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Abstract
$\rm Fe\sb3O\sb4$ films were grown by sputter deposition on a variety of substrates. X-ray diffraction studies of the films grown on MgO show they are uniformly strained epitaxial single crystal specimens. Conversion electron Mossbauer spectroscopy (CEMS) spectra for films on all substrates show the presence only of the stoichiometric $\rm Fe\sb3O\sb4$ phase. However, the CEMS spectra exhibit an anomalous out-of-plane moment distribution, although the moments are expected to be in the plane of the film due to the large shape anisotropy. Furthermore, the magnetization remains unsaturated in fields as large as 70 kOe, and torque measurements of films grown on MgO remain unsaturated at 21 kOe, where saturation is expected near 0.5 kOe. The extrapolated values for the anisotropy, derived from torque curves taken both in and out of the film plane, are much smaller than that required to cause the lack of saturation in high fields and the anomalous CEMS spectra, and are well explained as a combination of crystalline, magnetoelastic, and shape anisotropy of bulk single crystal $\rm Fe\sb3O\sb4$ subjected to in-plane tensile stress. The anomalous behavior is exhibited in films grown on other substrates and by other techniques, and is independent of thickness and deposition conditions. It appears to be an intrinsic property of all $\rm Fe\sb3O\sb4$ films. Structural characterization using transmission electron microscopy showed the presence of a high density of antiphase boundaries (APBs) in the single crystal films grown on MgO. Across the APB, the intrasublattice superexchange coupling is greatly strengthened, while the intersublattice superexchange coupling is weakened. Thus the APB's separate magnetic domains oppositely magnetized, which is consistent with Lorentz microscopy measurements. Thus the APB defects produce the very large saturation fields and anomalous, nearly random, remanent magnetization. Thin films of single crystal iron oxide were surface treated with cobalt ions using a wet chemical method. Rutherford backscattering measurements were consistent with approximate monolayer surface coverage with cobalt. The film anisotropy is strongly affected for treatment on the (110) surface and less on the (100) surface. A model considering the magnetoelastic energy of a such a layer is presented.
