SURFACE AND INTERFACE PROPERTIES OF EPITAXIAL FE3O4 FILMS STUDIED BY MOSSBAUER-SPECTROSCOPY

Stoichiometric Fe3O4 films have been formed epitaxially on alpha-Al2O3 and MgO single-crystal substrates by a reactive vapor deposition method. In order to apply conversion electron Mossbauer spectroscopy depth-selectively, a 5-7 angstrom-thick probe layer containing Fe-57 was formed at various depths in inactive (Fe3O4)-Fe-56 matrix films. At the topmost surfaces and also at the interfaces, the essential electronic features of bulk Fe3O4 are retained, including a rapid electron hopping between the Fe2+ and Fe3+ ions at B sites. Minor depth-dependent changes are confined to a few outermost atomic layers, and the changes depend on the orientation and the lattice mismatch with the substrate. For (111) growth on alpha-Al2O3, the surface layer seems to be strongly relaxed to reduce the electric polarization, while a high density of defects seems to be concentrated at the interface with alpha-Al2O3. For (001) growth on MgO, the surface retains the spinel lattice though slightly oxidized, while the interface with MgO has good crystallinity and stoichiometry. An enhanced thermal fluctuation of the Fe3+-spins in contact with the MgO substrate and in the topmost surface layer can be seen in their reduced magnetic hyperfine field at 300 K.