Depth selective Mössbauer spectroscopic study of Fe₃O ₄ epitaxial films

Several (111) oriented ⁵⁶Fe₃O₄ films containing a 5-Å-thick ⁵⁷Fe₃O₄ probe layer at or below the surface were grown epitaxially on the α-Al ₂O₃ (0001) surface by a reactive vapor deposition method. Conversion electron Mössbauer spectroscopy was applied at 6, 78, and 300 K by using a recently developed helium-filled proportional counter. The well-crystallized surface has been found to be surprisingly stable even in air as characterized by the Mössbauer parameters that are almost the same as for the bulk. Moreover, the Verwey transition was detected clearly even in the 5-Å-thick surface layer. However, the ferrous components seem to have changed their Mössbauer parameters probably because of their sensitivity to any crystalline field modifications in the surface. Generally speaking, the quality of epitaxial Fe₃O₄ films is very high: Any unusual surface state, if present, is confined in a shallow depth of ≤ 5 Å.