constants. A judicious incorporation of

the rules of packing of ions reveals that

1. The loose packing of the ions is surely a contributor of lathce constant

expansion.

2. The presence of crystal field distortions is unavoidable, as there are CU*'

ions in B site.

Therefore the occurrence of loose packing and crystal field distortions is

estimated as the cause mechanism of the gap between the theoretical and

experimental lattice constants. It is also understood that the increasing trend of

lattice constants with concentration is solely attributed to that part of zn2' ions

replacing ~ i ~ ' ions in the B site. Replacement of ~ e ~ ' ions by the doped zn2' ions in

the A sites contributes insignificantly for the expansion of the lattice

Figure-2.3A shows the thermal variation of XT / XRT with temperature for the

N i 0 . s - ~ Z n ~ C u ~ , ~ F e ~ 0 ~ system. It is seen from Figure-2.3A that there is a sudden fall of

susceptibility ratio at the transition point followed by a plateau region when the

temperature increases from the raom temperature values. It is explained as a

standard phase transition of second order in which the fenites undergo a change of

phase ftom paramagnetic to ferrimagnetic phases. It is believed that the height of the

plateau region of the curve is the measure of the magnetic energy in femtes [38]. It

is observed from the set of curves for 111.1 XRT versus temperature ordered magnetic

phase is well settled in the members.of the spinel femtes.

Generally a polycrystalline magnetic material may consist of four types of

domain states namely 1.Multidomine (MD) 2. Single

domain(SD).3.Superparamagnetic particles(SP) and 4.Cation deficient phases(CD).

~athakrishnamoorthy et.a1[39] and Bean[40] systematically characterized magnetic

materials and postulated different nature of curves as the indirect evidences for the

presence of varying domain states. According to the postulate of Bean[40], it is

stated that the nature of the curves is indicative of the presence of multidomain

states.

Other important result of thermal variation of A.C susceptibility is the

measurement of Curie temperature which is the phase transition point. Measured

values of Curie temperatures in TABLE-2.5A show a proportionate decrease with

the increase of concentration. The parameter Curie temperature is the sincere

function total magnetic moment contribution of the spinel. The decrease of Curie

temperature with concentration is explained to be due to the increase in the

concentration of zn2' ion, which is non magnetic [7].

From the TABLE-2dA it is seen that the Tc values range from 771% to

463%. The values of Curie temperature of the end member ferrites is 585 OC and

455 OC for NiFe204 and CuFe204 femtes respectively[l]. For the ZnFe104 ferrite

the transition temperature is 10 O K [41] also for the Ni-Zn ferrite the transition

temperature is 770 k [5]. The transition temperature of Cu-Ni fe* is 797 OK [4]

and for Cu-Zn ferrite Tc is 473 OK (221. It is clear that the range of values of Curie

temperature is well within the values reported earlier. Figure-2.4 shows the DSC

spectra for the samples of present study up to 5 0 0 ~ ~ from room temperature. The

values of Tc estimated from DSC charts are presented in TABLE-2.4A. They show

a good agreement with the ones measured using low field susceptibility technique. It

1s established that the substitution of zn2' sufficiently reduces the transition

temperature of Ni-Zn-Cu spinel ferrites. From the knowledge of ions present in the

A and B sites of the fenite it is positively interpreted that the crystal field faces a

concise reduction due to cu2' ions as the neighbor of the ions in B sites. Thus the

reduction of Tc with concentration explains that the interaction of crystal field and

Jahn-Teller splitting.

In the present study the parameters of primary importance are saturation

magnetization, coercive field, remanance magnetization and magnetic moment. The

shape of magnetic hysteresis loop renders explanation for the nature of a magnetic

domain and type of the magnetic materials. Bean[40] has shown from theoretical

considerations that the hysteresis loops of dilute magnetic materials of various size

DSC spectra for the sintered Nio.sxZnxCuo~SFe20,, system

Temperature (K)

TABLE - 2.4A

Curie temperature of the samples of

Sintered Nio.S-xZnxCuo.sFe204 system

of particles would be different. It has been worked out that the shapes of the loops

will be divided into four categories. Each kind of loop is found to be indicative of

one type domain or mixed types of domains. The type of the domains so categorized

are single-domain (SD), multidomain (MD), superparamagnetic domain (SP) and

cation deficient states (CD). The reason for the occurrence of differCFR domain states

is well established in the literature[42].

According to the fairly well established criteria for domains, the loops obtained

for the Nio s.xZnFuo.~Fe20~ system (Figure-2.6A) of the present study are indicative

of multidomains. The value of coercive field varies between 21.25 Oe and

25.010e. It is seen from the TABLE-2.5A that the coercive field is found to be

fairly low and there is no much change with respect to concentration. Low value of

coercive field is effected due to soft nature of the ferrite. It is found to be insensitive

to concentration variation. The values of remanance ratio range from 0.352 to 0.457.

Therefore it is understood that the ferrites prepared in the present study are

niultidomain cases. The values of saturation magnetization(M,) values vary from

341 Gauss to 424 Gauss and the specific magnetization (a) values are form 83 to

102 emu 1 g . These values are in good agreement with the values reported by Smit

and Wijn [I], Wisvanathan and V.R.K. Murthy [43 ] and Derek Craik eta1 [44] for

the similar types of femtes.

According to Neel's two sublamce molel [45] of femmagnetism, theoretical

magnetic moments can be calculated by the relation

56

Hysteresis loops of the quenched Nias,Zn,C~osFqO~ system

Saturation induction, remanance induction, B,\ Bs ratio and coercive

field of sintered Nio.s.xZnxCuo.5Fe204 system

~ B ( N ~ ) = M B - MA . . ..(2.2)

Mg and MA are the B and A sublattice magnetic moments respectively. In the

present studies nB values are calculated using ionic magnetic moments of

Fe7',~i2',~n2',and ~ d ' . The observed magnetic moments are compared with the

theoretical peel ' s model) magnetic moment and are summarized& TABLE-2.6A.

In the earlier studies 1461, agreement between the observed and theoretical magnetic

moment values have been found confirming the collinear spin ordering and

disagreement also found indicating the non-collinear order. The present discrepancy

between observed and theoretical n~values can be understood in terms of significant

non-collinear behavior.

For x>0.3 the discrepancy between observed and theoretical n~ values are

increased with the dopant concentration (x). With the increase of x , the increased

dtscrepancy of ne values suggests the dominant role of canted spin (non-collinear)

on B sites existing and Yafet-Kittel angles having strong influence on the Ms

variation with x. The values of canting angle (OYK) have been obtained from the

observed ng variation with x by the relation [47]

$B(x)(,~) = MB(X)COS~YK - MA@) ....( 2.3)

These values are also given in TABLE-2.6A. Thus the observed Ms variation

has been explained on the basis of Yafet-Kittel angles existing on the B site spin

besides two sub lattice models. zn2' ions going into B sites decrease the magnetic

Variation of Curie ternperstun, with concentrstlon

Concentration (x) Figure-2.SA

for the sintered Nia,ZnlCua,Fe,O, system

Variation of Magnetic moment with concentration for the sintered samples of Ni,,JnxCu,,Fe,O, system

850 - MY)-

750 - 7W - 650 - 600-

550 - 5W - 450 -

4 W , -0 1 0.0 0.1 0.2 0 3 0 4 0.5 0 6

1

I

. , . , . , . , . , . , .

15 -

14 - 13 - 12 - ":

t l o - E 9 -

B 8 1

7 : U 6 -

5 '

7

Experimental

g '-. --J-

1 - o i - , . . . . - . - m . .

-0.1 0.0 0.1 0.2 0.3 0.4 0.5 0.6

concentration(x) Figure-2.7A

moment of the B sub lattice and also mod$ the exchange interactions of A and B

sites. Hence, the net magnetization, Ms decreases.

Surprisingly for xC0.3 the theoretical n~ values are smaller than their

experimental values. Thus these studies indicate the modification of A-B and B-B

exchange interactions. However, studies using Mossbauer spectrollcter would give

clear information about the magnetic structure of the fenites to understand in terms

of Neel's two sub lattice model and canting spin on B sites.

Magnetic moment contribution and magnetic energy bear direct relation to na

and curie temperature. From Figures-3.5A and 2.7A it is confirmed that T, and

magnetic moment decrease with concentration.

In order to check the validity of molecular field theory for the present system

of fenites, the values of exchange energy, molecular field coefficient and exchange

field are obtained and reported in TABLE-2.6A. The value of exchange energy lies

between 16.772~ 10"~ and 40.604~10- '~ ergs. The molecular field coefficient ranges

between 194.26 and 470.28, and the exchange field varies from 8 2 . 2 6 ~ 1 0 ~ to

163.16x10' Gauss. The order of values of these parameters shows that the spinel

ferrite prepared is fenimagnetic in nature.

The TABLE-2.7A shows that the values of conductivity measured in the

present study lie between 9 .343~10 '~ and 1.24~10" at 1 5 0 ' ~ and 10 kHz frequency.

The dielectric constant and tan6 values, at 1 5 0 ~ ~ and 10 kHz, ranges from 112.633

to 405.186 and 1.491 to 5.505 respectively. The order of the values is indicative of

well organized grain formation for the sintering condition adopted in the present

work. The activation energy E, measured at 10kHz frequency spreads from 0.266 to

0.347 eV. The site preference factor 'n' lies between 0.397 and 0.732. Similarly the

gain boundary conductivity 'B' ranges from 2.935~10" to 1.991~@' s cm".

The frequency dependence of conductivity, dielectric constant and tan6 are

p e n t e d in the Figures-2.8A,2.9A and 2.10A respectively for the present spinel

system. Figure-2.8A shows a slow change of the values of conductivity at low

frequencies and a steady rising up at higher frequencies. Conversely Figure-2.9A

shows a clear fall of values of permittivity with frequency. Figure-2.10A shows that

the tan6 values rise up at lower frequencies. The trend of conductivity explains that

the mixed femtes have more electrons available for transport at higher frequencies.

The variation of dielectric constant with frequency in these materials is explained to

be due to the effective value of polarization caused by inter-ionic, molecular and

grain boundary effects at low frequencies. The low values of dielectric constant at

high frequencies are attributed to a less contribution of polarization due to

exclusively inter ionic type.

Curves of tan6 in Figure-2.10A provide a picture of the absorption of energy

carried by electromagnetic field when it propagates through the femte. The decrease

Variation of conductivlty(ac) at lSO°C with frequency for the sintered samples of Ni,,.xZnzCu,,Fe,O, symtem

+ r O . 0 -- x=0.1 +-- x=0.2 --t x=0.3

Varfation of dklectrlc constant with frequonoy for the s i n t e d sampks of Ni,,aZnxCu,Fe,O, system

Variation of tans with frequency for the sintered samples of Ni Zn Cu Fe 0 system

at I 50°c - P D . 0 - x=O.l --e x=0.2 -t ~ 0 . 3

100 -C x=0.4 -4- r O . 5

of tans with fnquency reflects the subsiding nature subsequently leadmg to a

p d u a l limping up of effect of conduction at high fnquencies.

It may be explained so because of the release of charge camiers from the

clutches of polarization at high frequency. The behavior of polarization with

frequency can be explained using a principle laid down by &ap [48] i.e. the

dielectric constant is reciprocal of root mean square of conductivity. Verwey and de

Boer [49] have established that in oxides, containing one ion of variable valence, the

conduction takes place by hopping via activation of states involving cations

changing valence as

Fe2+ u Fe3+ t e- . . ..(2.4)

and vice versa. The presence of nickel ion on the octahedral sites favours the

conduction mechanism as proposed by Van Uitert [50], viz.

NiZ' t Fez' o Ni3' t Fez' t 2e' . . ..(2.5)

which explains the predominant conduction mechanism of Ni-Zn-Cu system of

present investigation. The conduction mechanism for the samples of present study is

due to hole transfer from ~ i " to Ni " ions [5 1,521. According to Mossbauer study

carried out by Daniels and Rosencwaig [53], the cation distribution of Ni-Zn

ferrites is given as

znZ; Fe1,3' [~i",, Fe3'1+, oZh] . . . .(2.6)

As zinc is a non-magnetic element and ZnFez04 is a normal femte, znZ' ions

occupy tetrahedral A-site, whde in. NiFe204, a predominautly inverse ferrite, ~ i "

60

ions occupy octahedral B-site. In samples sinbred at highw temperature, Ni3' may

also be present along with Ni2' and hopping of holes from ~ i ~ ' to Ni2+ is also

probable according to the mechanism given above. So, in samples sintercd at higher

temperature, the conduction mechanism is enhanced and the conductivity of such

samples is changed by an order of lo5.

The curves of log(o) versus 10001 T at 10 kHz for different concentration of

Z" in the mixed ferrites are shown in the Figure-2.11A. It is seen from the Figure-

2.11A that conductivity is a direct exponential function of temperature, which is the

inherent property of a semi-conductor. It is explained on the basis of hopping of

electrons between localized 3d bands of ions in these mixed fenites. The slope of

the lines in the Figure-2.11A should be a measure of activation energy (E,,). A

hopping activation energy of the order of 0.1 eV is associated with the drift mobility

of electrons [54]. However, if the observed values of activation energies are s 0.2

eV suggest [55,56] that the conduction mechanism observed is due to hopping of

electron of the type ~ e ~ ' o ~ e ~ ' . For femtes possessing Cu and Ni in octahedral

sites the value of activation energy has been reported to be = 0.3797 eV[56]. Thus

there is a good agreement of the values of activation energy of the spinel system of

the present study.

The measured values of activation energy versus concentration (Figure-2.12A)

would be able to throw more light on the changes in the position of the energy bands

due to incorporation of zn2'. This-range of values is suggestive of hopping and the

6 1

Variation of conductivfty with temp.mture forthe 8Intered Ni,,+axCu,,Fe,04 rystem

at lOkHz --t ~ 0 . 0 --e x-0 1 -& -0.2 -----. - ~ ~ 0 . 3 -4- ~ 1 0 . 4 -+- FO.5

------.

1E-4 7

4 - . - . . , . . . . . , \

1.5 1.6 1 7 8 1.9 2.0 2.1 2.2 2.3 2 4

1 0 0 0 ~ Graph-2.llA

Variation of activation energy(€-) with concentration for the sintered samples of Ni,,xZnxCu,,Fe,04 system 1 .o

at 1OkHz

present system behaves like a semiconductor with intermittent energy levels in the

localized d bands of neighboring Fe ions of B site. Based on these results we

conclude that the Fe3d electrons are important in the conduction process.

Conductivity measurement is indicative of the presence of N?' and ~e ' ' ions in

B site. The rising up of activation energy (E,) with concentration is ahbuted to the

widening of the band gap of 3d band of ions mainly in B site. The increase of

concentration alters the composition of Ni and Zn. Generally CU" ion in B site is the

cause of Jahn-Teller effect which makes the neighboring ions to go to the exited

state.

Normally electrical conduction is explained by mobile charge carriers i.e.

electrons or holes in semiconductors. The mobility of excess charge carriers

accounts for conductivity. From the curves of concentration dependence of

conductivity and activation energy, it is inferred that Cu and Zn ions does not

remain as passive ions but behave in a trivial manner in bimming the energy band

beparation.

The variations of 'B' and 'n' with respect to concentration (Figures-2.13A and

1.14A) show difference in the particle formation as the materials are prepared by

ceramic techniques. Phenomenologically conductivity is explained to be dependent

on the composition of ions making ferrites and the grain size of the samples. The

explicit expression relating these physical quantities is

Q(O) =Ban

62

Variation of grain boundary conductivity with concentration for the sintered NiO,+-rZn,C~I,,Fe,O, system

3 ,..lo*:

a

2.7~10. % ? 3'6x104;

8 r.e*io4- i?

3 B.0flo.r:

1 . C 3 0.0 - 0

Variation of site preference factor with concentration for the sintered samples of Ni O,,.x ZnxCu,,,Fe,O, system

0.1 0.0 0 1 0.2 0.3 0 4 0 5 0 6

./

--- . ' . 1 . , . , - 1 - , . , . ,

0 6

1.0.

0.B - 0.8 - - '

.f 0 7 - L 0 ' V 0.6 - $

0.5 - 0.4 -

E 0.3;

g 0.2:

0.1 - 0 . 0

ui

Goncentration (x) Figure-Z.14A

. ------L--

m m

L

. , . , . . , . , . 0.1 0.0 0.1 0.2 0.3 0.4 0 5

where, 'B' is the parameter decided by the organization of grains, a, is the frequency

and ' n' is the parameter depending on the composition of the samples. It is inferred

form the Figure-2.13A the size of the grain increases with the concenh.ation of Zn.

~ u t in Figure-2.14A the site preference factor 'n' indicates gradual fall with respect

to Zn concentration. Of the two parameters 'B' is found to change sensitively with

the variation of Zn. However the deviations of 'B' and 'n' are attributed to the

ceramic method of preparation of the system of spinel femtes.

The fenites prepared in the present study crystallize in the natural spinel form

w ~ t h the space group Fd3m-(Oh) On the basis of group theoretical calculations, spinel

ferrites exlubit four IR active fundamentals (Ti,,) in the vibrational spectra of normal

as well as inverse spinel femtes. It has been reported that the first three 1R bands are

due to the tetrahedral (Td) and octahedral (Oh) coordination compounds, while the

fourth one is due to some type of l a the vibrations involving tetrahedral cation [57].

The infrared spectra of Nio,S.xZn,C~,~Fe204 mixed ferrite samples are shown

i the Figure-2.15A for different concentrations. The absorption bands obtained in

the present investigation are found to be in the range reported for similar type of

ferrites[58,22]. The position of the bands together with their shoulders are given in

the TABLE-2.9A.

From Figure-2.15A, it can be seen that the IR spectra of the present system

exhibit five principal bands. The principal bands v, and vz shift gradually towards

63

TABLE - 2.8A

FTIR spectral frequencies of some end member ferrites*

TABLE - 2.9A

Center frequency of the bands of FTIR spectra of the

sintered Nio.s.xZn,Cuo.5Fe204 system,

Concentration X

0.0

0.1

0.2

671.0 569.0 468.75 381.9

668.38 567.0 385.7

668.75 563.2

Center frequency cm-I

v 1

671.0

671.1

v2

578.6

576.7

572.8

"3

469.23

466.66

48 1.25

v4

407.69

401.11

" 5

-- 372.2

372.3

395.4 -

low 6equency side mainly the high-frequency brmd(vl) and second absorption

band(vz) an found to be in the range 660-670 and 580-560 cm" respectively. A

comparison of FTIR charts at different concentrations provides information about

decrease in transmittance and increase in broadness.

These features are explained on the basis of cation distribution in A and B sites

of mixed Ni-Zn-Cu femtes. Waldron [59] has attributed the occurrence of v, and vz

bands to the intrinsic vibrations of the tetrahedral (Td) and octahedral (Oh)

coordination compounds. Both these hgh frequency bands have been attributed to

the intrinsic lattice vibrations of E-symmety. The tiurd absorption band v3 is

attributed to the CU" - 0'- complexes at octahedral sites. The frequency of the

(v4and vs ) bands depend on the mass of tetrahedral metal ion complexes[60] and

hence it is attributed to the vibrations of ions at the tetrahedral site. The low

frequency bands are attributed to vibrations of T2* -symmety.

The cation distribution of mixed Ni-Zn-Cu fenites is:

( ~ n ~ ~ x.y Fe3+ I . ~ ~ ~ )A [t4iZt 0.5.x znZt , Cu 0.5 F$+ I+x.~]B oZ.q. ....( 2.8)

Wherex=0.1,0.2,0.3,0.4and0.5,y=25%ofx.

As thc content of 5 increases Zn2' ions consistently replace ~ e j ' ions from A

to B site. At the same time ~ i ~ ' ions on the octahedral site decrease. This disturbs

the order on the octahedral site with increase in zinc. The disordered systems give

rise to broad bands in their spectrum[61]. TABLE-2.9A shows high frequency band

in the range 670660 cm which is found to decrease to a small extent with the

*crease of concentration. Consequently the changes in the high frequency band vl

are comelatable to znZf-d' bond stretching. The agreement of the value of

frequency of v l band with the literature value is illustrated in TABLE-2.9A. On the

other hand second absorption band v l (580-560 cm-') which may be attributed for

zn2'- 0'' at octahedral sites as it clearly indicates an appreciable decrease of

frequency with increase of Zn concentration. The sites A and B are explained to

posses other ions also in this mixed fenites along with Zn. But the shifting of the

band frequency is explained to be due to Zn ion concentration affecting the

distribution of other ions. Based on these observations it is accounted for by that the

expansion of B sites is larger than the expansion of A site. It is in agreement with

the suggestion made out of X-ray diffraction studies of the present samples. Third

absorption band v3 explains John-Teller effect due to the presence of cu2 ' ion in

octahedral sites. The presence of CU" ions and the John-Teller splitting makes the

companions ions of B site to exists in their exited states. It means that ~ i ~ ' and ~ e ' '

ions in B sites may be shifted to ~ i ~ ' and I?e3' states. The explanation of this kind is

in accordance with the Inference of conductivity measurements. Consequently these

results are in support of hopping of charge caniers from 3d bands of Ni and Fe ions.

The variation of activation energy with concentration(x) has established that the

band gap of localized 3d band widens. Widening of the band gap of 3d bands with x

is correlatable to the relative strength of Jahn-Teller splitting and crystal field in the

octahedral site of spinel ferrite.

On the similar lines, the discrepancy in the magnetic moment values for x<O.3

may be attributed to the change in the moment caused by differing magnetic

moments of the moment bearers in B sites. It is seen from the T A B ~ E - 2 . 8 ~ that the

frequencies of the bands v.4 and vs are found to be shifted from their corresponding

values of end member fenites namely NiFe204, CuFe204, ZnFez04 and Fe304. The

larger difference-of the frequency of v~ band found between end members and the

present spinel femte may be attributed to the presence of many ion complexes and

their masses in B site.

SECTION: B

CHARECTRElSATlON OF THE

N ~ O . S ~ Z ~ , C U ~ . ~ F Q O ~ QUENCHED SYSTEM

The interestmg physical and chemical properties of spinel fenites arise from

the ability of these compounds to distribute the cations a m the available

tetrahedral and octahedral sites. This distribution of cations depends on the method

of preparation of samples. The femtes synthesized by different techniques have been

found to exhibit different chemical, structural and magnetic properties 1621.

Particularly the structural and magnetic properties of femtes depend upon the

cooling rate employed during the synthesis of samples.

We study the change in the properties of the spinel ferrite Nio.s.xZn,Cuo sFe204

by changing the rate of cooling. This section deals with the characterization of the

quenched samples of Nio.s.xZn,Cuo.5Fe204 spinel system. For the quenching process

the same procedure is adopted as the one followed for sintering method (section-A)

up to pre-sintering process. The pressed samples are fired at 1100 "C for 15 hours

and they are immersed in the ice bath immediately after taken out of the furnace.

The ice quenching operaion is canied after the pressing operation.

2.3B EXPERIMENTAL

2.3.1 B X-RAY DIFFRACTION STUDY

The XRD spectra for all the samples, of the quenched femtes of the present

study, are noted. Experimental determination of lattice constant, molecular density,

size and oxygen parameter is taken up following the procedwe highlighted

In SECTION-A of this chapter.

PROPOSAL FOR THE SITES OF CATIONS

The theoretical lattice constants for the quenched samples of the present study are

calculated as per the procedwe mentioned in SECTION-A. ~hztheoretical and

experimental lattice constants are tabulated in TABLE-2.2B. The porosity, particle

slze, molecular density and u- parameter are also listed in the same table. In order to

appreciate the variation of lattice constant with concentration a graph is drawn

between them, as shown in Figure-2.2B. The proposed cation distribution is given

In TABLE-2.3B.

2.3.2B LOW FIELD MEASUREMENTS

Curie temperature is measured for the quenched samples using the Low Field

Susceptibility Bridge as explained in Chapter-I. A graph is plotted between

susceptibility ratio i.e. XT I XRT versus temperature in OK for different concentrations

and it is shown in the Figure-2.3B. The temperature corresponding to the minimum

value of XT 1 XRT gives the curie temperature of the sample. The values of Curie

temperature for different-concentrations are given in the TABLE 2.4B. A graph is

drawn between concentration and Curie temperature and it is given in the

Figure-2.5B.

TABLE - 2.3B

Proposed Cations distribution for the

quenched N ~ o . s . ~ Z ~ , C U ~ . ~ F ~ ~ O ~ System

1 conce;tratim 1 I

Tetrahedral site (A) Octahedral site [B]

0.2

0.3

0.4

0.5

Fee YS Zno 05

Feu925 Zn0075

Feo 9 Zno I

Fee 875 Zno 125

Cuo s Nla 3 Zno 15 Fe1o5

Cuo s N ~ o 2 Zno225 Fel 075

Cuo 5 Nio I Zno 3Fe1 I

CUO rNlooZn0~7sFe1 I25

2.3.3B HIGH FIELD MEASUREMENTS

Hysteresis loops are traced using the A.C High Field Hysteresis Loop Tracw in

the field ranging up to 3600 Oe. The representative loops for the present study is

given in Figure-2.6B. The parameters calculated from the hysteresis loop are given

in TABLES-2.5B and 2.6B.

2.3.6B A.C CONDUCTIVITY MEASUREMENTS

A.C conductivity values are measured at different frequencies i.e. from 40 Hz

to 100 kHz with a desired interval of frequency steps at different temperatures for all

the concentrations of N&,~,Zn,Cuo,~Fe~O~ quenched system. The variation of

conductivity (0,) with frequency log (o), for all the concentrations, at one particular

temperature, is shown in the Figure-2.8B.

The dielectric loss factor tan6 and the dielectric constant of the materials are

measured at different temperatures and at different frequencies. The values of these

quantities measure by electrical studies are reported for 10 kHz frequency and at

150°c, for all the concentrations, which are given in the TABLE-2.7B. In order to

appreciate the behavior of dielectric constant and tan6 with frequency, curves are

drawn and are shown Cthe Figures-2.9B and 2.10B respectively.

The values of activation energy (E,) are estimated, from the slopes of the set

of plots of log (0,) versus 1000/T( Figure-2.11B) drawn for a particular frequency

of lo&, for all the concentrations. The values of activation energy are tabulated as

in TABLE-2.7B. A gaph is drawn between concentration and activation energy

69

(E,), at IOlrHz, which is shown in the Figure-2.12B. The site preference factor 'n'

and the grain boundary conductivity 'B' are calculated from the log (a, - ode) versus

log (o) plot for all the concentrations. Variation of 'B' and 'n' with concentration is

sllown in the Figures-2.13B and 2.14B respectively.

2.3.7B FTIR STUDY

The FTlR spectra are taken for all the samples of Nio,5.,Zn,C~sFe204

quenched system using SHIMADZU-8700 FTlR Spectrometer. The spectra are

shown in the Figure-2.15B. The vibration frequencies are given in TABLE-2.8B.

2.4B RESULTS AND DISCUSSION

The observed XRD patterns of the samples of the present study are shown in

the Figure-2.1B. it is seen from Figure-2.1B that there are separated and

distinguished peaks with varying intensities. This observation simply ensures that

tlie samples prepared are in single phase and polyc~ystalline nature. The measured

values of diffraction angles vary between 34' and 35' for the characteristic peak of

3 1 1 plane. The presence of 3 11 reflection plane for these ferrites provides the first

evidence of the phase centered cubic structure (FCC). The values of experimental

lattice constants given in TABLE-2.2B are found to range between 8 .494~ ' to

8.557~' for the spinel femtes. The variation of experimental lattice constants is well

depicted from Figure-2.2B. Figure-2.2B clearly indicates that there is an increase of

lamce constants with increase of concentration (x). According to the plan of

preparation of the spinel ferrite System ~ i ' ' ions are replaced by a proportionate

70

Variation of lattice concentration with concentration for the quenched samples of NI,,Zn~Cu,,Fe,O, system

I Experimental Theoret~cal

Variation of susceptibility ratio with temperature for the quenched Nio,s.xZnxCu,~6Fe,0, system

TABLE - 2.4B

Curie temperature of the samples of

Quenched Nio.~-,ZnxC~0.5Fe204 system

Concentration <XI

0.0

0.1

0.2

0.3

0.4

0.5

Curie temperature Tc ( K)

750

713

660

6 10

532

463

substitufion of Zn2' ions when concentration x is increased. It is wise to relate the

increase of zn2' ions at the expense of ~ i ~ ' ions with the increase of lattice constant

as concentration increases. Actually the size of zn2' ions ( 0 . 7 4 ~ 4 is larger than that

of the ~ i ~ ' ions ( 0 . 6 9 ~ 4 which legitimately conforms the increase of lattice

constant as znZ' ions increase. The measured values of particle size of the spinel

ferrite lie in-between 2 8 4 ~ ' and 428~' . They are in good agreement with the values

reported earlier for similar fenites prepared by double sintering methods[l]. The

values of % of porosity vary from 5.40 to 27.70. The range of values of % of

porosity of the present samples reflects a good schedule of heat treatment of

ceramics. Further the values of porosity are found to be well with in the range of

values reported by the earlier workers [6,10].

The values of molecular density determined using experimental lamce constant

data are also given in the TABLE-2.2B. Molecular density of the spinel femtes is

found to vary between 5055 ~ ~ / m ~ and 5 1 6 2 ~ ~ l m ~ which show good agreement

with the molecular density of similar femtes. It may be concluded that the

polycrystalline femte of the present study has a better compaction. The value of

Oxygen parameter is found to be 0.398 for all the conceneations, which is clearly

found to be close to the ideal value of Oxygen parameter 0.375. It means that the

spinel femtes of the present study have a better compaction with a small amount of

loose packing and the compaction remain uniform over concentration. This

expl~oation is in agreement with the ones suggested by porosity, molecular density

and particle size measurements.

The distribution of cations in the A and B sites is presented in TABLE-2.39.

It is noticed form the TABLEZ.3B that zn2' ions show more preference for B site.

The proposed cations distribution of spinel fenites of the present-dy is in vogue

with the criteria reported earlier[24]. Theoretical and experimental values of lattice

constants is plotted with respect to concentration and shown in the Figure-2.ZB for

comparison. Both the curves of theoretical and experimental lattice constants show

straight line behavior and increasing trend with concentration. Such an observation

on theoretical and experimental lattice constants certifies that the proposed cation

distribution is an equivalent of actual distribution of the ions in this spinel ferrites

system. It is also noticed in Figure-2.2B that there is an appreciable gap between the

two straight line curves of theoretical and experimental lattice constants.

The occurrence of loose packing and crystal field distortions is estimated as the

cause mechanism of the gap between the theoretical and experimental lattice

constants. It is also understood that the increasing trend of lattice constants with

concentration is solely attributed to that part of zn2' ions replacing ~ i ~ ' ions in the B

site. Replacement of ~ e ~ ' ions by the doped zn2' ions in the A sites contributes

insignificantly for the expansion of the lattice

Figure-2.3B shows the thermal variation of XT I XRT with temperature for the

Nb.,.xZn,Cuo,5~e204 system. It is seen fiom Figure-2.3B that there is a sudden fall

77

of susceptibility ratio at the transition point followed by a plateau region when the

temperature increases fiom the room temperature values. It is explained as a

standard phase transition of second order in whch the femtes undergo a change of

phase from para magnetic to fenimagnetic phases. It is believed that the height of

the plateau region of the curve 1s the measure of the magnetic energy in ferritesl381.

~t is observed from the set of curves for XT I XRT versus temperature ordered

magnetic phase is well settled in the members of the spinel ferrites According to the

postulate of Bean [40], it is stated that the nature of the curves is indicative of the

presence of multidomine states.

Other important result of thermal variation of A.C susceptibility is the

measurement of Curie temperature which is the phase transition point. Measured

values of Curie temperatures in TABLE-2.4B show a proportionate decrease with

the increase of concentration. The parameter Curie temperature is the sincere

function of total magnetic moment contribution of the spinel. The decrease of Curie

temperature with concentration is explained to be due to the increase in the

concentration of zn2' ion, which is non-magnetic [7]. From the TABLE-2.4B it is

seen that the Tc valuesrange from 7 5 0 ' ~ to 463'~. It is clear that the range of

values of Curie temperature is well within the values reported earlier for similar

fenites [1,4,5,22,41]. It is established that the substitution of zn2' sufficiently

reduces the transition temperature of the Ni-Cu spinel ferrites. Also the Curie

temperature of the quenched system is found to be lower than that of the sintered

73

Variation of Curie temperature with concentration for the quenched Ni,,,ZnxCu,,Fe,O, syst.m

1100

750

700

650

6cm

550

5W

450

400

Variation of Magnetic moment with concentration for the quenched samples of Ni,,,lZnlCu,,,Fe,04 system

15

c

-

-

-

- - -

-

-

. ' . '

14 - 13 - 12 - 11 i

Experimental

one because of the excess substitution of zn2' ion on B site which dilutes the

magnetic linkages and results in lower values of Curie temperature [24].

In the present study the parameters of primary importance are saturation

magnetization, coercive field, remanance magnetization and magnetic moment. The

shape of magnetic hysteresis loop render explanation for the noiture of a magnetic

domain and type of the magnetic materials. According to the fairly well

established considerations, the loops obtained for the Nio r.xZnxC~,sFez04 quenched

system (Figure-2.6B) of the present study are indicative of multidomains. The value

of coersive field varies between 21.15 Oe and 23.6 Oe. It is seen from the TABLE-

2.5B that the coercive field is found to be fairly low and there is no much change

with respect to concentration. Low value of coercive field is effected due to soft

nature of the ferrite. It is found to be insensitive to concentration variation. The

values of remanance ratio ranges from 0.324 to 0.363. Therefore it is understood

that the fenites prepared in the present study are multidomain cases. The saturation

magnetization(M,) values ranges from 48 Gauss to 247 Gauss and the specific

magnetization (IS) values are form 11 to 52 emu 1 gm. These values are in good

agreement with the valies reported by Smit and Wijin [I], Viswanathan and V.R.K.

Murthy [43] and Derek Craik et.al [44] for the similar types of fenites.

The theoretical magnetic moment (nB) values are calculated as per the formula

given in SECTION-A of this chapter. In the present studies, discrepancies between

observed and theoretical ns values are increased with the dopant concentration 6).

74

Hysteresis loop of the sintered Ni~.s.Zn,CqsFolO~ system

TABLE - 2.5B

Saturation induction, remanance induction, B,\ Bs ratio and coercive

field of quenched Nio.~,Zn,Cuo,5FezO~ system

With the increase of x, the increased discrepancy of n~ values suggest the dominant

role of canted spin (non-collinear) on B sites existing, and Yafet-Kittel angles

having strong influence on the Ms variation with x.

The values of canting angle are also given in TABLE-2.6B. Thus the observed

M~ variation has been explained on the basis of Yafet-Kittel anglm%sting on the B

site spin besides two sublattice models. znZ' ions going into B sites decrease the

magnetic moment of the B sublattice and also modify the exchange interactions of A

and B sites. Hence, the net magnetization, Ms decreases. Thus these studies indicate

the modification of A-B and B-B exchange interactions However, studies using

Mossbauer spectrometer would give clear information about the magnetic structure

of the ferrites to understand in terms of Neel's two-sublattice model and canting spin

on B sites.

In order to check the validity of molecular field theory for the present system

of ferrites, the values of exchange energy, molecular field coefficient and exchange

field are obtained and reported in TABLE-2.6B. The value of exchange energy lies

between 74.0x10-'~ and 2468x10.'~ ergs. The molecular field coefficient ranges

between 856 and 28584, and the exchange field varies from 21 1 x lo3 to 1372x 10'

Gauss. The order of values of these parameters shows that the spinel ferrite prepared

is ferrimagnetic in nature.

The TABLE-2.7B shows that the conductivity values of the present system lie

between 9,138 x105 and 4.87.x103 at 1 5 0 ~ ~ and at 10kI-I~ frequency. The

75

Conductivity, dielectric constant, tanb, activation energy, site preference factor and grain boundary

conductivity of quenched Nio.s,Zn,Cuo.5Fez04 system

Dielectric constant

Activation energy

Eac eV at IOkHz

Site preference Factor

n

0.045

Grain boundary Conductivity

B s cm"

7 . 2 6 ~ 1 0 ~

Variation of conducttvity(ac) at 15O0C with frequency for the quenched samples of Ni,,-,Zn,Cu,,Fe,O, system

Variation of tan6 with frequency for the quenched samples of Ni,,xZnxCu,,Fe,O, system

3W0

2WO

2 I

1m

1 W O lDOOO

loo(m) Figure-2.106

Variation of dielectric constant with frequency for the quenched samples of Ni,,Zn,Cu,,Fe,O, system

1 W O at 1 6 0 ~ ~ + X-0.0

--C A 1 A- -0.2 +-- x-0.3 -e x10.4 4- ~ 1 0 . 5

- #.,

F -

l a . . - -%#

dielectric constant and Eans values, at 150 '~ and at IOltHz, ranges from 70.11 to

196.29 and 2.46 to 44.58 respectively. This order is indicative of well-organized

grain formation for the samples in the present work. The activation energy E,,

measured at 10kHz frequency spreads from 0.238 to 0.344 eV. The site preference

factor 'n' lie between 0.045 and 0.500. Similarly the grain boun&y conductivity

.B' range form 3 . 3 9 ~ 1 0 ' ~ to 7 . 2 6 ~ 1 0 ~ s cm".

The frequency dependence of conductivity, dielectric constant and tan6 are

pesented in the Figures-2.8B, 2.9B and 2.10B respectively for the present spinel

system. Figure-2.8B shows a slow change of the values of conductivity at low

frequencies and a steady rising up at higher frequencies. Conversely Figure-2.9B

shows a gradual fall of values of permittivity with frequency. Figure-2. IOB shows

that the tan6 values rise up at lower frequencies. The trend of conductivity explains

that the mixed ferrites have more electrons available for transport at higher

frequencies. The variation of dielectric constant with frequency in these materials is

explained to be due to the effective value of polarization caused by inter-ionic,

molecular and grain boundary effects at low frequencies. The low values of

dielectnic constant at high frequencies are attributed to a less contribution of

polarization due to exclusively inter ionic type.

Curves of tan6 in Figure-2.1OB provide a picture of the absorption of energy

canied by electromagnetic field when it propagates through the ferrite. The decrease

of tan6 with frequency reflects the subsiding dielectric naturc subsequently leading

to a gradual limping up of effect of conduction at high frequencies. It may be

explained so because of the release of charge carriers from the clutches of

at high frequency. The behavior of polarization with frequency can be

explained using a principle laid down by Koop [48]. The presence of nickel ion on

the octahedral sites favors the conduction mechanism as proposed by Van Ilite~t

[50], which explains the predominant conduction mechanism of Ni-Zn-Cu system of

present investigation. The conduction mechanism for the samples of present study is

due to hole transfer from Ni3' to Nilt ions [51,52]. According to Mossbauer study

carried out by Daniels and Rosencwaig [53], in samples sintered at higher

temperature, Ni3' may also be present along with ~ i " and hopping of holes from

N?' to Ni" is also probable. So, in samples sintered at higher temperature, the

conduction mechanism is enhanced and the conductivity of such samples is changed

by an order of 10".

The curves of log(o) versus 10001 T at 10 kHz for different concentration of

Zn in the mixed ferrites are shown in the Figure-2.1 IB. It is seen from the Figure-

2.1 IB that conductivity is a direct exponential function of temperature, which is the

inherent property of a semi-conductor. It is explained on the basis of hopping of

electrons between localized 3d ban& of ions in these mixed femtes. The slope of

the lines in the Figure-2.11B should be a measure of activation energy(E,). A

hopping activation energy of thLorder of 0.1 eV is associated with the drift mobility

77

Variation of conductivity with temperature for the quenchad Ni,,Zn

I 1

I --c- x = 0.0 at 10 kHz - e x -0.1

Variation of activation energy(Eac) with concentration for the quenched samples of Ni,,xZnxCu,6Fe,0, system

of electrons [54]. However, if the observed values of activation energies are - 0.2

eV suggest [55,56] that the conduction mechanism observed is due to hopping of

electron of the type ~ e " ~ e ~ ' . For fenites possessing Cu and Ni in octahedral

sites the value of activation energy has been reported to be = 0.3797 eV[56]. Thus

there is a good agreement of the values of activation energy of the spinel system of

the present study.

The measured values of activation energy versus concentration (Figure-2.12B)

would be able to throw more light on the changes in the position of the energy bands

due to incorporation of znZ'. This range of values is suggestive of hopping and the

present system behaves like a semiconductor with intermittent energy levels in the

localized d bands of neighboring Fe ions of B site. Based on these results we

conclude that the Fe3d electrons are important in the conduction process.

Conductivity measurement is indicative of the presence of N?' and Fe2' ions in

B site. The rising up of activation energy (E,) with concentration is attributed to the

widening of the band gap of 3d band of ions mainly in B site. The increase of

concentration alters the composition of Ni and Zn. Generally cu2' ion in B site is the

cause of Jahn-Teller effect which makes the neighboring ions to go to the exited

state.

Normally electrical conduction is explained by mobile charge carriers i.e.

electrons or holes in semiconductors. The mobility of excess charge carriers

accounts for conductivity. From the curves of concentration dependence of

78

VaIJation of grain boundary conductlvlty with concentration for the quenched Ni,

Concentration (x) Figure-2.14B

5.010'

3 4m10-: a

f 3'"'041 z 2.(b104 ,

3 E

O.O:

2 -1.0x1o4-

C - $j - 2 . 0 ~ 1 0 ~ -

Variation of site preference factor with concentration for the quenched samples of Ni,,xZnxCu,,Fe,O, system

0.9 - 0.8 -

= 0 7 - - L . s 0.6 - 0 3 ' 8 C e! 0.4 - f 0.3:

3 . 5 0.2 - 0.1 - 0 0

Concentratlon(x) Figure-2.1 3B

----t-------C---t------ t - -

l , , . , . , . , . , . , . -01 0.0 0 1 0.2 0.3 04 0 5

- m A

--__

. , . , . , . , . , . , . % -0.1 0.0 0.1 0.2 0.3 0.4 0.5 0 6

0 6

conductivity and activation energy, it is inferred that Cu and Zn ions dots not

remain as passive ions but behave in a trivial manner in trimming the energy band

separation.

The variations of 'B' and 'n' with respect to concenhation (Figures-2.13B and

2.14B) show difference in the particle formation as the materials are prepared by

ceramic techniques. Phenomenologically conductivity is explained to be dependent

on the composition of ions making ferrites and the grain size of the sanlples. The

explicit expression relating these physical quantities is

rr(o)= Bw" . . . .(2.9)

where, 'B' is the parameter decided by the organization of grains, w is the frequency

and ' n' is the parameter depending on the composition of the samples.

It is inferred form the Figure-2.13B the size of the grain remains almost

constant with the concentration of Zn. But in Figure-2.14B the site preference factor

'n' indicates gradual fall with respect to Zn concenhation. Of the two parameters 'n'

is found to change sensitively with the variation of Zn. However the deviations of

'B' and 'n' are attributed to the ceramic method of preparation of the system of

spinel ferrites.

The ferrites prepared in the present study crystallize in the natural spinel form

with the space group Fdt,-(Oh). On the basis of group theoretical calculations, spinel

fenites exhibit four IR active fundamentals (Ti,,) in the vibrational spectra of normal

as well as inverse spinel ferrites., It has been reported that the first three IR bands are

79

due to the tetrahedral (Td) and octahedral (Oh) coordination compounds, while the

fourth one is due to some type of lattice vibrations involving tetrahedral cation [57].

The infrared spectra of Nio.s.Zn,Cuo.sFe~O~ mixed femte samples are shown

ln the Figure-2.15B for different concentrations. The absorption bands obtained in

the present investigation are found to be in the range reported for similar type of

fenites[58,22]. The position of the bands together with their shoulders is given in

the TABLE-2.8B.

From Figure-2.15B, it can be seen that the 1R spectra of the present system

exhibit five principal bands. The principal bands v , and v2 shift gradually towards

low frequency side mainly the high-frequency band(vl) and second absorption

hand(v2) are found to be in the range 670-660 and 580-555 cm" respectively. A

comparison of FTIR charts at different concentrations provides information about

decrease in transmittance and increase in broadness.

These features are explained on the basis of cation distribution in A and B sites

of mixed Ni-Zn-Cu fenites. Waldron [59] has attributed the occurrence of vl and v2

bands to the intrinsic vibrations of the tetrahedral (Td) and octahedral (Oh)

coordination compounds. Both these high frequency bands have been attributed to

the intrinsic lattice vibrations of E-symmetry. The third absorption band v3 is

attributed to the cu2+ - 0'. complexes at octahedral sites. The frequency of the

(band v~ ) bands depend on the mass of tetrahedral metal ion complexes[60] and

TABLE - 2.8B

Center frequency of the bands of FTIR spectra of

quenched N ~ o . ~ , Z ~ , C U O . ~ F ~ , O ~ system

hence it is attributed to the vibrations of ions at the tetrahedral site. The low

frequency bands are attributed to vibrstions of Tzr -Symmetry.

The cation distribution of quenched Ni-Zn-Cu ferrites is:

(zn2+ ,, ~ e " I.,, )A p i2 ' 0 . 5 . ~ zn2+, cu o.5 ~ e " I+x.y]R 02;. ....( 2.10)

Wherex=0.1,0.2,0.3,0.4and0.5andy=75%ofx.

As the content of Zn increases zn2' ions consistently replace ~ e j ' ions from A

to B site. At the same time ~ i ~ ' ions on the octahedral site decreases. This disturbs

the order on the octahedral site with increase in zinc. The disordered systems give

rise to broad bands in their spechum[61]. TABLE-2.8B shows high frequency band

in the range 670-660 cm 'I which is found to decrease to a small extent with the

Increase of concentration. Consequently the changes in the high frequency band vl

i s correlatable to Z ~ ' ~ - O ~ ' bond stretchng. The agreement in the value of frequency

of v , band with the literature value is illustrated in TABLE-2.8A. On the other hand

second absorption band vz (580-555 cm-I) which may be attributed for zn2'- 02- at

octahedral sites as it clearly indicates an appreciable decrease of frequency with

increase of Zn concentration. The sites A and B are explained to posses other ions

also in this mixed feniies along with Zn. But the shifting of the band frequency is

explained to be due to Zn ion concentration affecting the distribution of other ions.

Based on these observations it is accounted for by that the expansion of B sites is

larger than the expansion of A site. It is in agreement with the suggestion made out

of X-ray difiaction studies of the present samples. Third absorption band v3

R1

explains John-Teller effect due to the presence of cu2' ion in octahedral sites. The

presence of cu2" ions and the John-Teller splitting makes the companion ions of B

site to exist in their exited states. It means that Ni2' and ~ e " ions in B sites may be

shifted to Ni3' and ~ e j ' states. The explanation of this kind is in accordance with the

inference of conductivity measurements. Consequently these results are in support

of hopping of charge carriers from 3d bands of Ni and Fe ions. The variation of

activation energy with concentration(x) has established that the band gap of

localized 3d band widens. Widening of the band gap of 3d bands with x is

correlatable to the relative strength of Jahn-Teller splitting and crystal field in the

octahedral site of spinel femte.

It is seen £rom the TABLE-2.8A that the frequencies of the bands v4 and v5 are

found to be shifted from their corresponding values of end member femtes ferrites

namely NiFe204, CuFe204, ZnFe204 and Fes04. The larger difference of the

frequency of vs band found between end members and the present spinel femte may

be attributed to the presence of many ion complexes and their masses in B site.

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