The (001) Surface of Fe₃O₄ Grown Epitaxially on MgO and Characterized by Scanning Tunneling Microscopy

Abstract

Spinel Fe₃O₄ contains two sites for iron: tetrahedrally coordinated sites containing Fe³⁺ ions and octahedrally coordinated sites containing a mixture of Fe²⁺ and Fe³⁺ ions. Scanning tunneling microscopy performed on the (001) surface of Fe₃O₄, grown epitaxially on MgO, shows localized charge density at the tetrahedral sites. The images show that the p(1×1) surface reconstruction (also observed during molecular beam epitaxy of Fe₃O₄) is produced by a displacement of the two tetrahedrally coordinated Fe ions on the unit cell surface from their bulk positions toward each other. The octahedral Fe ions are imaged as extended rows of charge density, with no resolution of atom-size features along the rows. This slight corrugation of electron charge density along the octahedral sites is consistent with the original conjectures explaining the high electrical conductivity in bulk Fe₃O₄: electrons move by hopping between the Fe³⁺ and Fe²⁺ atoms along the octahedral rows of Fe ions.