Materials Sciences and Applications, 2011, 2, 226-236
doi:10.4236/msa.2011.24029 Published Online April 2011 (
Copyright © 2011 SciRes. MSA
Direct and Alternate Current Conductivity and
Magnetoconductivity of Nanocrystalline
Cadmium-Zinc Ferrite below Room Temperature
Somenath Ghatak1, Goutam Chakraborty1, Monika Sinha2, Swapan Kumar Pradhan2,
Ajit Kumar Meikap1*
1Department of Physics, National Institute of Technology, Deemed University, Durgapur, India; 2Department of Physics, University
of Burdwan, Burdwan, India
Received December 15th, 2010; revised January 4th, 2011; accepted March 4th, 2011.
Nanocrystalline cadmium-zinc ferrite samples were prepared by ball milling method and its electrical transport prop-
erty were investigated within a temperature range 77 K T 300 K in presence of a magnetic field up to 1 T and in a
frequency range 20 Hz to 1 MHz. The investigated samples follow a simple hopping type charge transport. The dc mag-
netoconductivity has been explained in terms of orbital magnetoconductivity theory. The alternating current conductiv-
ity follows the universal dielectric response σ/(f)
Tnfs. The values of ‘s’ have a decreasing trend with temperature. The
temperature exponent ‘n’ depends on frequency. The dielectric permittivity of the samples depends on the grain resis-
tance and interfacial grain boundary resistance. The ac magnetoconductivity is positive which can be explained in
terms of impedance of the sample.
Keywords: Nanostructures, X-Ray Diffraction, Electrical Properties, Dielectric Properties
1. Introduction
The potential application and unusual properties of
nanocrystalline materials has made them an object of
interest for many researches. The unique properties of
these classes of materials come due to the presence of its
atoms at the grain boundaries or interfacial boundaries in
comparison to coarse grained polycrystalline counter-
parts. Nanocrystalline spinel ferrite is a group of techno-
logically important nanomaterials that has a potential
application in magnetic, electronic and microwave fields
[1-5]. Due to their relatively insulating behaviour, they
are used as high frequency magnetic materials [6]. In
various fields like magnetic recording medium, informa-
tion storage, colour imaging, bio-processing magnetic
refrigeration and magneto optical devices [7-9], the use
of nanoferrites has made them a very important material
for industrial application where as the reduction of parti-
cle size to nanometre scale level, different new mecha-
nism like super paramagnetism, quantum mechanical
tunnelling, spin canting etc. makes them interesting
among scientists to study their transport properties
The general formula of ferrite are represented as
Fe O, where M is a divalent metallic ion. The
spinel structure has a unit cell that consists of a cubic
close-packed array of 32 oxygen ions with 64 tetrahedral
sites (T sites) and 32 octahedral sites (O sites); but only
eight of the T sites and 16 of the O sites are filled. A
large number of investigations on structural and magnetic
properties like magnetisation measurement, Mossbauer
spectroscopy, neutron scattering etc. have been done on
the spinel oxide nanoparticles over the last few years
[10-23]. The dielectric behaviour on high energy ball
milling ultrafine Zinc ferrite above room temperature
was reported by Shenoy et al. [17]. They suggested that
the defects caused by the milling, produce traps in the
surface layer contributes to dielectric permittivity via
spin polarised electron tunnelling between grains. Rav-
inder et al. [24-26] studied the electrical conductivity of
cadmium substituted manganese ferrite and cadmium
substituted copper ferrites and nickel ferrites above room
temperatures. The electrical conduction in these ferrites
was explained on the basis of the hopping mechanism.
Direct and Alternate Current Conductivity and Magnetoconductivity of Nanocrystalline Cadmium-Zinc Ferrite 227
below Room Temperature
They show a transition near the Curie temperature in the
conductivity versus temperature curve and also suggested
the activation energy in the ferromagnetic region is in
general less than that in the paramagnetic region. But a
systematic analysis on the electron transport mechanism
of cadmium substituted zinc ferrite below room tem-
perature is still lacking.
Thus, the detailed electrical transport properties like ac
and dc conductivity, ac and dc magnetoconductivity and
dielectric properties of nanocrystalline Cd-Zn ferities re-
ported within a temperature range 77 to 300 K and a fre-
quency range 20 Hz to 1 MHz in presence as well as ab-
sence of a magnetic field up to 1 T.
2. Experimental
Accurately weighed starting powders of CdO (M/S
Merck, 98% purity), ZnO (M/S Merck, 99% purity) and
α-Fe2O3 (M/S Glaxo, 99% purity) taken in 0.5:0.5:1 mol%
were hand-ground by an agate mortar pestle in a doubly
distilled acetone medium for more than 5 h. The dried
homogeneous powder mixture was then termed as un-
milled (0 h) stoichiometric homogeneous powder mix-
ture. A part of this mixture was ball milled for 3 h, 8 h,
20 h and 25 h duration at room temperature in air in a
planetary ball mill (Model P5, M/S Fritsch, GmbH,
Germany), keeping the disk rotation speed = 300 rpm
and that of the vials~450 rpm respectively. Milling was
done in hardened chrome steel vial of volume 80 ml us-
ing 30 hardened chrome steel ball of 10 mm dia, at ball
to powder mass ratio 40:1.
The X-ray powder diffraction profiles of unmilled and
all ball milled samples were recorded (step size = 0.02˚
(2), counting time = 5 s, angular range = 15˚ - 80˚ 2)
using Ni-filtered CuKα radiation from a highly stabilized
and automated Philips generator (PW1830) operated at
40 kV and 20 mA. The generator is coupled with a Phil-
ips X-ray powder diffractometer consisting of a PW 3710
mpd controller, PW1050/37 goniometer and a propor-
tional counter.
The Rietveld’s analysis based on structure and micro-
structure refinement method [27-31] has been employed
which is considered to be the best method for micro-
structure characterization and quantitative estimations of
multiphase nanocrystalline material containing several
number of overlapping reflections of ferrite phase for
both the unmilled and ball milled powder sample.
The electrical conductivity of the samples was meas-
ured by a standard four probe method by using 81/2-digit
Agilent 3458 multimeter and 6514 Keithley Electrometer.
The ac measurement was carried out with a 4284A
“Agilent Impedance” analyzer up to the frequency 1
MHz at different temperatures. Liquid nitrogen cryostat
was used to study the temperature dependent conductiv-
ity by the ITC 502S Oxford temperature controller. To
measure the ac response, samples were prepared as 1cm
dia pellets by pressing the powder under a hydraulic
pressure of 500 MPa. Fine copper wires were used as the
connecting wire and silver paint was used as coating ma-
terials. The experimental density of the pressed pellets
has been calculated from the relation ρexp = m/r2h,
where, m, r and h are mass, radius and thickness of the
pellet respectively. The measured density lies in the
range 5.12 g/cc to 6.38 g/cc. The percentage of error in
determining the density is very small (0.21%). On the
other hand density has been calculated from X-ray data
by using the relation ρtheo = 8M/NAVcell, where, M is the
molar mass of the sample, NA is the Avogadro’s number
and Vcell is the unit cell volume. It is observed that the
dexp is 80% to 84% of dtheo for the investigated samples.
The capacitance (CP) and the dissipation factor (D) were
measured at various frequencies and temperatures. The
real part of ac conductivity, real and imaginary part of
dielectric permittivity have been calculated using the
relations σ/(f) = 2fε0ε//(f), ε/(f) = CPd/ε0A and ε//(f) =
ε/(f)D respectively, where ε0 = 8.854 10–12 F/m, A and
d are the area and thickness of the sample respectively.
CP is the capacitance measured in Farad; f is the fre-
quency in Hz. The magnetoconductivity was measured in
the same manner varying the transverse magnetic field B
1 T by using an electromagnet.
3. Results and Discussion
The XRD powder patterns recorded from unmilled and
ball milled powder mixture of CdO, ZnO and α-Fe2O3
are shown in Figure 1. The powder pattern of unmilled
mixture contains only the individual reflections of ZnO,
CdO and α-Fe2O3 phases. The intensity ratios of individ-
ual reflections are in accordance with the stoichiometric
composition of the mixture. It is evident from the figure
that the particle size of the starting materials reduces very
fast as their peaks become broadened rapidly in the
course of milling. The ferrite reflections were appeared
clearly in 3 h milled sample and the content of ferrite
phase increases continuously with milling time as noticed
up to 25 h of milling. The CdO phase was not used up
completely in the process even after 25 h of milling,
whereas the other two starting phases vanishes com-
pletely in the course of milling. This indicates to the fact
that the formed ferrite phase is a non-stoichiometric in
composition. There must be a number of vacancies in the
tetrahedral sites of the spinel ferrite lattice due to these
unreacted Cd2+ ions.
Figure 2 shows the Rietveld’s fitting outputs of un-
milled and all ball milled powder patterns. Peak positions
opyright © 2011 SciRes. MSA
Direct and Alternate Current Conductivity and Magnetoconductivity of Nanocrystalline Cadmium-Zinc Ferrite
below Room Temperature
20 30 40 50 60 70 80
(1010) (201)
(200) (311)
(110) (220)
(113) (200)
Intensit y(a r b. un it )
Figure 1. X-ray diffraction patterns of unmilled (0 h) and
ball-milled CdO + ZnO + α-Fe2O3 powder mixture for dif-
ferent durations of ball-millin g.
of all reflections of all four phases are marked and shown
at the bottom of the plot. Residual of fitting (I0-Ic) be-
tween observed (I0) and calculated (Ic) intensities of each
fitting is plotted under respective patterns. In the present
analysis, all recorded XRD patterns of ball milled sam-
ples were fitted well only with normal spinel ferrite
20 30 40 50 60 70 80
(Z n,Cd )Fe2O4
Inte ns ity(ar b.unit )
Figure 2. Observed () and calculated (-) XRD patterns of
unmilled (0 h) and different ball-milled samples revealed
from Rietveld’s powder structure refinement analysis.
Residues of fittings are shown under the respective patters.
Peak positions of the phases are shown at the base line.
phase. It indicates that there are tetrahedral vacancies in
normal spinel structure of (Zn,Cd) ferrite which are sup-
posed to fill with Cd2+ ions.
Figure 3 shows the variation of relative phase abun-
dances of different phases with increasing milling time.
The content (mole fraction) of ZnO phase decreases very
rapidly and after 3 h of milling it becomes almost nil,
whereas the variation of CdO content shows that initially
the phase was utilized rapidly in ferrite phase but at the
higher milling time, the rate of inclusion becomes very
slow and till 25 h milling the phase was not completely
incorporated in the ferrite matrix and ~0.04 mole fraction
of the phase remained unreacted. The α-Fe2O3 phase con-
tent decreases in a moderate rate and after 8 h of milling it
almost vanishes. The ferrite phase content increases
sharply up to 8 h of milling and then approaches towards a
saturation to ~0.96 mole fraction (Ta ble 1). It may be no-
ticed that the ferrite content increases considerably until
the α-Fe2O3 phase was used up completely and after that a
slight increment up to 25 h milling is due to a very slow
diffusion of Cd2+ ions into the ferrite matrix. All these
variations in contents indicate that Zn2+ ions have occupied
the tetrahedral positions quite rapidly but the Cd2+ ions
took longer time and even after 25 h of milling some tet-
rahedral positions remained vacant due to insolubility of
CdO in ferrite matrix. It is therefore obvious that the pre-
pared ferrite phase is a Zn-rich non-stoichiometric
(Zn,Cd)Fe2O4 normal spinel with tetrahedral vacancies.
The nature of variation of lattice parameter of the cu-
bic (Zn,Cd)Fe2O4 phase with increasing milling time is
shown in Figure 4. It can be seen from the plot that the
lattice parameter of ferrite phase formed after 1h of mill-
ing reduces rapidly within 3 h of milling from the value
0.859 nm to ~0.849 nm (Table 1) and then remained
almost invariant up to 25 h of milling. In the course of
milling, ZnO phase utilized completely in tetrahedral
vacancies but CdO phase was not utilized completely and
some tetrahedral sites remained unoccupied even after 25
h of milling.
The scanning electron micrograph of the samples
CZF3h and CZF20h are shown in Figures 5(a) and 5(b).
The pictures show that the samples are not closely
packed and consist of several grains. The grains are well
resolved and have almost circular (spherical) shape. It is
clearly seen in the micrographs that the grains are at
nanoscale. The average grain size determined from SEM
was noted as 35 nm - 40 nm.
The dc conductivity of the Cd-Zn ferrite samples have
been measured in the temperature range 77 K T 300
K. It is observed that the incorporation of Cadmium atom
within zinc ferrite reduces the dc conductivity in compare
to our previous study [32]. The room temperature con-
opyright © 2011 SciRes. MSA
Direct and Alternate Current Conductivity and Magnetoconductivity of Nanocrystalline Cadmium-Zinc Ferrite 229
below Room Temperature
0510 15 20 25
(Zn,Cd) Fe2O4
Mole fraction
Milling time (h)
Figure 3. Variations of mole fraction of different phases in
ball-milled CdO + ZnO + α-Fe2O3 powder mixture with
increasing milling time.
ductivity σ(300 K) and conductivity ratio σr [= σ(300 K)/
σ(77 K)] of the investigated samples increases with the
increasing of milling time. The value of conductivity
ratio changes between 1.02 102 to 2.48 103 which is
smaller than Zn-ferrite (σr = 1.12 103 to 1.21 106)
[32]. The contacts between the nanoparticles are the rea-
son behind this larger conductivity ratio. The conductiv-
ity variation of all the samples has the characteristic of a
semiconductor i.e. their conductivity increases with in-
0510 15 20 25
Lattice parameter (nm )
Milling time (h)
Figure 4. Variation of lattice parameter of the (Zn,Cd)-
Fe2O4 phase with increasing milling time.
Figure 5. Scanning electron micrograph of (a) CZF3h and
(b) CZF20h.
creasing temperature. The variation of conductivity with
temperature is indicated in Figure 6. Lu et al. [18] re-
ported the similar type of variation of conductivity in
case of nanocrystalline Zinc ferrite samples. Figure 6
shows the linear variation of ln[σ(T)] with 1/T indicating
a simple hopping type charge transport in all the investi-
gated samples [33]. The values of activation energy of
different samples have been obtained from the slopes of
different straight lines. The values of Ea with milling
time are shown in the inset of Figure 6. The values of Ea
increase with increasing milling time and hence with
decreasing particle size of the samples. The increasing
milling time may decrease the size of the metal core
which in turn increases the activation energy.
The influence of magnetic field on the dc conductivity
of the samples has been observed under a magnetic field
of strength < 1 T. The magnetoconductivity ratio of the
milled samples increases with increasing milling time
which may be explained in terms of a simple phenome-
nological model named as orbital magnetoconductivity
theory (Forward interference model) [34,35]. Again, for
the unmilled sample, there is a decrease in conductivity
opyright © 2011 SciRes. MSA
Direct and Alternate Current Conductivity and Magnetoconductivity of Nanocrystalline Cadmium-Zinc Ferrite
below Room Temperature
Copyright © 2011 SciRes. MSA
Table 1. Microstructure parameters of unmilled and ball-milled CdO, ZnO and α-Fe2O3 powder mixture as revealed from
Rietveld’s analysis of XRD data.
Milling Phase present Mole Lattice parameter (10–5 - 10–3)* Particle
Time fraction a(nm) c(nm) size(nm)
(10–3 - 10–2)* (10–2 - 10–1)*
α-Fe2O3 0.4951 0.5031 1.3734 59.37
0 h CdO 0.2377 0.4691 230.77
CZF0h ZnO 0.2672 0.3247 0.5200 193.59
α-Fe2O3 0.1953 0.5067 1.3638 37.31
3 h CdO 0.1229 0.4638 2.71
CZF3h ZnO 0.0082 0.3189 0.5375 50.00
(Zn,Cd)Fe2O4 0.6734 0.8492 3.99
α-Fe2O3 0. 0114 0.5036 1.3749 50.00
8 h CdO 0.0910 0.4670 5.98
CZF8h (ZnCd)Fe2O4 0.8976 0.8494 6.88
20 h CdO 0.0393 0.4685 4.95
CZF20h (Zn,Cd)Fe2O4 0.9607 0.8494 7.02
25 h CdO 0.0361 0.4684 7.17
CZF25h (Zn,Cd)Fe2O4 0.9639 0.8490 6.29
*Error limits.
where t1= 5/2016. The variation of magnetoconductivity
ratio with magnetic field intensity is shown in Figure 7.
The different points in Figure 7 are the experimental
data for different samples and the solid lines are the
theoretical best fit obtained from Equation (1) for milled
samples and from Equation (2) for unmilled sample. For
milled samples the value of Csat and Bsat can be obtained
as a fitting parameter of Equation (1) and the values are
ratio with increasing magnetic field strength which may
be explained by wave function shrinkage model [36].
The orbital magnetoconductivity theory predicts the for-
ward interference among the random paths in the hop-
ping process between the two sites spaced at a distance
equal to optimum hopping distance resulting in positive
magnetoconduction and can be expressed as
0, 1
where Csat is a temperature independent parameter and
Bsat is the magnetic field where the magnetoconductivity
is saturated [= 0.7(h/e)(8/3) 3/2(1/L2
loc)(T/Tmott)3/8]. Where
Lloc is the localization length and Tmo t t is the Mott char-
acteristic temperature. In wave function shrinkage model,
the average hopping length reduces due to the contrac-
tion of wave function of electrons under the influence of
a magnetic field. As a result, the conductivity decreases
with increasing magnetic field. Under a small magnetic
field, the magnetoconductivity ratio can be expressed as
ln 0,
 
 
loc Mott
(2) Figure 6. Variation of dc conductivity of the different sam-
ples with temperature. Inset shows the variation of activa-
tion energy of different samples with milling time.
Direct and Alternate Current Conductivity and Magnetoconductivity of Nanocrystalline Cadmium-Zinc Ferrite 231
below Room Temperature
0.0 0.2 0.4 0.6 0.8
Figure 7. Variation of dc magnetoconductivity of different
samples with magnetic field strength at T = 300 K. The solid
lines of milled samples are fitted with Equation (1) and the
solid line of unmilled sample is fitted with Equation (2).
0.045 to 0.104 for Csat and 0.311 to 0.731 T for Bsat, re-
spectively. The values of Lloc of different investigated
samples have been calculated. For the milled samples,
the values of Lloc are 71.44 nm to 100.19 nm and for the
unmilled sample, the localization length has been calcu-
lated as 26.79 nm. Due to large localization length in
milled samples conductivity is greater, which is con-
firmed by the dc conductivity result.
The ac conductivity of Cd-Zn ferrite samples are in-
vestigated in the frequency range 20 Hz to 1 MHz and in
the temperature range 77 K T 300 K. A general fea-
ture of the amorphous semiconductors or disordered sys-
tems is that, in addition to the dc conductivity contribu-
tion σdc, the real part of complex ac conductivity σ/(f) is
found to follow the so called universal dielectric re-
sponse behavior, which can be expressed as [37-39]
 
dc acdc
 
 (3)
where σdc is the dc conductivity, α is the temperature
dependent constant and the frequency exponent s 1.
The value of σac(f)(the frequency dependent of conduc-
tivity) has been determined upon subtraction of the dc
contribution from the total frequency dependent conduc-
tivity σ/(f). Figure 8 shows the linear variation of
ln[σac(f)] with ln(f) at different temperatures for the sam-
ple 25 h. All the other samples behave in a similar man-
ner. The value of ‘s’ has been calculated from the slope
of these linear variation. Figure 9 shows the variation of
s’ with temperature. A weak variation of ‘s’ with tem-
perature is observed up to 200 K but at higher tempera-
ture (T > 200 K), the value of ‘s’ decrease with increas-
ing temperature. In general, the conduction process of
Figure 8. Variation of ac conductivity of CZF25h sample
with frequency at different constant temperature.
systems is governed by two physical processes such as
correlated barrier hopping (CBH) [39] and quantum me-
chanical tunneling (electron tunneling [40], small polaron
tunneling [39] and large polaron tunneling [38]. For dif-
ferent conduction process, the nature of temperature de-
pendency of ‘s’ are different. So the exact nature of
charge transport may be obtained experimentally from
the temperature dependence of ‘s’. According to the elec-
tron tunneling theory ‘s’ is independent of temperature,
whereas for small polaron tunneling ‘s’ increases with
increasing accordance with the CBH model. According
to this model, the charge carrier hops between the sites
over the potential barrier separating them and the fre-
quency exponent ‘s’ can be written as temperature. But
in case of correlated barrier hopping model ‘s’ only de-
50100 150 200 250 300
Figure 9. Variation of frequency exponent ‘s’ with tem-
perature for different samples. The different solid lines are
fitted with Equation (4).
opyright © 2011 SciRes. MSA
Direct and Alternate Current Conductivity and Magnetoconductivity of Nanocrystalline Cadmium-Zinc Ferrite
below Room Temperature
creases with the increasing temperature. The trend of
variation of ‘s’ for the investigated samples show that it
is in [39]
 
where WH is the effective barrier height and
0 is the
characteristic relaxation time. For large value of WH/kBT,
the value of ‘s’ can be considered as independent of fre-
quency as there is a very small variation of ‘s’ with fre-
quency [41]. Again for the investigated samples, the lin-
ear variation of ln[σac(f)] with ln(f) indicates that ‘s’ is
frequency independent. Therefore the experimental data
has been fitted with Equation (4) with WH and
0 as the
fitting parameters. In Figure 9, the points are the ex-
perimental data and solid lines are the theoretical best fit
values obtained from Equation (4). The best fitted values
of the parameters WH and
o (at a fixed frequency of f =
1 MHz) are in the range 0.82 to 1.34 eV and 1.36 10–14
to 7.63 10–13 s, respectively for different samples. WH
has a greater value than the activation energy measured
from grain and grain boundary contribution and the val-
ues of characteristic relaxation time
0 has a similar value
as expected for typical inverse phonon frequency.
Therefore the trend of variation ‘s’ with temperature in-
dicates that the ac conductivity of the investigated sam-
ples can be explained by the CBH model.
The temperature dependence of ac conductivity is
shown in Figure 10 for the sample CZF25h for different
some frequencies. A weak variation is observed at lower
temperature (T < 200 K) in comparison to high tempera-
ture (T > 200 K). At a particular frequency the real part
of complex conductivity increases with temperature and
is found to follow a power law σ/(f) Tn, which is shown
as the solid lines in Figure 10 . The values of n have been
calculated from the power law fitting and found to be
strongly frequency dependent for all samples. For dif-
ferent frequency ranging from 1 kHz to 1 MHz, the val-
ues of ‘n’ vary between 15.4 to 10.6 for CZF25h. Ac-
cording to the CBH model [37-40] the ac conductivity
σ/(f) is expressed as σ/(f) T2Rω
6 Tn with n = [2 + (1 –
0)] for broad band limit and σ/(f) Rω
6 Tn
with n = (1 – s)ln(1/ω
0) for narrow band limit, where Rω
= e2/{εεo[WHkBTln(1/ω
0)]}. The theoretical values of
‘n’, have been calculated taking s = 0.39 at 300 K and
0.88 at 77 K for different frequencies and the value of 0
= 3.26 10–14 s. The variation of the calculated values of
‘n’ are in the range 15.61 to 11.40 at 300 K and 4.68 to
3.84 at 77 K for broad band limit and 13.61 to 9.40 at
300 K and 2.68 to 1.84 at 77 K for narrow band limit
with frequency variation from 1 kHz to 1 MHz. The ex-
Figure 10. Variation of ac conductivity of CZF25h sample
with temperature at different constant frequency. The lines
are fitted with the equation σ/(f) Tn.
perimental values are close to the theoretical values of
broad band limit at the higher temperature range but at
lower temperature, there is a discrepancy between theo-
retical and experimental result.
In general, interfacial polarization is exhibited by the
ferrites due to structural inhomogenities and existence of
free charges [42]. The hopping electrons at low frequen-
cies may be trapped by the inhomogeneties. At a particu-
lar frequency, the increase in ε/(f) with temperature is due
to the drop in the resistance of the ferrite with increasing
temperature. Electron hopping is promoted by the low
resistance and hence resulting a larger polarizability or
larger ε/(f). The variation of real part of dielectric permit-
tivity ε/(f) with frequency for different samples are shown
in Figure 11 at T = 300 K. The dielectric permittivity
increases sharply with decreasing frequency for all the
samples and such behaviour can be attributed to the
presence of large degree of dispersion due to charge
transfer within the interfacial diffusion layer present be-
tween the electrodes. The magnitude of dielectric disper-
sion depends on the temperature. At lower temperature
the relaxation process becomes easier due to the freezing
of the electric dipoles and thus there is decay in polariza-
tion with respect to the applied electric field. So a sharp
increase in ε/(f) is observed at lower frequency region.
Therefore the inhomogeneous nature of the samples con-
taining different permittivity and conducting regions,
governs the frequency behaviour of ε/(f) were the charge
carriers are blocked by the poorly conducting regions.
The effective dielectric permittivity of this type of in-
homogeneous systems is explained in terms of Maxwell
Wegner capacitor model [43,44] as which the complex
impedance can be modelled by an equivalent circuit con-
opyright © 2011 SciRes. MSA
Direct and Alternate Current Conductivity and Magnetoconductivity of Nanocrystalline Cadmium-Zinc Ferrite 233
below Room Temperature
Figure 11. Variation of real part of dielectric permittivity of
different samples with frequency at T = 300 K.
sisting of resistance and capacitance due to grain and
interfacial grain boundary contribution and can be ex-
pressed as
gggb gb
 
gg gbgb
gggb gb
where sub-indexes ‘g’ and ‘gb’ refer to the grain and
interfacial grain boundary respectively, R is the resis-
tance, C is the capacitance, ω is 2πf and C0 is the free
space capacitance. The real part of the complex imped-
ance for different samples have been calculated from the
experimental data for real (ε/) and imaginary (ε//) part of
the dielectric permittivity. Figure 12 shows the variation
of the real part of the complex impedance of different
samples with frequency at room temperature and Figure
13 shows the same variation for 20 h and 25 h samples in
presence of a magnetic field. The points in both figures
are the experimental data and the solid lines are the
theoretical best fit obtained from Equation (6). It is ob-
served from both figures that the experimental data are
reasonably well fitted with the theory. The grain and
grain boundary resistance and capacitance have been
extracted from the analysis at room temperature whose
Figure 12. Variation of real part of impedance of different
samples with frequency at room temperature. The solid
lines are fitted with Equation (6).
value lie within the range 19.8 k to 0.32 M for Rg,
0.33 M to 2.79 M for Rgb, 0.16 to 1.94 nF for Cg and
0.20 to 1.62 nF for Cgb for different samples Q without
magnetic field and 22.03 k to 0.24 M for Rg, 0.35 M
to 2.05 M for Rgb, 0.18 to 1.99 nF for Cg and 0.21 to
1.69 nF for Cgb for different samples in presence of a
magnetic field. As the resistance due to interfacial grain
boundary is much larger than the grain resistance, it may
conclude that the grain boundary contribution dominates
over the grain contribution.
Figure 13. Variation of real part of impedance of CZF20h
and CZF25h samples with frequency at room temperature
in presence of magnetic field of 0.76 T. The solid lines are
fitted with Equation (6).
opyright © 2011 SciRes. MSA
Direct and Alternate Current Conductivity and Magnetoconductivity of Nanocrystalline Cadmium-Zinc Ferrite
below Room Temperature
The variation of real part of the ac conductivity for dif-
ferent samples at room temperature and f = 1 MHz under
the influence of magnetic field is shown in Figure 14.
With increasing magnetic field, there is an increase in
conductivity for different milled samples, but for un-
milled sample opposite behaviour has been observed. At
present, no theoretical model is found in literature which
can explain directly the behaviour of ac conductivity in
presence of magnetic field. The SEM micrograph reveals
that the investigated samples are heterogeneous in nature
with spherical grains. Thus the materials consist of grain
and interfacial grain boundary regions. For such hetero-
geneous samples, it has been already discussed that the
dielectric property and impedance depend on the grain
and grain boundary resistance and capacitance. The real
part of ac conductivity is related to the dielectric re-
sponse by the relation σ/(B,f) = ωε0ε//(B,f) .As the value
of ε//(B,f) is dependent on the grain and grain boundary
resistance and capacitances, the ac conductivity can be
written as [45]
 
gbg gb
 
g = CgRg,
gb = CgbRgb and
= RgRgb(Cg + Cg)/(Rg
+ Rgb). The change in the value of any of these resistance
by the magnetic field will affect the value of the conduc-
tivity. From the analysis of the real part of complex im-
pedance in presence of constant magnetic field at 0.76 T,
it has been found that the value of grain and grain
boundary resistance increases by the application of mag-
netic field for unmilled sample, whereas it decreases for
milled samples. Hence the total contribution due to grain
and grain boundary resistance (R = Rg + Rgb) decrease
with increasing magnetic field for milled samples. Thus
the influence on ac conductivity by magnetic field is due
to the change in grain and grain boundary resistance by
the applied magnetic field. But due to the inability of the
analytical expression, the measured data can not be
compared with the theory. Thus a more explicit theoreti-
cal and experimental study is required to reveal the true
mechanism of magnetic field dependent ac conductivity.
4. Conclusions
The different Cd-Zn ferrite samples had been prepared
by the high energy ball milling method. The samples
were characterized by XRD which confirms the forma-
tion of normal spinel structure with tetrahedral vacancies
with particle size 7 nm. SEM picture reveals that the dif-
ferent samples consist of grains of almost spherical shape.
The dc conductivity of different Cd-Zn–ferrite follows a
simple hopping time of charge conduction mechanism.
The magnetic field dependent conductivity of the differ-
Figure 14. Variation of change in ac magnetoconductivity
[σ/(f,B) = σ/(f,B) – σ/(f,0)] with magnetic field intensity (B)
at T = 300 K and frequency f = 1 MHz.
ent investigated samples increases with increasing mag-
netic field for milled samples whereas decreases with
increasing magnetic field for unmilled sample and those
can be explained in terms of orbital magneto conductiv-
ity theory and wave function shrinkage model respec-
tively. The real part ac conductivity follows the power
law σ/(f ) α f
s. The temperature dependence of universal
dielectric response parameter ‘s’ was found to follow
correlated barrier hooping change transfer mechanism.
At a particular frequency the conductivity of the investi-
gated samples follows σ/(f ) α T
n, where the values of n
strongly depend on frequency. The frequency dependent
real part of complex permittivity shows large degree dis-
persion at low frequency , but rapid polarization at high
frequencies which can be interpreted by Maxwell
Wegner capacitor model. The grain resistance and ca-
pacitance was found to be smaller than the grain bound-
ary resistance and capacitance and the total resistance
due to grain and grain boundary decrease due to the ap-
plication of a magnetic field. The ac resistivity of milled
samples was found to show positive (increasing with
increasing magnetic field) variation in presence of mag-
netic field which may be due to the variation of grain and
grain boundary resistances by the application of a mag-
netic field.
5. Acknowledgements
This work has been carried out under Grant nos.
F.27-1/2002.TS.V dated 19.03.2002 and F.28-1/2003. T-
S.V dated 31-03.2003 sanctioned by the MHRD, Gov-
ernment of India. The authors gratefully acknowledge the
principal assistance received from the above organization
opyright © 2011 SciRes. MSA
Direct and Alternate Current Conductivity and Magnetoconductivity of Nanocrystalline Cadmium-Zinc Ferrite 235
below Room Temperature
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