i get the DSC results which shows two exothermic peak first around 250 C and second around 500 C .does it mean the magnetite transform to maghemite and maghemite transform to hematite respectively?
Actually, the conditions of thermal transformation are for magnetite strongly dependent on granulometry and the synthetic route. Some clues are provided in the following paper:
Journal of Thermal Analysis and Calorimetry, June 2003, Volume 72, Issue 3, pp 777-789