A specific heat study of natural haematite around the Morin transition and the effects of entrapped water

Abstract

The Morintransition in samples of natural haemitite has been examined by specificheat and M�ssbauer measurements. Specificheat measurements on as-received samples reveal an anomaly at ? 273.2 K and irregular behaviour at higher temperatures. However, heat treatment of samples demonstrated that these effects were due to water trapped inside the natural specimens.

No anomaly due to the Morintransition was observed in the specificheat although the expected changes in the M�ssbauer hyperfine parameters were observed for the different magnetic phases below and above the Morintransition. The results are consistent with a coexistence of the two phases and a spread of temperatures ? 40 K over which the transitions take place throughout the material of purity 99.6 wt%.