Electrical resistivity and Mössbauer studies of Cr substituted Co nano ferrites

Nano structured CoCrxFe2-xO4 Ferrites with x = 0, 0.1, 0.3, 0.5, 0.7, 0.9 and 1.0 were prepared through Citrate-gel chemical synthesis. DC electrical resistivity (ρ) of the prepared samples was measured in the temperature range of 200-700 °C using two probe method and room temperature Mössbauer measurements were studied. A decreasing trend ρ with an increase in temperature was observed ensuring the semiconducting nature of the ferrites under test. From the Arrhenius plots (log ρ vs 1000/T), corresponding activation energies for conduction of all the compositions were obtained that show a straight line with two slopes. With an increase in Cr composition, resistivity (ρ) of the ferrite samples was found to increase from 1.4 × 106 Ω-cm to 46.2 × 103 Ω-cm at 300 K. Drift mobility was found to increase with increasing temperature due to a decrease in DC electrical resistivity. Curie temperature (TC) was decreased from 740 K to 641 K and the Activation energy of hopping (ΔE) was increased from 0.075 to 0.222 ev with an increase in Cr composition. This suggests that conduction process in the present Co-Cr nano ferrite system is due to hopping of polarons. The results indicate that the activation energy in the paramagnetic region is higher than that in the ferrimagnetic region. The presence of a paramagnetic doublet in Mossbauer spectra of the samples was attributed to the presence of very small sized nanoparticles in the superparamagnetic regime.