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

A ferromagnetic-type behavior has been found in sputtered FeO and α-Fe2O3 thin films at low temperatures. This observation is in contrast to the bulk antiferromagnetic properties of FeO and α-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 α-Fe2O3 samples) and thickness dependent. Mössbauer spectra of α-Fe2O3 films could be analyzed by two magnetic components, one very close to bulk α-Fe2O3 and the other one identified as surface α-Fe2O3. Mössbauer 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 α-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.