Electrical properties of nanocrystalline magnetite with large non-stoichiometry, near Verwey transition

DC electrical measurements were carried out on compacted powders of magnetite with an average particle diameter of 50 nm over the temperature range 10-300 K. The non-stoichiometry was estimated from Mossbauer spectroscopy analysis. High-resolution X-ray diffraction studies in the temperature range 93-300 K did not show any phase transition. There was a drastic change in resistivity around 80 K but no discontinuity thereof. Electrical resistivity vs. temperature data were analysed on the basis of Mott's small polaron and variable-range hopping models, respectively. The Verwey temperature as estimated from this analysis was 93 K. From voltage-current characteristics it was concluded that there was a small intrinsic gap at the Fermi level above the transition temperature and the same increased drastically below the transition temperature. This was ascribed to a transition from short-range order to long-range order as the temperature was lowered.