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 meth od. In order to apply conversion electron Mossbauer spectroscopy depth -selectively, a 5-7 angstrom-thick probe layer containing Fe-57 was fo rmed 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 hoppin g between the Fe2+ and Fe3+ ions at B sites. Minor depth-dependent cha nges are confined to a few outermost atomic layers, and the changes de pend on the orientation and the lattice mismatch with the substrate. F or (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. Fo r (001) growth on MgO, the surface retains the spinel lattice though s lightly oxidized, while the interface with MgO has good crystallinity and stoichiometry. An enhanced thermal fluctuation of the Fe3+-spins i n contact with the MgO substrate and in the topmost surface layer can be seen in their reduced magnetic hyperfine field at 300 K.