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CHAPTER 7

SYNTHESIS AND CHARACTERIZATION OF COPPER

DOPED COBALT-NICKEL MIXED FERRITE

NANOPARTICLES BY CO-PRECIPITATION METHOD

7.1 INTRODUCTION

In recent years, magnetic properties of nanoparticles are of great

interest because of the development of high-density magnetic storage

media with nanosized constituent particles or crystallites (Kodama et al

1996, Aparna Roy et al 2004). Also, trivalent ion substituted spinel ferrites

have shown to be promising candidates for applications in high-density

magnetic recording and enhanced memory storage, magnetic fluids and

catalysts (Davidenko 2004).

Cobalt ferrite is a well-known hard magnetic material with

relatively high coercivity and saturation magnetization while nickel ferrite

is a soft material with low values of these parameters. Many of these hard

and soft magnetic properties make them very promising candidates for a

variety of applications in biomedical and electronic devices as well as in

recording technologies (Ross 2001, Wood et al 2002, Tartaj et al 2003,

Bader 2006). Among the spinel ferrites, CuFe2O4 exhibits switching and

changing semiconductive properties (Sawant & Patil 1982). Copper is very

interesting owing to its great number of coordination, octahedral,

pyramidal and tetrahedral and square planar. From the application point of

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view, the magnetic character of the nanoparticles depends crucially on size,

shape, purity and magnetic stability of these nanoparticles. These particles

should have single domain, pure phase, suitable coercivity and moderate

magnetization values. It is therefore very important to tailor the various

magnetic properties of these materials according to the requirements.

This can be done either by varying the sizes of these nanoparticles or by

adjusting the concentrations of soft and hard ferrites in these materials.

Several reports are available in the literature on the electrical conductivity,

dielectric and magnetic properties of bulk mixed Co-Ni ferrites. However,

any such reports are not available for the mixed Co-Ni ferrites prepared by

chemical method especially with the doping of copper. Hence, in the

present investigation, an attempt has been made to study the structural,

magnetic and dielectric properties of copper doped cobalt-nickel mixed

ferrite (Co0.4Ni0.4Cu0.2Fe2O4) nanoparticles synthesized by co-precipitation

method.

7.2 MATERIALS AND METHODS

Copper doped cobalt-nickel mixed ferrites

(Co0.4Ni0.4Cu0.2Fe2O4) nanoparticles are successfully prepared by co-

precipitation method. The desired composition is obtained by using

stoichiometric amounts of cobaltous chloride [CoCl2.6H2O], nickel

chloride [NiCl2.6H2O], cupric chloride [CuCl2.2H2O] and anhydrous ferric

chloride [FeCl3] dissolved in distilled water. The neutralization is carried

out with sodium hydroxide solution and the reaction temperature is

maintained at 60 C. The pH of the solution is maintained at 8 and it is

stirred for 2 hrs. The precipitate is thoroughly washed with distilled water

until it is free from impurities. The product is dried at a temperature of

100 C to remove the water contents. The dried powder is sintered at

130 C, 600 C and 900 C. These samples are characterized using FT-IR,

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XRD, SEM, EDX, TEM, VSM and LCRZ meter and the results are

discussed.

7.3 RESULTS AND DISCUSSION

7.3.1 FT-IR Spectral Analysis

The infrared absorption spectra of the cobalt-nickel-copper

mixed ferrites of composition Co0.4Ni0.4Cu0.2Fe2O4 sintered at 130 C,

600 C and 900 C respectively and recorded at room temperature in the

wave number range 4000 cm-1 - 400 cm-1 is as shown in Figure 7.1.

Figure 7.1 FT-IR spectra of Co0.4Ni0.4Cu0.2Fe2O4 nanoparticles

sintered at (a) 130 C (b) 600 C (c) 900 C

The appearance of the absorption band around 590 cm-1 is

attributed to the stretching vibration of tetrahedral sites and the depth

increases as the sample is sintered at 600 C. It is also observed that the

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depth decreases as the sample is sintered at 900 C and the subsidiary band

appears around 543 cm-1 as reported in Chapter 6 for nickel-copper mixed

ferrites. A small band appears around 649 cm-1 - 641 cm-1 as reported in

Chapter 4 for pure nickel ferrites as well as for nickel - copper mixed

ferrites. The band around 435 cm-1 - 427 cm-1 is attributed to the stretching

1 and

2 shift to lower values on sintering the samples. The absorption band

around 3600 cm-1 is assigned to the stretching vibration of the free or

absorbed water. The small absorption band around 2350 cm-1 is due to

traces of adsorbed or atmospheric CO2 as reported in Chapter 5 for cobalt-

copper mixed ferrites.

7.3.2 Structural Analysis

XRD patterns of cobalt-nickel-copper mixed ferrites of

composition (Co0.4Ni0.4Cu0.2Fe2O4) synthesized at pH equal to 8 and

sintered at 130 C, 600 C and 900 C are shown in Figure 7.2 (a, b & c)

respectively. The peaks are indexed by comparing the interplaner distance

with JCPDS data (JCPDS card no 22-1086, 74-2081 and 34-0425). All the

peaks of the samples can be indexed to the major peaks of the spinel

ferrites (220), (311), (222), (400), (422), (511), (440), (533), (622) and

(642) which are identifying the planes of a cubic unit cell, corresponding to

the spinel structure. The diffraction patterns show amorphous nature for the

sample sintered at 130 C. The degree of crystallinity of the sample

increases with increasing sintering temperature to 600 C and 900 C,

suggesting the enhancement of crystallinity due to sintering. No additional

peaks are found, ensuring phase purity.

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Figure 7.2 XRD pattern of Co0.4Ni0.4Cu0.2Fe2O4 nanoparticles

sintered at (a) 130 C (b) 600 C (c) 900 C

The average crystallite sizes of the synthesized samples are

determined by using the X-ray peak broadening of the (311) diffraction

peak, via the well-known Scherrer equation. The crystallite size is found to

be 12 nm for the sample sintered at 600 C and 32 nm for the sample

sintered at 900 C as shown in Table 7.1. The increase in crystallite size can

be attributed to fusing of two or more crystals because of the melting of

their surfaces due to sintering at higher temperature.

Table 7.1 Structural parameters of Co0.4Ni0.4Cu0.2Fe2O4 nanoparticles

Sintering Temp °C Lattice Parameter (a) Å Crystallite Size (D) nm

600 8.3237 12

900 8.3579 32

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The crystallite size is observed to increase in comparison with

that for Ni-Cu system (as reported in Chapter 6) but found to decrease in

comparison with that for Co-Cu system (as reported in Chapter 5).

This may be due to the fact that the larger ionic radius of Co2+ in

comparison to Ni2+ and also the inclusion of copper obstruct the grain

growth thereby causing reduction in the particle size.

The value of the lattice parameter of Co-Ni-Cu ferrite powders

is higher than that of NiFe2O4 and lower than that of CoFe2O4, which is

expected due to the bigger ionic radius of Co2+ ion when compared to that

of the Ni2+ ion as observed from Table 7.1. (Evgeny V. Rebrov et al 2011).

7.3.3 SEM Analysis

Figure 7.3 SEM micrographs of Co0.4Ni0.4Cu0.2Fe2O4 nanoparticles

sintered at 600 C

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SEM images of different magnifications for the sample cobalt-

nickel-copper mixed ferrite nanoparticles Co0.4Ni0.4Cu0.2Fe2O4 sintered at

600 C are shown in Figure 7.3. It can be seen that the sample is highly

dense and agglomerated.

7.3.4 EDX Analysis

Figure 7.4 EDX spectrum of Co0.4Ni0.4Cu0.2Fe2O4 nanoparticles

sintered at 600 C

EDX analysis as shown in Figure 7.4 gives the qualitative

composition of nanoparticles of cobalt-nickel-copper mixed ferrites

Co0.4Ni0.4Cu0.2Fe2O4 sintered at 600 C. It indicates the quantitative

presence of Co, Ni, Cu, Fe and O in the samples. It also confirms the

absence of impurities in these samples.

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7.3.5 TEM Analysis

Figure 7.5 HR-TEM micrographs of Co0.4Ni0.4Cu0.2Fe2O4

nanoparticles sintered at 600 C

HR-TEM images of the sample Co0.4Ni0.4Cu0.2Fe2O4 sintered at

600 C are shown in Figure 7.5. It is clear that the particles are almost

spherical in shape and dispersed uniformly with a crystallite size of

approximately 12 to 16 nm. However, a few elongated particles and

cuboids are also present in the samples. Some moderately agglomerated

particles as well as separated particles are also seen in the images.

The particle size obtained from TEM is found to be in good agreement

with that obtained from XRD analysis.

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7.3.6 SAED Analysis

Figure 7.6 SAED pattern of Co0.4Ni0.4Cu0.2Fe2O4 nanoparticles

sintered at 600 C

Electron diffraction pattern of Co0.4Ni0.4Cu0.2Fe2O4 sintered at

600 C is shown in Figure 7.6. The position and intensities of lines in the

diffraction pattern confirm the spinel structure. The nanoparticles are found

to be in well-crystallized form as shown by the SAED analysis.

7.3.7 Magnetic Measurements using VSM

The magnetic properties of nanoparticles of cobalt-nickel-

copper mixed ferrites Co0.4Ni0.4Cu0.2Fe2O4 are studied using a VSM at

room temperature and are shown in Figure 7.7 (a, b & c) for the samples

sintered at 130 C, 600 C and 900 C respectively.

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(a) (b)

(c)

Figure 7.7 VSM measurements for Co0.4Ni0.4Cu0.2Fe2O4

nanoparticles sintered at (a) 130 C (b) 600 C (c) 900 C

The saturation magnetization (Ms), remanent magnetization

(Mr), coercivity (Hc) and squareness ratio values are listed in Table 7.2.

It is observed that the saturation magnetization (Ms), remanent

magnetization (Mr) and coercivity (Hc) increase on sintering the samples at

600 C and 900 C. The saturation magnetization values are very much

affected by the increase in the sintering temperature as a consequence of

the gradual increase in the crystallinity and particle size. A similar

behaviour has been reported for other magnetic materials (Waje et al

2010). The phenomena can be explained on the basis that the samples relax

back their spins by rotation on the removal of an applied magnetic field,

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giving a nearly zero net magnetic moment (Waje et al 2011). The variation

of magnetization with crystallite size is also explained on the basis of

domain structure, diameter of particles and crystal anisotropy.

Since sintering temperature causes changes by decomposition or

transformation of phases, these in turn brings about increase in grain size,

change in pore shape, pore size and number. The starting materials, being

in nanosize are not likely to have any grain boundary. However, sintering

introduces the effects of grain boundary, in addition to some

microstructural defects. These forms a source of flux B, thereby resulting

in higher values of corecivity and retentivity (Waje et al 2011).

Table 7.2 The saturation magnetization (Ms), remanent

magnetization (Mr), coercivity (Hc) and squareness ratio

values of Co0.4Ni0.4Cu0.2Fe2O4 nanoparticles

Sintering

Temp °C

Saturation

Magnetization

(Ms) emu/g

Remanent

Magnetization

(Mr) emu/g

Coercivity

(Hc)

G

Squareness

Ratio

130 0.61226 0.021881 421.43 0.0357

600 38.807 15.733 1091.4 0.405

900 48.377 20.847 1151.3 0.4309

According to Neel’s sublattice theory, the net magnetic

moments in ferromagnetic ferrite materials depend on the number of

magnetic ions occupying the tetrahedral and octahedral sites. The values of

saturation magnetization (Ms), remanent magnetization (Mr) and coercivity

(Hc) are found to be higher than that of Ni-Cu ferrite system and lower than

that of Co-Cu ferrite system as evidenced from Chapters 6 and 5. This can

be attributed to the fact that the Co2+ B

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have a tendency to replace Ni2+ B in the

octahedral sites causing an increase of magnetic moment of this sublattice

which results in turn in the increase of the total magnetic moment Ms

(Chikazumi 1997). The increase in Ms may also be related to the method of

preparation and change in magnetic structure of the surface of the particles.

Surface effects become significant due to the large surface/volume ratio of

the nanoparticles (Nathani et al 2005). The magnetic ions have a magnetic

interaction mediated by the electrons in their common nonmagnetic

neighbours, which is more important than their direct exchange interaction.

The presence of copper ions in the form of impurities or the absence of

oxygen ions at the surface leads to the breakage of the super-exchange

bonds between the magnetic cations, introducing a large surface spin

disorder (Caizer & Stefanescu 2002, Mallapur & Chougule 2010).

The squareness ratio as observed from the Table 7.2 exhibits magnetostatic

interaction for the cobalt-nickel-copper ferrite system sintered at 130 C,

600 C and 900 C.

7.3.8 Dielectric Properties

frequency (f) in the frequency range 100 Hz - 5 MHz for cobalt-nickel-

copper mixed ferrites Co0.4Ni0.4Cu0.2Fe2O4 nanoparticles sintered at 130 C,

600 C and 900 C is shown in Figure 7.8.

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Figure 7.8 Variation of dielectric constant with respect to log

frequency for Co0.4Ni0.4Cu0.2Fe2O4 nanoparticles

It is observed from the Figure 7.8 that the dielectric constant

decreases as the frequency increases. The decrease is rapid at lower

frequencies showing dispersion in the lower frequency region and remains

constant at higher frequencies. The decrease takes place when the jumping

frequency of electric charge carriers cannot follow the alternation of

applied AC electric field beyond a certain critical frequency. Dielectric

dispersion in ferrite can be explained on the basis of space charge

polarization, which is a result of the presence of higher conductivity phases

as grains and the insulation of matter as grain boundaries of a dielectric,

causing localized accumulation of charges under the influence of an

electric field (Mangalaraja et al 2002). The samples show dispersion due to

Maxwell Wagner type interfacial polarization in agreement with Koops

phenomenological theory (1951). The large value of dielectric constant at

lower frequencies is attributed to different types of polarization such as

electronic, atomic, interfacial and ionic and as frequency increases, the

contribution of ionic and orientation sources of polarizability decreases and

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finally disappears due to inertia of the molecules and ions. The mechanism

of this dielectric polarization may also be attributed to the dipoles resulting

from the change in valence of cations, such as Fe3+ / Fe2+ ions.

The polarization at lower frequencies may result from the electron hopping

between Fe3+ / Fe2+ ions in the ferrite lattice (Mallapur et al 2009). It is

further observed that on sintering the sample at 600 C and 900 C the

dielectric constant decreases drastically and reaches a constant value.

This may be attributed to the decrease in concentration of Fe2+ / Fe3+ ion

pairs at the B-site. The dielectric constant value is found to be greater than

that of Co-Cu system and Ni0.8Cu0.2Fe2O4 but lesser than that of

Ni(1-x)CuxFe2O4 (x = 0.4, 0.6) system.

The variation of dissipation factor as a function of frequency in

the range from 100 Hz to 5 MHz at room temperature for nanoparticles of

cobalt-nickel-copper mixed ferrites Co0.4Ni0.4Cu0.2Fe2O4 nanoparticles

sintered at 130 C, 600 C and 900 C is shown in Figure 7.9.

Figure 7.9 Variation of dissipation factor with respect to log

frequency for Co0.4Ni0.4Cu0.2Fe2O4 nanoparticles

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The dissipation factor decreases with increasing frequency

followed by the relaxation peak between the frequency 30 KHz to 1 MHz

for the sample sintered at 130 C. The appearance of relaxation peak can be

explained according to the Debye relaxation theory (Tridevi et al 2005).

It is further observed from the Figure 7.9 that on sintering the samples at

600 C and 900 C no relaxation peak appears, this may be attributed to the

high resistivity and the dissipation factor approaches very low values at

higher frequencies.

7.4 CONCLUSION

Copper doped cobalt-nickel mixed ferrites

(Co0.4Ni0.4Cu0.2Fe2O4) nanoparticles are prepared by co-precipitation

method and sintered at 130 C, 600 C and 900 C. The FT-IR spectra

confirm the absorption band around 590 cm-1 for the tetrahedral site and

around 435 cm-1 - 427 cm-1 for octahedral sites 1 2 shift to

lower values on sintering the samples. The XRD spectra reveal the average

crystallite size to be in the range 12 nm - 32 nm and it is found to be in

agreement with TEM results. The co-doping of three metal ions in spinel

ferrite decreases the crystallite size and it could be evidenced from XRD

analysis and TEM results. SEM images show the sample is highly dense

and agglomerated with a large number of pores. EDX analysis confirms the

presence of Co, Ni, Cu, Fe and O in the samples without any impurities.

The saturation magnetization (Ms), remanent magnetization (Mr),

coercivity (Hc) and squareness ratio increase with increase in sintering

temperature. The highest saturation magnetization and coercivity values

are found for these mixed ferrites. The dielectric constant decreases with

increase in frequency and reaches a constant value at higher frequencies.