Moessbauer study of the Morin transition

At hand of Moessbauer spectra on two commercial powder hematite samples, it was found that the finite slope shown by the Morin transition reveals the presence of an additional spectral component associated to a second-order phase transition. This extends over a temperature range much larger than the temperature width of the thermal hysteresis of the Morin transition, which is of first order. The new component is attributed to a change in the sign of the fourth-order single-ion anisotropy field due to crystal defects or cation substitution. The results of previous works are discussed from this point of view. (orig.)