Electrical Transport Properties of Bi 2 O 3-Doped CoFe 2 O 4 and CoHo 0 . 02 Fe 1 . 98 O 4 Ferrites

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 decrease 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 resistivity 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.


Introduction
The miniaturization of electrical and electronic gadgets demands new materials with nano-size particles for high frequency applications and high density recording.Bi 2 O 3 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 magnetostrictive coefficient amongst the oxide-based magnetostrictive materials.Cobalt ferrite nanoparticles have recently become the subject of research interest from the point of view of the synthesis, the magnetic characterization, and the applications [1][2][3][4].Among the various ferrite materials for magnetic recording applications, cobalt ferrite (CoFe 2 O 4 ) has been widely studied [5].Cobalt ferrite (CoFe 2 O 4 ) 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 introducing suitable divalent and trivalent oxides as dopants [9].It is reported [10] the dc electrical conductivity of the Bi 2 O 3 -doped ferrites was increased.Bi 2 O 3 is an alternative sintering aid for lowering sintering temperature of magnesium ferrites.The addition of Bi 2 O 3 has significant effect on the resistivity and dielectric properties of ferrites.Moreover the advantageous effect of Bi 2 O 3 is attributed to the formation of liquid phase layer due to the low melting point of Bi 2 O 3 [11] which enhances the resistivity of the ferrites.Due to the presence of Bi 2 O 3 the properties of grain boundaries of ferrite were changed and three dimensional network grain boundary structure formed at grain boundaries [12][13][14][15].Since Bi 2 O 3 doping enhances the magnetic and electrical properties of the ferrites, the purpose of this study is to investigate the influence of the Bi 2 O 3 doping from 0.

Experimental
Bi 2 O 3 doped CoFe 2 O 4, (Bi 2 O 3 :0.1 -0.3%) and CoHo 0.02 -Fe 1.98 O 4 (Bi 2 O 3 :0.1 -0.3%) ferrites were prepared by using co-precipitation method.The starting materials 99.9% pure, Co(C 2 H 3 O 2 ) 2 ,FeCl 3 and Ho 2 O 3 were used.The stoichometric amounts of selected salts were dissolved in de-ionized water in a 100ml beaker except Ho 2 O 3 which is insoluble in de-ionized water.Ho 2 O 3 was dissolved in HCl heated at 50 -60˚C and then added in the solution.The solution so obtained was stirred using magnetic stirrer for 10 hrs.During stirring calculated amount of Na 2 CO 3 /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 AgNO 3 test.The final product was then filtered with the help of suction flask having an outlet with pump operated on water.The filtered precipitates were dried in oven for 24h at 100ºC.The dried precipitates were then ground in mortar and pestle and Bi 2 O 3 was then doped from (0.1 -0.3) wt% in both set of samples 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 sintered in an electric furnace at 1150 -1200˚C for 10h followed by furnace cooling.For electrical measurements both surfaces of the pellets were polished on the micron paper.The phase formation of the samples was investigated by Shimadzu X-ray diffractometer using CuK α 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 ).

Lattice Constant
The lattice constants of Figures 2(a,b).It is observed that the lattice constants decrease as the doping of Bi 2 O 3 increases.The decrease in 'a' is due to the difference in ionic radii of Bi 3+ (0.74Å) as compared to Fe 3+ (0.78Å) [7].The decrease in lattice constant in both series can be explained on the basis of the fact that the Bi 2 O 3 enter into the lattice completely during sintering due to very small amount of Bi 2 O 3 doping.

Room Temperature Resistivity
Figures 3(a,b concentration leads to an increase in resistivity that might be due to the fact that Bi 5+ act as scattering centres for the carriers hopping between two octahedral sites [7,10], which hinders the hopping mechanism between the Fe 2+ and Fe 3+ ions.

Temperature Dependant Resistivity
where ΔE is the activation energy, T is the absolute temperature, k B 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 Co 2+ to their lowest valency and also produce Fe 2+ ions [20].The behavior of both type of electric charge carriers can be explained on the basis of Rezlescue model [21].According to this model, the exchanging of electrons between Fe 2+ and Fe 3+ ions and that of holes between Co 3+ to Co 2+ ions may be the likely conduction mechanism.Co 3+ ↔ Co 2+ (Hole conduction) (2) Fe 2+ ↔ Fe 3+ (Electronic conduction) (3)

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 Bi 2 O 3 -concentration for both CoFe 2 O 4 and CoHo 0.02 Fe 1.98 O 4 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 Bi 2 O 3 is substituted which can be thought of due to phonon-assisted small polaron hopping [21][22].

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][24][25].
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; where N A is Avogadro's number, C Fe is the number of iron atoms in sample, ρ b is the bulk density and M is the 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 hopping from one site to another as the temperature increases [26].The temperature dependence of resistivity and mobility shows that the samples are of degenerate type semiconductors.

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) respectively.The dielectric constant decreases with increasing frequency.At high frequencies the dielectric constant seems to be independent of frequency.This behavior of the samples is in accordance with the Maxwell Wagner model [27][28][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].tanδ Vs frequency for both series.The dielectric loss decreases substantially with increasing frequency and reaches a constant value later on [30][31][32].When the frequency of applied field is low than the hopping frequency 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 beyond certain critical frequency and the loss is minimum.

Scanning Electron Microscopy
Few representative SEM micrographs of CoFe 2 O 4 (Bi 2 O 3 : 0.1% -0.3%) are shown in Figures 9 (a,b,c  2) Room temperature dc resistivity of both series of ferrites increases due to the formation of Bi 5+ ions.

Conclusions
3) The temperature dependant resistivity and activation 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 model.
5) The SEM micrographs of the CoFe 2 O 4 (Bi 2 O 3 : 0.1% -0.3%) ferrites shows that the grain size increases from 141 -201 nm with the increase in Bi 2 O 3 concentration.
6) The activation energy shows that the hopping conduction mechanism is established in these samples.

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.
1-0.3% by weight on the microstructure, electrical and magnetic properties of CoFe 2 O 4 and CoHo 0.02 Fe 1.98 O 4 ferrites.The CoHo 0.02 -Fe 1.98 O 4 was chosen for comparison between CoFe 2 O 4 and the Rare earth substituted CoHo 0.02 Fe 1.98 O 4 ferrites after doping Bi 2 O 3 .
) shows the room temperature resistivity versus Bi 2 O 3 concentration for both CoFe 2 O 4 and CoHo 0.02 Fe 1.98 O 4 ferrite series.It can be observed that as Bi 2 O 3 -concentration increases, the resistivity increases from 18 × 10 4 to a maximum value of 42 × 10 4 Ω-cm for CoFe 2 O 4 , (Bi 2 O 3 : 0.1 -0.3%) samples and from 7 × 10 3 to 32 × 10 4 Ω-cm for CoHo 0.02 Fe 1.98 O 4 (Bi 2 O 3 : 0.1 -0.3%) ferrites.The increase in Bi 2 O 3 Temperature dependent electrical resistivity of CoFe 2 O 4 3%) ferrite series clearly indicate formation of spinel fcc single phase crystal structure.The lattice constant 'a' decreases as Bi 2 O 3 concentration increases for both series due to the difference in the ionic radii.