Low temperature reorientation of Fe spins in an irradiated hematite single crystal

Irradiation with alpha-particles of 29 and 50 MeV energies leads to essential changes in magnetic properties of hematite single crystals. When temperature decreases, the number of iron ions with high-temperature spin orientation decreases too but does not disappear completely, and at helium temperatures, in contrary, increases (the new transition). Appearance of the new transition is explained by superparamagnetic behaviour of the disordered zones produced by atom-atomic collision cascades. The splitting of the Müssbauer spectra in the temperature interval of the Morin transition as well as at the new transition undoubtedly shows that the both transitions are transitions of the first order.