The (001) surface of Fe_3O_4 grown epitaxially on MgO and characterized by scanning tunneling microscopy

摘要

Spinel Fe_3O_4 contains two sites for iron: tetrahedrally coordinated sites containing Fe~3+ ions and octahedrally coordinated sites containing a mixture of Fe~2+ and Fe~3+ ions. Scanning tunneling microscopy performed on the (001) surface of Fe_3O-4, grown epitaxially on MgO, shows localized charge density at the tetrahedral sites. the images show that the p(1x1) surface reconstruction (also observed during molecular beam epitaxy of Fe_3O_4) 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 corrugationof electron charge density along the octahedral sites is consistent with the original conjectures explaining the high electrical conductivity in bulk Fe_3O_4: electrons move by hopping between the Fe~3+ and Fe~2- atoms along the octahedral rows of Fe ions.