Phase stability in non-stoichiometric iron-cobalt oxides prepared by mechanochemical activation

Mechanochemical processing has been used to produce magnetic nanocomposite fine powders based on cobalt doped magnetite (Fe3O4) and maghemite (gamma-Fe2O3). Characterisation of morphological and structural transformations during milling and after annealing processes was performed via x-ray diffraction and thermal analysis measurements were employed. Wet milling in oxygen free conditions of mixtures of hematite (alpha-Fe2O3) and cobalt hydroxide (Co(OH)(2)) resulted in several different phases, depending on Co:Fe ratio in the starting powder mixture. In the concentration range 3.7-13.3 atomic % the milled product shows a spinel type structure which can be -transformed into Co-doped maghemite under isothermal annealing at 550 K in air. For Co concentrations above this range multiphase Co -and Fe-oxides were formed directly by milling in the same conditions.