Surface and interface properties of epitaxial Fe₃O₄ films studied by Mössbauer spectroscopy

Stoichiometric Fe₃O₄ films have formed epitaxially on α-Al₂O₃ and MgO single-crystal substrates by a reactive vapor deposition method. In order to apply conversion electron Mössbauer spectroscopy depth-selectively, a 5-7 Åthick probe layer containing ⁵⁷Fe was formed at various depths in inactive ⁵⁶Fe₃O₄ 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 α-Al₂O₃, the surface layer seems to be strongly relaxed to reduced the electric polarization, while a high density of defects seems to be concentrated at the interface with α-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.