Magnesium interdiffusion and surface oxidation in magnetite epitaxial films grown on MgO(100).

Epitaxial Fe3O4(001) thin films were grown on MgO(001) using molecular beam of Fe-57 at the oxygen partial pressure of about 1 x 10(-6) mbar. LEED patterns indicated perfect growth with a p(1 x 1) reconstructed surface (as labeled with respect to the bulk unit cell of magnetite). In situ UHV conversion electron Mossbauer spectroscopy (CEMS) was al,plied to investigate the film stoichiometry and composition. GEMS spectra taken for films in the 10 nm range revealed a magnesium rich magnetite MgxF3-xO4 phase formed at the MgO/Fe3O4 interface. Ex situ Mossbauer measurements proved that the magnetite surface oxidizes when exposed to the atmosphere for a couple of weeks. The oxidized phase was identified as a few nanometers thick surface layer of gamma -Fe2O3 formed on Fe3O4. By virtue of the gamma -Fe2O3 GEMS pattern, the oxidation can be easily misinterpreted as a surface non-stoichiometry for films studied only ex situ. The oxidation process is reversible and annealing at 600 K in UHV restores the magnetite surface structure. At elevated annealing temperature Fe also diffuses into MgO forming a wustite phase. (C) 2001 Elsevier Science Ltd. All rights reserved.