Semiconductor to metallic transition and polaron conduction in nanostructured cobalt ferrite

In this paper we report semiconductor to metal transition and polaron conduction in nanostructured cobalt ferrite. The material was prepared by the coprecipitation technique. Average particle size was determined to be similar to 10 nm by x-ray diffraction and transmission electron microscope analysis. A detailed electrical characterization was performed in the frequency range 20 Hz-2MHz and temperature range between 300 and 400 K. Nanostructured cobalt ferrite exhibits semiconductor behaviour from 300 to 330 K. From 330 to 400K it has metallic behaviour. The change at similar to 330K is attributed to a change in cation distribution as obtained from Mossbauer spectroscopy. The ac conductivity of cobalt ferrite followed sigma(ac) proportional to omega(s) dependence. The observed variation of the exponent 's' with temperature suggests that overlapping large polaron tunnelling is the dominant conduction mechanism from 300 to 400K and in the frequency range 20 Hz-2 MHz.