Interpretation of electrical conductance transition of hematite in the spin-flip magnetic transition temperature range

Using polycrystalline pure hematite at frequencies of 100 Hz-100 kHz and temperatures of 190-350 K impedance spectroscopy was employed to investigate the electrical conductance transition and correlate it with magnetic transition occurring within this temperature range. A background of slight impurity donor or acceptor semiconductivity was observed above which a peak of conductivity appeared in the temperature range of magnetic transition. The results suggested that the transition from antiferromagnetically to weak ferromagnetically coupled Fe3+ and vice versa takes place through their transformation to uncoupled Fe3+ and an equilibrium between these types of coupled and uncoupled pairs of Fe3+ is established. A model, thermodynamically sustained, involving bulk concentration of both types of coupled and of uncoupled Fe3+ was formulated precisely predicting the dependence of magnetic transition on temperature and the appearance of peaks in both the uncoupled Fe3+ concentration and conductivity within the transition temperature range. The conductivity, mainly due to intrinsic semiconductance coming from the activation of uncoupled Fe3+, depends on both temperature and concentration of uncoupled Fe3+. The heretofore elusive semiconductive character of hematite is explained. (C) 2004 The Electrochemical Society.