Field-induced phase transitions in neutron-irradiated haematite

Abstract

Mossbauer spectra of three neutron-irradiated polycrystalline haematite ( alpha -Fe2O3) samples have been recorded at 4.2 K in applied fields of up to 10 T. Field-induced phase transitions were observed, and were found to differ markedly from those that occur in non-irradiated haematite. In two lightly irradiated samples, transitions were detected from a state with Fe3+ spins near the (111) axis ('antiferromagnetic phase') to one where the spins are almost perpendicular to both the applied field and to the (111) axis ('weak ferromagnetic phase'). In the case of a heavily irradiated sample, in which the spins were initially in the weak ferromagnetic phase, the spins were observed to lie almost perpendicular to applied fields of greater than approximately 4.5 T. The difference in behaviour in irradiated and non-irradiated haematite is attributed to a smaller difference in anisotropy energy, in the irradiated samples, between the antiferromagnetic and weak ferromagnetic phases at 4.2 K.