Unusual magnetic behavior in sputtered FeO and alpha-Fe2O3 thin films

A ferromagnetic-type behavior has been found in sputtered FeO and alpha-Fe2O3 thin films at low temperatures. This observation is in contrast to the bulk antiferromagnetic properties of FeO and alpha-Fe2O3. At room temperature the coercivity of the films is negligible, but below 150 K it increases rapidly and at 10 K it is larger than 3 kOe for both kinds of iron oxide films. The saturation magnetization of the films is large (with a maximum of 270 emu/cc in FeO and 140 emu/cc in alpha-Fe2O3 samples) and thickness dependent. Mossbauer spectra of alpha-Fe2O3 films could be analyzed by two magnetic components, one very close to bulk alpha-Fe2O3 and the other one identified as surface alpha-Fe2O3. Mossbauer spectra of FeO films showed that the samples were off-stoichiometric, with about 10% Fe deficiency. Clusters of tetrahedrally coordinated Fe3+ ions were observed in an environment similar to that in Fe3O4. The hysteresis behavior in FexO films can be explained by the defect clusters of Fe3+ ions, and by uncompensated spins in the surface component of alpha-Fe2O3 films. Shifted hysteresis loops were observed in field-cooled samples, indicating a strong exchange coupling between the regions with the net magnetic moment and the antiferromagnetic magnetic component. (C) 1997 American Vacuum Society.