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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,
185
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
186
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.
187
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
189
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.
190
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.
191
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.
192
(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,
193
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
194
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.
195
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
196
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
197
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.