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CHAPTER 4

EFFECT OF CADMIUM SUBSTITUTION IN

NICKEL COBALT FERRITES

4.1 INTRODUCTION

The effect of cadmium substitution in ferrite was done by many

people like Kamilov et al (1979), who substituted nickel in cadmium ferrite

and observed that the spontaneous magnetization disappears at 230 K.

Mossbauer studies, also confirmed the same. Upadhyay and Kulkarni (1983)

studied the cation distribution in A and B site by substituting Cd2+ in

MnFe2O4. Rajesh Iyer et al (2009) interpreted cation distribution from FTIR

studies in Mn2+ doped cadmium Fe2O4 by co-precipitation method. Saha and

Hakim (2008) reported Cadmium substitution in Ni-Co perminvar ferrite.

They found that the dielectric constant of the prepared compound in bulk

form increases to 3.5 times when cadmium is substituted in nickel cobalt

ferrite. This leads to the present work to study the effect of cadmium in nickel

cobalt spinel nanoferrite, hence nanoparticles of CdxNi(0.5-x)Co0.5Fe2O4 were

synthesized by co-precipitation method. The structural, magnetic and

dielectric properties of the prepared compound are discussed in this chapter.

4.2 PREPARATION OF CdxNi(0.5-x)Co0.5Fe2O4 NANOPARTICLES

Nanoparticles of CdxNi(0.5-x)Co0.5Fe2O4 (x = 0.0 to 0.3) were

synthesized by chemical co-precipitation method. Analytical grade FeCl3,

CoCl2.6H2O, 3CdCl2.8H2O, NiCl2.6H2O and NaOH.6H2O were the starting

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materials. Stoichiometric proportions of aqueous solutions of FeCl3, CoCl2,

CdCl2 and NiCl2 are taken separately and dissolved in distilled water. Then

the solutions are mixed thoroughly using magnetic stirrer at 80 0C. It is then

transferred immediately into a boiling solution of NaOH. After pouring the

solution in alkaline NaOH, a precipitate is formed immediately and the entire

solution is stirred using mechanical stirrer for about 60 minutes until the

reaction is complete. The pH of the solution is maintained at 12 throughout

the reaction. Washing and filtering are repeated with deionised water until

the pH of the solution becomes neutral and finally the sample is dried in a

heating oven for about 5 hours. The dried powder is grounded using agate

mortar; a portion of the material is pelletized at 2 ton pressure for 3 minutes.

The pellet and powder are then sintered in furnace at 550 0C for 5 hours. The

pelletized samples were painted on either side with silver paste followed by

heating the samples upto 2000C for 1 hour to ensure good electrical contact.

4.3 STRUCTURAL ANALYSIS

4.3.1 XRD-analysis

4.3.1.1 Lattice parameter of CdxNi(0.5-x)Co0.5Fe2O4 nanoparticles

Figure 4.1 shows the X-ray diffraction pattern of

Ni(0.5-x)CdxCo0.5Fe2O4 (x = 0.0, 0.1, 0.2, 0.3). All diffraction peaks match very

well with those of the spinel ferrite type structure. The peak broadness can be

attributed to the nanocrystalline nature of the prepared samples. All the peaks

in the diffraction pattern have been indexed and the refinement of the lattice

parameter done using powderX software. The average crystallite size for each

composition were calculated from XRD line width of the (311) peak using

Scherrer equation t=0.9 cos (where t is the crystalline size, is the

wavelength of the Cu K ( =1.5406 Å) and is the FWHM of the XRD peak

at 2 . The values of lattice parameter ‘a’ and particle size as deduced from the

X-ray data are given in Table 4.1. The lattice parameter for Co0.5Ni0.5Fe2O4 in

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the present investigation is 8.354 Å. However the values are found to be

slightly less than the reported lattice parameter value by Sonal Singhal et al

(2005). The values for the reported compound are Co0.6Ni0.4Fe2O4 (8.3682 Å)

and Co0.4Ni0.6Fe2O4 (8.3566 Å) in the particle size range 76nm.

Figure.4.1 XRD patterns for the composition Ni(0.5-x)CdxCo0.5Fe2O4

Table 4.1 Lattice parameter and average particle size for the

Ni(0.5-x)CdxCo0.5Fe2O4 compound

CompositionLattice parameter

(Å)

Average particle size

(nm)

Co0.5Ni0.5Fe2O4 8.354(1) 10.5

Cd0.1Ni0.4Co0.5Fe2O4 8.385(2) 10.3

Cd0.2Ni0.3Co0.5Fe2O4 8.405(2) 9.5

Cd0.3Ni0.2Co0.5Fe2O4 8.425(1) 8.5

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However this small decrease in lattice parameter is due to smaller

particle size of 10.5 nm when compared with the particle size 76nm for the

compound reported by Sonal Singhal et al (2005). The lattice parameter for

the prepared compound was found to increase with increase in cadmium

content for all the samples. This can be explained on the basis of cation

stoichometry. The ionic radius of cadmium ion (0.97 Å) is greater than ionic

radius of nickel (0.79 Å); replacement of larger cadmium ions instead of

smaller Ni ions leads to increase in lattice constant obeying Vegard’s law

(Denton and Ashcroft 1991).

4.3.1.2 Cation Distribution

The distribution of divalent and trivalent cations among octahedral

and tetrahedral sites in the Ni(0.5-x)CdxCo0.5Fe2O4 compound was determined

from the ratio of X-ray diffraction intensities I(220)/I(400), I(220)/I(440) and

I(400)/I(440) and given in Table 4.2. It was found that there is a deviation in

cation preferences in nanosized particles when compared with bulk particles.

Usually Cd2+ and Fe3+ occupies A-site and Co2+, Ni2+ and Fe3+ and Fe2+

occupies B-site, but in the nanoscale some of the nickel, cobalt and cadmium

occupy A-site and B-site leading to a metastable state and statistically

disordered cation distribution.

4.3.2 Electron microscopic studies

4.3.2.1 Transmission electron microscopic analysis for the compound

Ni0.3Cd0.2Co0.5Fe2O4

The particle size and morphology of the sample

Ni(0.5-x)CdxCo0.5Fe2O4 with x = 0.2 is shown in figure 4.2. The average

particle size is around 10 nm. TEM analysis revealed that the particles are

nearly spherical. The particle size determined from TEM was found to be in

close agreement with that obtained from XRD studies.

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Table 4.2 Cation distribution analysis for as prepared

Ni(0.5-x)CdxCo0.5Fe2O4 compound

A- Site B- Site

I(220)/

I(440)

I(220)/

I(400)

I(400)/

I(440)

obs cal obs cal obs cal

x = 0.0

Ni2+0.8

Co2+0.38Fe

3+0.54

Ni2+0.42

Co2+0.12Fe3+

1.18Fe2+0.28

0.66 0.65 1.21 1.18 0.54 0.52

x = 0.1

Cd2+0.02Ni

2+0.04Co2+

0

.38Fe3+0.56

Cd2+0.08

Ni2+0.36

Co2+0.12Fe3+

1.19Fe2+0.25

0.67 0.66 1.34 1.32 0.50 0.49

x = 0.2

Cd2+0.02Ni

2+0.02Co2+

0

.38Fe3+0.58

Cd2+0.18Ni2+

0.28 Co2+0.12

Fe3+1.2Fe2+

0.

22

0.70 0.68 1.38 1.34 0.50 0.48

x = 0.3

Cd2+0.01Ni

2+0.01Co2+

0

.38Fe3+0.6

Cd2+0.29Ni2+

0.19Co2+0.12F

e3+1.3Fe2+

0.1

0.63 0.62 1.42 1.41 0.44 0.42

Figure 4.2 TEM images for the composition Cd0.2Ni0.3Co0.5Fe2O4

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4.3.2.2 Surface morphology and compositional analysis

Figure 4.3 SEM images and EDX spectra of Ni(0.5-x)CdxCo0.5Fe2O4

(a)

(b)

(c)

(d)

(e)

(f)

(g)

(h)

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The structural features and elemental composition of the powder

samples coated with gold to posses good conductivity were observed by using

Hitachi S-3400 scanning electron microscope with energy dispersive X-ray

analysis setup. The SEM images and EDX spectrum of the samples are shown

in Figure 4.3 (a-h). The SEM images show that the materials are smaller in

size in the nanoregion. Chemical analyses through EDX were also carried out

on powder samples Ni(0.5-x)CdxCo0.5Fe2O4 with x = 0 to 0.3 compounds.

The EDX analysis in Figure 4.3 (e-h) reveals that the wt% of nickel

and cadmium for the powder sample is 13.29, 10.45, 6.97, 5.04 and 4.71,

8.66, 12.22, respectively. This shows the decreasing trend in nickel and

increasing behavior of cadmium. The wt% of iron and cobalt remain almost

constant. The EDX spectrum of the samples shows that all the samples are of

nearly expected elemental composition. It also reveals the presence of the

substitution elements Ni, Co, Cd, Fe and Oxygen for the compound

Ni(0.5-x)CdxCo0.5Fe2O4 (x = 0 to 0.3).

4.3.3 Fourier Transform Infrared Studies for CdxNi(0.5-x)Co0.5Fe2O4

FTIR spectra recorded for the nanocrystalline CdxNi(0.5-x)Co0.5Fe2O4

(x=0.0, 0.1, 0.2, 0.3) are shown in Figure 4.4. The presence of bands in the

range 591 cm-1 is due to the tetrahedral vibration '. The readings are taken in

the range from 450 to 4000 cm-1 but the octahedral vibrations " are observed

in the range of 420cm-1 since wave number below 450cm-1 is absent in this

studies. Similar observation has been reported in literature by Brabers (1969).

The presence of tetrahedral vibrations indicates the formation of ferrite phase.

The band at 590 cm-1 corresponds to intrinsic stretching vibrations of the

metal ion at the tetrahedral site.

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Figure 4.4 FTIR spectrometer graph for the composition

Ni(0.5-x)CdxCo0.5Fe2O4

4.4 MAGNETIC STUDIES

4.4.1 Variation of magnetic moment with applied field for

Ni(0.5-x)CdxCo0.5Fe2O4 nanoparticles at room temperature

Figure 4.5 shows the hysteresis loop curves for the samples

Ni(0.5-x)CdxCo0.5Fe2O4 (x=0.0 to 0.3). As the normal behavior of ferrite the

magnetization increases with the increase in applied magnetic field and attains

saturation magnetization MS after 17 KOe. It is noted that the saturation

magnetic moment decreases with the increase in Cd concentration from 25.5

to 20.39 emu/g. Amin Azizi et al (2010) reported 25 emu/g at 10 KOe in Ni-

Co ferrite. Similar observation is observed in the present investigation.

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Figure 4.5 Hysteresis loop at room temperature for the composition

Ni(0.5-x)CdxCo0.5Fe2O4

The decrease in maximum magnetization with concentration is

explained from the cation distribution derived from X-ray analysis. The

increase in the concentration of x (i.e., Cd2+ ion with 0 B) at Ni-site with 2 B

forces Fe3+ (with 5 B) from B site to A site, hence decrease in the saturation

magnetization values at B-site. This gradual decrease in the saturation

magnetization is attributed to the substitution of non-magnetic Cd2+ ion at

Ni2+ ion in A site which decreases the AB interactions and hence decrease in

the saturation magnetization according to the relation MS=MB-MA. The

saturation magnetization values also find similar trend as that of the particle

size, the decrease in saturation magnetization is due to decrease in particle

size for all the compositions. Similar behavior was reported in

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Mg0.95Mn0.05Fe2O4 by Sharma (2007) and also by Ichiyanagi (2005). In

addition to cation distribution there exists random spin canting of particles,

that the asymmetric environment of the spin creates antiferromagnetic

interaction; thus, the saturation magnetic field is less than the bulk materials

(Coey 1971). Pankhurst and Pollard (1991) claimed that the lower

magnetization values are due to non-saturation effects because of random

distribution of the small particles with enhanced values of magneto-crystalline

anisotropy.

Size effect in nanoparticles causes a reduction in the magnetization

value in comparison with the bulk counterpart. Smaller grains have larger

surface to volume ratio. Spin disorder at the surface of the nanoparticles

increases, especially when the surface to volume is large. According to the

core-shell morphology of nanoparticles, the core consists of ferrimagnetically

aligned spins and a surface with disordered spins. The core magnetic moment

aligns with the applied magnetic field and gets saturated at a particular value

of magnetic field. Any further increase in magnetic field has an effect only on

the surface causing a slowdown of magnetization. This is the reason all our

samples do not reach saturation magnetization under an applied field of

17 KOe.

4.4.2 Temperature dependence magnetic effect on

Ni(0.5-x)CdxCo0.5Fe2O4 nanoparticles at constant field 500Oe.

Temperature dependence of magnetization at constant applied field

500 Oe for the Ni(0.5-x)CdxCo0.5Fe2O4 (x = 0.0, 0.1, 0.2, 0.3) samples are

shown in Figure 4.6. Magnetization decreases with increase in temperature.

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Figure 4.6 M3-T curve for the composition Ni(0.5-x)CdxCo0.5Fe2O4

The Tc values of the samples were deduced from the relation of the

spontaneous magnetization M (Tc – T) with = 1/3 (Belayachi et al 1998;

Bhowmik 2006). There is a systematic decrease in the value of the Curie

temperature (Tc) as the cadmium concentration increases except for the

Cd content x = 0.1. This is due to the substitution of the non-magnetic Cd2+ in

place of Ni2+ ion. The deviation of Tc for the Cd content x = 0.1 may be due to

variation in cation distribution. In Ni0.5Co0.5Fe2O4 ferrite Ni occupies B-site,

the substitution of larger size Cd2+ instead of Ni2+ ions occupies A-site. The

value of the Tc is found to be higher than that of its bulk counterpart. The

change in Tc may be positive or negative depending on the grain boundaries,

geometry and interaction. It is also possible that Tc decreases due to some

unknown surface effect. For nanosized particles, a significant fraction of

atoms is on the surface and hence the difference in average Curie temperature.

The values of the variation of the magnetization are shown in the Table 4.3.

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Table 4.3 Magnetic and Dielectric parameter for the

Ni(0.5-x)CdxCo0.5Fe2O4 compounds

Composition

Average

particle

size

nm

Saturation

magnetization

(emu/g)

MS

Curie

temperature

Tc (K)

Dielectric

constant

at 1 kHz

'

Co0.5Ni0.5Fe2O4 10.5 25.5 515 15.11407

Cd0.1Ni0.4Co0.5

Fe2O410.3 25.29 413 27.66978

Cd0.2Ni0.3Co0.5

Fe2O49.5 23.33 497 35.13088

Cd0.3Ni0.2Co0.5

Fe2O48.5 20.39 385 48.85256

4.5 DIELECTRIC STUDIES OF Ni(0.5-x)CdxCo0.5Fe2O4

NANOPARTICLES

Figures 4.7 and 4.8 show the variation of dielectric constant ( ') and

dielectric loss factor tan ( ) as a function of applied angular frequency ( ) for

the composition Ni(0.5-x)CdxCo0.5Fe2O4 (x = 0.0, 0.1, 0.2 and 0.3) respectively

at room temperature 303 K. It is seen from Figure 4.7 that the dielectric

constant for Ni0.5Co0.5Fe2O4 is lower than the corresponding bulk value at

room temperature. Kambale et al (2009) have reported a dielectric constant

of 200-400 at 300 K, measured at a frequency of 1 kHz for Ni-Co in bulk,

prepared by double sintering ceramic method. In the present investigation a

dielectric constant of 15 is obtained, at the frequency of 1 kHz at 303 K. This

low value of dielectric constant is attributed to homogeneity, better symmetry,

uniform and smaller grains. Smaller grains contain large surface boundaries

and are the regions of high resistance. This reduces the interfacial

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polarization, which decreases the dielectric constant in the present

investigation as compared to the bulk material.

Figure 4.7 Variation of dielectric constant with frequency for the

composition Ni(0.5-x)CdxCo0.5Fe2O4

Figure 4.7 shows that the value of dielectric constant decreases as

frequency increases describing usual dielectric dispersion which is due to

Maxwell and Wanger type interfacial polarization. The decrease is quite

rapid at low frequencies and becomes quite slow at higher frequencies.

The decrease in dielectric constant at higher frequency can be

explained on the basis of Maxwell and Wagner theory, which is a result of the

homogeneous nature of dielectric structure. This dielectric structure is

supposed to be composed of two layers, where first layer is the large ferrite

grains of fairly well conducting materials which is separated by the second

thin layer (grain boundaries) of relatively poor conducting grain boundaries.

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The electrons reach the grain boundary through hopping and if the resistance

of the grain boundaries is high enough, electrons pile up at the boundaries and

produce polarization. However as the frequency of the applied field is

increased beyond a certain value, the electrons cannot follow the applied

alternating field. This decreases the probability of electrons reaching the

grain boundary and as a result polarization decreases.

Figure 4.8 Variation of dielectric loss with frequency for the

composition CdxNi(0.5-x)Co0.5Fe2O4

The composition dependence of dielectric constant is observed to

decrease with increase in cadmium concentration for all the samples. In Cd-

Ni-Co ferrites Cd2+ ions occupies B-site instead of Ni2+ ions in B-site, while

Fe ion occupies both A and B sites. When Cd2+ is replaced in place of Ni2+, it

increases the migration of Fe3+ from B-sites to A-sites, thereby reducing the

electron hopping between Fe2+ and Fe3+ ions. This is the reason for increase in

polarization and hence increases in dielectric constant.

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The variation of dielectric loss factor (tan ) is shown in Figure 4.8,

it is observed that the loss tangent decreases initially with frequency followed

by appearance of the relaxation peak. The appearance of relaxation peak can

be explained according to the Debye relaxation theory. The loss peak occurs

when the applied field is in phase with the dielectrics and the condition = 1

is satisfied, where = 2 ƒ, ƒ is the frequency of the applied field, Navneet

singh et al (2010) have observed a similar relaxation at a frequency of 1 KHz

for Cd substituted Ni-Co ferrites synthesized by the co precipitation. The

shifting of relaxation peak towards lower frequency side with increase in

cadmium content (x) is due to strengthening of the dipole-dipole interactions

causing hindrance to the rotation of the dipoles. Hence the resonance between

rotation of the dipoles and applied field takes place at lower frequency.

Figure 4.9 Variation of a.c conductivity with frequency for the

composition Ni(0.5-x)CdxCo0.5Fe2O4

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Figure 4.9 shows the variation of ac conductivity log( ac) with

frequency at 308 K. It is seen that the ac conductivity increases with increase

in frequency and increase in cadmium ion concentration. The conduction

mechanism can be explained on the basis of hopping in B-site between the

charge carriers Fe2+ and Fe3+. The increase in ac with increase in frequency of

the applied field increases the hopping mechanism and hence there is an

increase in ac conductivity. The increase in ac with the increase in cadmium

concentration may be due to the decrease in Fe2+ in B-site. The decrease in

Fe2+ in B-site increases the hopping between the charge carriers Fe2+ and Fe3+

and hence there is an increase in conductivity ac with increase in cadmium

concentration. The decrease in the Fe2+ in B-site is also observed in the cation

distribution, analyzed from XRD data.

A consolidated summary of the above studies is given in chapter 6.