Reentrant Morin Transition in Natural Haematite Crystal at Low Temperature

We investigate the magnetic properties of natural haematite ($\alpha$-Fe${}_{2}$O${}_{3}$) crystal at low temperature using a SQUID and ${}^{57}$Fe nuclear magnetic resonance (NMR). The M (T) curve shows that the net magnetization in the (111) plane vanishes at Morin transition occurring around 260 K but reappears as temperature decreases below 30 K. The M(H) curve shows that the hysteresis exists in the (111) plane and the spin-flop transition also occurs when strong external field is applied in parallel with the [111] direction at low temperature. We obtain the NMR spectra in external magnetic field of up to 7 T at 4.2 K. Comparison of the M(H) curve and the field dependence of the NMR resonance frequency suggests that the antiferromagnetic and the weak-ferromagnetic phases coexist at low temperature. Assuming that the weak- ferromagnetic state at low temperature is the same with that above Morin transition, the amount of the weak-ferromagnetic phase in the antiferromagnetic bath at low temperature is about 3 $\%$.