Stable Thermoremanence and Memory in Hematite

Thermoremanent magnetization (TRM) has been studied for synthetic hematites having grain sizes between 0.04 and 0.4 micrometer and a natural single crystal of hematite (10mm), before and after low-temperature demagnetization (LTD), which consists of zero field cycling through the Morin transition Tm. At Tm, a spin-flop transition occurs, the magnetocrystalline anisotropy constant passes through zero and changes sign, and spin-canted remanence vanishes. We measured stepwise thermal demagnetization curves of 1 mT TRM, with and without prior LTD. In submicron hematites, as in the natural crystal, TRM has no unblocking temperatures below 600 C. LTD demagnetized about 55-60% of the initial TRM for the submicron hematites. The memory fraction of TRM recovered after LTD was very stable against thermal demagnetization. Absolutely no decrease, in fact a very slight increase, in memory occurred up to 600 C. In the natural hematite crystal, TRM memory was only 15% of the original TRM. The thermal demagnetization curve after LTD was flat between room temperature and 500 C. In further demagnetization above 550 C the memory increased, peaking just below the Neel temperature, indicating a reversely magnetized fraction of TRM which constitutes about 50% of the TRM memory. Low-temperature demagnetization is an effective way of removing the spin-canted remanence and isolating the defect moment in hematites. Crystal defects and resulting stress centers are likely responsible for the TRM memory in submicron and multidomain (MD) hematites. The reversed fraction of TRM memory in the large MD crystal is probably controlled in part by the phase coupling between the antiferromagnetic phase and weakly ferromagnetic phase which resides in defect centers.