Enhancement of Impedance by Chromium Substitution and Correlation with   DC Resistivity in Cobalt Ferrite
  Chromium substituted cobalt ferrite with grain size less than the single domain (approx. 70 nm) has been prepared by the sol-gel method. XRD analysis reveals that the samples crystallize to cubic symmetry with spacegroup number 227. Two transition temperatures (TD (approx. 450 K) and TM (approx. 600 K) have been observed from the impedance verses temperature measurement. TD increases with the increase in frequency due to dipole response to the frequency. TM is comparable with the para-ferrimagnetic transition temperature of cobalt ferrite, which is independent of frequency. This result is well supported by the temperature dependent DC conductivity measurement. The modified Debye relaxation could be explained the impedance spectra of CoFe2-xCrxO4. The grain and grain boundary effect on impedance spectroscopy has been observed from Cole-Cole analysis. The ac conductivity follows Arrhenius behavior at different frequencies. All the samples exhibit the negative temperature coefficient of resistance behavior which reveals the semiconducting behavior of the material. The Mott VRH model could explain the DC electrical conductivity. Both ac impedance and DC resistivity are well co-related each other to explain the electron transport properties in Cr substituted cobalt ferrite. The electrical transport properties could be explained by the electron hopping between different metal ions via oxygen in the material. 
Sweety Supriya, Sunil Kumar and Manoranjan Kar
cond-mat.mtrl-sci