Transformation of hematite to maghemite as observed by changes in magnetic parameters: Effects of mechanical activation?

A natural hematite of hydrothermal origin shows evidence of time‐dependent behavior related to the sample preparation procedure (involving significant crushing and milling). A strongly magnetic phase with a Curie temperature (∼630°C) corresponding to that of maghemite is present after storage for one‐and‐a‐half‐years, while being absent in the original sample material. The magnetic properties of the hematite‐maghemite in relation to mechanical treatment are compared and possible mechanisms of the hematite transformation are discussed. The hematite‐maghemite transformation is suggested to have resulted primarily from recrystallization induced by shear stress resulting from crushing and milling.