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 (similar to 630 degrees 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.