Magnetic properties and rotational hysteresis losses of oxidized approximate to 250 nm Fe3O4 particles

The magnetization and rotational hysteresis were measured for dispersed Fe3O4 particles that were partly oxidized at temperatures of 600 and 700 degrees C in air. It was found that after oxidation for 4 min at 600 degrees C the magnetic component was in fact gamma-Fe2O3, as deduced from saturation magnetization data sigma(s) and Mossbauer spectra. A drop in the relative saturation remanence sigma(rs)/sigma(s) and coercive force H-c took place compared to the original Fe3O4. Further heating depresses sigma(s) more and more while sigma(rs)/sigma(s) and H-c remain rather stable. The rotational hysteresis W-RH shows a characteristic increase at large magnetic fields after prolonged heating which is interpreted in terms of exchange anisotropy by gamma-Fe2O3-alpha-Fe2O3 interfaces.