Materials Sciences and Application, 2011, 2, 1083-1089
doi:10.4236/msa.2011.28146 Published Online August 2011 (
Copyright © 2011 SciRes. MSA
Electrical Transport Properties of Bi2O3-Doped
CoFe2O4 and CoHo0.02Fe1.98O4 Ferrites
Hasan Mehmood Khan*, Misbah-ul-Islam, Irshad Ali, Mazhar-ud-din Rana
Department of Physics, Bahauddin Zakariya University Multan, Multan, Pakistan.
Email: *
Received March 19th, 2011; revised April 26th, 2011; accepted May 6th, 2011.
Two series of CoHoxFe2-x O4 (x = 0.0, 0.02) ferrites with Bismuth oxide doping from (0.1 - 0.3)% were prepared by
Co-precipitation technique. X-ray diffraction analysis revealed fcc structure. The lattice constants were found to de-
crease as the doping of Bi2O3 increases in both series. An increase in Bismuth oxide concentration from (0.1 - 0.3%) in
CoFe2O4, and CoHo0.02Fe1.98O4 ferrites leads to an increase in room temperature resistivity. Temperature dependent re-
sistivity decreases as the temperature increases following the Arrhenius equation. The activation energy increases with
the increase of Bi2O3-concentration for both CoFe2O4 and CoHo0.02Fe1.98O4 series. The frequency dependant dielectric
constant follows the Maxwell-Wagner type interfacial polarization. The dielectric loss indicates the normal behavior of
these ferrites. SEM analysis shows an increase in grain size with increasing Bismuth concentration.
Keywords: Cobalt Ferrites, Co-Precipitation, Bi Doping, Electrical Properties
1. Introduction
The miniaturization of electrical and electronic gadgets
demands new materials with nano-size particles for high
frequency applications and high density recording. Bi2O3
is a potential dopant for improving the magnetic and
electrical properties of ferrites. The properties of ferrite
materials are known to be strongly influenced by their
composition and microstructure which are sensitive to
the processing methods used to synthesize them. Over
the last decade, the magnetostrictive materials for smart
sensors have attracted a great interest due to their wide
range of applications in the automotive industry. The
cobalt ferrites are well known for its highest magne-
tostrictive coefficient amongst the oxide-based magne-
tostrictive materials. Cobalt ferrite nanoparticles have
recently become the subject of research interest from the
point of view of the synthesis, the magnetic characteriza-
tion, and the applications [1-4]. Among the various fer-
rite materials for magnetic recording applications, cobalt
ferrite (CoFe2O4) has been widely studied [5]. Cobalt
ferrite (CoFe2O4) possesses excellent chemical stability,
good mechanical hardness and a large positive first-order
crystalline anisotropy constant, which made this ferrite a
promising candidate for magneto-optical recording media
[6]. High electrical resistivity and low eddy current
losses make these ferrites an excellent core material for
power transformers in electrical and electronic industry,
recording heads, antenna rods, loading coils, microwave
devices and telecommunication applications [7,8].The
magnetic properties of ferrites can be modified by intro-
ducing suitable divalent and trivalent oxides as dopants
[9]. It is reported [10] the dc electrical conductivity of the
Bi2O3-doped ferrites was increased. Bi2O3 is an alterna-
tive sintering aid for lowering sintering temperature of
magnesium ferrites. The addition of Bi2O3 has significant
effect on the resistivity and dielectric properties of fer-
rites. Moreover the advantageous effect of Bi2O3 is at-
tributed to the formation of liquid phase layer due to the
low melting point of Bi2O3 [11] which enhances the re-
sistivity of the ferrites. Due to the presence of Bi2O3 the
properties of grain boundaries of ferrite were changed
and three dimensional network grain boundary structure
formed at grain boundaries [12-15]. Since Bi2O3 doping
enhances the magnetic and electrical properties of the
ferrites, the purpose of this study is to investigate the
influence of the Bi2O3 doping from 0.1-0.3% by weight
on the microstructure, electrical and magnetic properties
of CoFe2O4 and CoHo0.02Fe1.98O4 ferrites. The CoHo0.02-
Fe1.98O4 was chosen for comparison between CoFe2O4
and the Rare earth substituted CoHo0.02Fe1.98O4 ferrites
after doping Bi2O3 .
Electrical Transport Properties of BiO-doped CoFeOand CoHoFe O Ferrites
1084 2324 0.021.984
2. Experimental
Bi2O3 doped CoFe2O4, (Bi2O3:0.1 - 0.3%) and CoHo0.02-
Fe1.98O4 (Bi2O3:0.1 - 0.3%) ferrites were prepared by
using co-precipitation method. The starting materials
99.9% pure, Co(C2H3O2)2 ,FeCl3 and Ho2O3 were used.
The stoichometric amounts of selected salts were dis-
solved in de-ionized water in a 100ml beaker except
Ho2O3 which is insoluble in de-ionized water. Ho2O3
was dissolved in HCl heated at 50 - 60˚C and then added
in the solution. The solution so obtained was stirred us-
ing magnetic stirrer for 10 hrs. During stirring calculated
amount of Na2CO3/NaOH were used in the solution as
reagent or as precipitating agents to precipitate the metals
and hydroxides. The precipitates were thoroughly
washed with distilled water until free from chloride ions,
which were checked by AgNO3 test. The final product
was then filtered with the help of suction flask having an
outlet with pump operated on water. The filtered precipi-
tates were dried in oven for 24h at 100ºC. The dried pre-
cipitates were then ground in mortar and pestle and Bi2O3
was then doped from (0.1 - 0.3) wt% in both set of sam-
ples during grinding. The Pellets of ground powders were
formed using Paul-Otto Weber hydraulic press under the
pressure of (~35 KN·mm2). The pellets were then sin-
tered in an electric furnace at 1150 - 1200˚C for 10h fol-
lowed by furnace cooling. For electrical measurements
both surfaces of the pellets were polished on the micron
paper. The phase formation of the samples was investi-
gated by Shimadzu X-ray diffractometer using CuKα
radiations (λ = 1.5406Å). SEM analysis of the one series
of samples was investigated by JEOL-Japan Model JSM
5910 and it was used to observe the microstructure of the
sintered specimens. Electrical resistivity was measured
by two probe method using source meter model 2400
(Keithley).The dielectric properties were measured using
Digi Bridge (GenRad 1689 ).
3. Results and Discussions
3.1. X-Ray Diffraction
X-ray diffraction patterns of Bi2O3 doped CoFe2O4,
(Bi2O3:0.1 - 0.3%) and CoHo0.02Fe1.98O4 (Bi2O3:0.1 -
0.3%) ferrites are shown in Figures 1(a,b) respectively.
The X-ray diffraction analysis shows the formation of
single phase fcc spinel crystal structure for all the sam-
ples. The d-values were compared using JCPDS (1998)
cards (card No. 01 - 1121) [16]. Apparently no traces of
secondary phase were observed [17-19]. The 220, 311,
400, 333, 440 reflections were observed in X-ray diffrac-
tion patterns.
3.2. Lattice Constant
The lattice constants of CoFe2O4 (Bi2O3: 0.1 - 0.3%) and
CoHo0.02Fe1.98O4 (Bi2O3: 0.1 - 0.3%) samples are shown in
Figure 1. (a) XRD pattern for CoFe2O4 (Bi2O3: 0.1 - 0.3%).
(b) XRD pattern for CoHo0.02Fe1.98O4, (Bi2O3: 0.1 - 0.3%)
Figures 2(a,b). It is observed that the lattice constants
decrease as the doping of Bi2O3 increases. The decrease
in ‘a’ is due to the difference in ionic radii of Bi3+ (0.74Å)
as compared to Fe3+ (0.78Å) [7]. The decrease in lattice
constant in both series can be explained on the basis of
the fact that the Bi2O3 enter into the lattice completely
during sintering due to very small amount of Bi2O3 dop-
3.3. Room Temperature Resistivity
Figures 3(a,b) shows the room temperature resistivity
versus Bi2O3 concentration for both CoFe2O4 and
CoHo0.02Fe1.98O4 ferrite series. It can be observed that
as Bi2O3-concentration increases, the resistivity increases
from 18 × 104 to a maximum value of 42 × 104 -cm for
CoFe2O4 , (Bi2O3: 0.1 - 0.3%) samples and from 7 × 103
to 32 × 104 -cm for CoHo0.02Fe1.98O4 (Bi2O3: 0.1 - 0.3%)
ferrites. The increase in Bi2O3 concentration leads to an
increase in resistivity that might be due to the fact that
Bi5+ act as scattering centres for the carriers hopping be-
tween two octahedral sites [7,10], which hinders the
hopping mechanism between the Fe2+ and Fe3+ ions.
3.4. Temperature Dependant Resistivity
Temperature dependent electrical resistivity of CoFe2O4
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Electrical Transport Properties of BiO-doped CoFeOand CoHoFe O Ferrites 1085
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8. 32
8. 33
8. 34
8. 35
8. 36
8. 37
8. 38
8. 39
0.05 0.15 0.25 0.35
Bismuth oxide Concentration
a (Angstrom)
8. 2
8. 3
8. 4
8. 5
8. 6
00.1 0.20.3 0.4
Bismuth oxide Concentaration
a ( Angstrom )
(a) (b)
Figure 2. (a) Lattice constant for CoFe2O4, (Bi2O3: 0.1 - 0.3%); (b) Lattice constant for CoHo0.02Fe1.98O4, (Bi2O3: 0.1 - 0.3%).
0.05 0.15 0.25 0.35
Bismuth oxide Concentration
0.05 0.150.25 0.35
Bismuth oxide Concentration
(a) (b)
Figure 3. (a) Room temperature resistivity (ρ) for CoFe2O4, (Bi2O3: 0.1% - 0.3%); (b) Room temperature resistivity (ρ) for
CoHo0.02Fe1.98O4, (Bi2O3: 0.1% - 0.3%).
(Bi2O3:0.1 - 0.3%) and CoHo0.02Fe1.98O4 (Bi2O3:0.1-0.3%)
ferrite series was measured in temperature range (40 -
200˚C). The Arrhenius plots of these samples are shown
in Figures 4(a) and (b) respectively. Each sample fol-
lows Arrhenius equation,
ρ = ρ exp/B
where ΔE is the activation energy, T is the absolute tem-
perature, kB is the Boltzmann’s constant. The plots show
that the resistivity decreases as the temperature increases
indicating the semi conducting nature of the samples [10].
This decrease in resistivity may be due to the excess of
electrons released from both sites which reduces the Co2+
to their lowest valency and also produce Fe2+ ions [20].
The behavior of both type of electric charge carriers can
be explained on the basis of Rezlescue model [21]. Ac-
cording to this model, the exchanging of electrons be-
tween Fe2+ and Fe3+ ions and that of holes between Co3+
to Co2+ ions may be the likely conduction mechanism.
Co3+ Co2+ (Hole conduction) (2)
Fe2+ Fe3+ (Electronic conduction) (3)
3.5. Activation Energy
The activation energy obtained from Arrhenius plots are
shown in Figures 5(a,b) respectively. The activation
energy increases with the increase of Bi2O3-concentration
for both CoFe2O4 and CoHo0.02Fe1.98O4 samples. It can be
observed that the samples having high resistivity value
also have high activation energy and vice versa [22] .The
result indicates the presence of conduction dependant to
the structure [23]. As the activation energy is high and so
the resistivity is high due to which conductivity will be
lower as Bi2O3 is substituted which can be thought of due
to phonon-assisted small polaron hopping [21-22].
3.6. Drift Mobility
Figures 6(a) and (b) respectively shows the variation of
drift mobility with temperature for both series. The drift
mobility was calculated by using the resistivity data and
is calculated with the help of the given formula [23-25].
µd = 1/neρ (4)
where ρ is electrical resistivity, e is charge on an electron
and ‘n’ is the concentration of charge carriers and it is
calculated by following relation;
n = NACFeρb/M (5)
where NA is Avogadro’s number, CFe is the number of
iron atoms in sample, ρb is the bulk density and M is the
Copyright © 2011 SciRes. MSA
Electrical Transport Properties of BiO-doped CoFeOand CoHoFe O Ferrites
1086 2324 0.021.984
Bi =0.1 %
Bi =0.2 %
Bi =0.3 %
1000/T (K
Log ρ(oh m -cm)
(a) (b)
Figure 4. (a) Temperature dependant resistivity for CoFe2O4, (Bi2O3: 0.1% - 0.3%); (b) Temperature dependant resistivity
for CoHo0.02Fe1.98O4, (Bi2O3: 0.1% - 0.3%)
0. 2
0. 4
0. 6
Bismuth oxide Concentrations.
Bismuth oxide Concentrations.
Ea (eV)
(a) (b)
Figure 5. (a) Activation Energy for CoFe2O4, (Bi2O3: 0.1% - 0.3%); (b) Activation Energy CoHo0.02Fe1.98O4, (Bi2O3: 0.1% -
300 400 500
T(K )
×Mobility (cm
300400 500
×Mobility (cm
(a) (b)
Figure 6. (a) Drift Mobility for CoFe2O4, (Bi2O3: 0.1% - 0.3%); (b) Drift Mobility for CoHo0.02Fe1.98O4, (Bi2O3: 0.1% - 0.3%).
molar mass of the samples. It is however observed that
drift mobility increases with the increase in temperature.
This may be due to the fact that charge carriers start hop-
ping from one site to another as the temperature increases
[26]. The temperature dependence of resistivity and mo-
bility shows that the samples are of degenerate type
3.7. Dielectric Properties
3.7.1. Dielectric Constant and Loss Tangent
The dielectric constant Vs frequency of both the series at
room temperature 30˚C are shown in Figures 7(a,b) re-
spectively. The dielectric constant decreases with in-
creasing frequency. At high frequencies the dielectric
constant seems to be independent of frequency. This be-
havior of the samples is in accordance with the Maxwell
Wagner model [27-29]. In this model the dielectric
structure of ferrite material is assumed to be made up of
two layers. First layer being conducting, contains large
number of grains and other being grain boundaries which
are poor conductor. This bi-layer formation is resulted by
high temperature sintering [23]. Figures 8(a,b) shows
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Electrical Transport Properties of BiO-doped CoFeOand CoHoFe O Ferrites 1087
2324 0.021.984
050100 150
Frequency (kH z
Dielectric Constant
Bi =0.1%
Bi =0.2%
Bi =0.3%
Dielectric Constant
(a) (b)
Figure 7. (a) Dielectric Constant vs Frequency for CoFe2O4, (Bi2O3: 0.1 - 0.3%); (b) Dielectric Constant vs Frequency for
CoHo0.02Fe1.98O4, (Bi2O3: 0.1 - 0.3%).
050100 150
Frequency(kHz )
Dielectric Loss
Dielectric Loss
Bi =0.1%
Bi =0.2%
Bi =0.3%
(a) (b)
Figure 8. (a) Dielectric vs Frequency loss for CoFe2O4, (Bi2O3: 0.1% - 0.3%); (b) Dielectric Loss vs Frequency for
CoHo0.02Fe1.98O4, (Bi2O3: 0.1% - 0.3%).
tanδ Vs frequency for both series. The dielectric loss
decreases substantially with increasing frequency and
reaches a constant value later on [30-32]. When the fre-
quency of applied field is low than the hopping fre-
quency of electrons between ferrous and ferric ions at
adjacent octahedral sites, the electron follow the applied
field and hence loss is maximum. At higher frequencies
the hopping frequency of the electron exchange between
ferrous and ferric ions can not follow applied field be-
yond certain critical frequency and the loss is minimum.
3.8. Scanning Electron Microscopy
Few representative SEM micrographs of CoFe2O4 (Bi2O3:
0.1% - 0.3%) are shown in Figures 9 (a,b,c) respectively.
The grain boundaries and grains can be clearly distin-
guished at 50,000 magnification. The SEM micrographs
of the CoFe2O4 (Bi2O3: 0.1% - 0.3%) sintered at 1150˚C
for 10 h are shown for various contents of Bi2O3. It is
however observed that the grain size increases from 141 -
201 nm with the increase in Bi2O3 concentration in
CoFe2O4 (Bi2O3: 0.1% - 0.3%) ferrites. Uneven grain
boun-daries were also observed. These non-uniform
grain boundaries seems to be a diffusion induced grain
boundary migration [33,34].
4. Conclusions
1) X-ray diffraction analysis reveals that CoFe2O4
(Bi2O3:0.1% - 0.3%) and CoHo0.02Fe1.98O4 (Bi2O3: 0.1% -
0.3%) ferrite series clearly indicate formation of spinel
fcc single phase crystal structure. The lattice constant ‘a’
decreases as Bi2O3 concentration increases for both series
due to the difference in the ionic radii.
2) Room temperature dc resistivity of both series of
ferrites increases due to the formation of Bi5+ ions.
3) The temperature dependant resistivity and activa-
tion energy Vs Bi content follows same trend indicating
that the samples with high resistivity have high activation
energies and vice versa .
4) The behavior of dielectric constant and loss tangent
for both series of ferrite follows the Maxwell Wagner
5) The SEM micrographs of the CoFe2O4 (Bi2O3: 0.1%
- 0.3%) ferrites shows that the grain size increases from
141 - 201 nm with the increase in Bi2O3 concentration.
6) The activation energy shows that the hopping con-
duction mechanism is established in these samples.
5. Acknowledgements
Authors are thankful to Higher Education Commission of
Pakistan for providing financial assistance under 5000
indigenous fellowship programme. We are grateful to Dr.
Amir Bashir Ziya for his help in taking XRD patterns of
the samples.
Copyright © 2011 SciRes. MSA
Electrical Transport Properties of BiO-doped CoFeOand CoHoFe O Ferrites
1088 2324 0.021.984
Figure 9. (a) SEM for CoFe2O4 (Bi2O3: 0.1%); (b) SEM for
CoFe2O4 (Bi2O3: 0.2%); (c) SEM for CoFe2O4, (Bi2O3:
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