MAGNETIC PHASE-TRANSITIONS IN IRRADIATED HEMATITE

In haematite the magnetic phase change from a weak ferromagnetic state to an antiferromagnetic state that occurs as the temperature is decreased through the critical value is known as the Morin transition. The different magnetic states give rise to distinct Mossbauer spectra and this technique was used to study the phase changes in samples of haematite that had been exposed to different levels of neutron irradiation. At relatively low levels of irradiation the normal Morin transition was observed at a temperature lower than that for non-irradiated haematite. In samples with higher levels of irradiation, as the temperature was decreased, the normal weak ferromagnet to antiferromagnet transition occurred, but on further decrease of temperature a reverse phase transition was observed back to the weak ferromagnetic state. The magnetic phase transitions occurring in all the irradiated samples were studied and the critical temperatures measured. The mechanism and conditions required for the normal and reverse Morin phase transitions are discussed in terms of a model of competing single-ion and magnetic dipole anisotropy energy. It is found that the model can account for the sequences of transitions and the critical temperatures. The nature of the transitions is seen to be a mixture of first and second order.