T. Fujii et al., SURFACE AND INTERFACE PROPERTIES OF EPITAXIAL FE3O4 FILMS STUDIED BY MOSSBAUER-SPECTROSCOPY, Journal of magnetism and magnetic materials, 130(1-3), 1994, pp. 267-274
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.