J.S. Ghodake et al. / Journal of Alloys and Compounds 486 (2009) 830–834
833
Table 4b
Data on Curie temperature by dc resistivity and permeability measurements and kTc
values for ferrite system Zn0.55Ni0.40−xCoxFe2.05O4.
◦
x
Curie temperature Tc ( C) from
dc resistivity
Permeability
kTc (eV)
0
0
0
0
0
0
0
.00
.01
.02
.03
.04
.05
.06
175
170
165
160
155
150
140
177
170
167
163
161
150
142
0.039
0.038
0.038
0.038
0.037
0.037
0.036
Fig. 6. Variation of ac resistivity (ꢀac) with frequency for Zn0.55Ni0.40−xCoxFe2.05O4
composition.
dilution of A lattice magnetic moment and subsequent weakening
of AB super-exchange interaction. Similar observations have been
reported by Murthy et al. [34].
non-magnetic Zn2+ ion strongly prefers the occupation of A-sites
2
+
3+
[
28–32]. The electron hopping between Fe and Fe ions (n-
4. Conclusion
3+
2+
type) and hole hopping between Ni and Ni (p-type) on B-sites
are responsible for electric conduction and dielectric polarization
Nanocrystalline Ni–Co–Zn ferrites have been successfully syn-
2
+
2+
[
33]. As Co ion substitution increases replacing Ni ions, some
of Fe ions are likely to be forced to migrate from B-sites to A-
thesized by chemical co precipitation method, using oxalate
precursors. The phase formation of the sintered ferrite was con-
firmed by X-ray diffraction study. The resistivity was found to
3+
2+
3+
sites [34]. As a result, the number Fe and Fe ions increases
while the number of Ni3 and Ni ions decreases as Co ion sub-
stitution increases. Therefore, the local displacement (dielectric
polarization) in the direction of applied electric field (for electrons)
+
2+
2+
2+
increase with the doping of Co content in the Ni–Zn ferrite
matrix. The variation of resistivity is almost linear up to the
Curie temperature where a break occurs indicating a change of
magnetic ordering from ferrimagnetism to paramagnetism. The
values of activation energy in paramagnetic region are found to
be greater than those in ferrimagnetic region, which suggests that
the process of conduction is affected by the change in magnetic
ꢀ
2+
decreases. This explains the decrease in ε and tan ı as Co ion
substitution increases.
Fig. 6 shows the variation of ac resistivity (ꢀac) as a func-
tion of applied frequency measured at room temperature for
Zn0.55Ni
CoxFe2.05O4 series. In all the compositions of this
ꢀ
0.40−x
ordering. Dielectric constant (ε ) and dielectric loss tangent (tan ı)
series the ac resistivity decreases with increasing frequency and
then it remains invariant at high frequency (>100 kHz). From Fig. 6,
it is clear that in the low frequency region ac resistivity increases
with increase in the composition parameter x. The comparison
between lower frequency regions of Figs. 3 and 6 show opposite
trends for dielectric constant and ac resistivity with respect to the
composition.
2+
decreases with the addition of Co content in the mixed Ni–Zn
ferrites.
References
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The Curie temperature data for the compositions
Zn0.50Ni
CoxFe2.05O4 and Zn0.55Ni
CoxFe2.05O4 deter-
0.45−x
0.40−x
599.
mined from the dc resistivity and initial permeability
measurements are represented in Tables 4a and 4b. It was
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Co concentration for both the series. This can be explained on
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4
2
4
◦
Curie temperature of CoFe O (525 C) is less than that of NiFe O
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2
4
2
4
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[
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(
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2
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2
3+
exchange interaction from FeA –O –FeB
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words, the thermal energy required to offset the spin alignment
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[
[
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Table 4a
Data on Curie temperature by dc resistivity and permeability measurements and kTc
values for ferrite system Zn0.50Ni0.45−xCoxFe2.05o4.
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◦
x
Curie temperature Tc ( C) from
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[
[
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dc resistivity
Permeability
kTc (eV)
0
0
0
0
0
0
0
.00
.01
.02
.03
.04
.05
.06
225
215
210
200
185
180
175
223
216
210
199
187
180
175
0.043
0.042
0.042
0.041
0.040
0.039
0.039
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