Self Reversal of Chemical Remanent Magnetization On the Transformation of Maghemite to Haematite

Abstract

SUMMARY Self-reversed chemical remanent magnetization (CRM) has been observed in haematite formed on heating maghemite which has been produced by the dehydration of acicular crystals of synthetic lepidocrocite, 1 to 2 pm in length. Our experimental evidence suggests that self reversal of haematite remanence only occurs when the parent maghemite is still blocked at the temperature of its transformation to haematite; when the transformation temperature is above the blocking temperature of the parent maghemite and it is unblocked, the resulting haematite remanence is normally magnetized. It is suggested that the strong dependence on remanent state supports exchange control of the self-reversal process. We propose that the self reversal is probably a general feature of the maghemite to haematite transition, and the significance of the source lepidocrocite in our experiments is that it produces maghemite of a suitable grain size so that much of it remains blocked at the elevated temperatures required to make the transformation to haematite occur over the short time scale of the laboratory experiments. In nature, transformation of maghemite to haematite will occur at much lower temperatures due to the much longer time scales involved, and self reversal of the resulting CRM may occur over a much less restricted grain size range of parent maghemite.