The surface termination, structure, morphology and composition of Fe3O4(001) were investigated using scanning tunnelling microscopy, low-energy electron diffraction, low-energy He+-ion scattering and X-ray photo-electron spectroscopy. The samples consisted of ≈5000Å-thick epitaxial films of Fe3O4(001) grown by oxygen-plasma-assisted molecular-beam epitaxy onto MgO(001) substrates. The (√2 x √2)R45° surface reconstruction that was present on the as-grown surface was recovered by heating the sample in oxygen (10−6 to 10−7mbar) at between 420 and 770K after through-air transfer from the molecular beam epitaxy chamber. The scanning tunnelling microscopy results were best interpreted by assuming an autocompensated B-layer termination, which consisted of a layer of octahedrally coordinated iron and tetrahedrally coordinated oxygen, together with one oxygen vacancy per unit cell. Evidence for a vacancy-induced lateral relaxation of the adjacent octahedral iron ions was presented. Further annealing in ultra-high vacuum caused a transformation to either a (1 x n) or a (2√2 x √2)R45° structure. These surfaces could be reproducibly transformed back to the (√2 x √2)R45° reconstruction by annealing in oxygen. At no time was the other autocompensated termination observed which consisted of one-half of a monolayer of tetrahedrally coordinated Fe(III), in spite of its observation on the as-grown surface. Thus, it appeared that the surface termination was critically dependent upon the method of surface preparation.

Surface Reconstruction of Fe3O4(001). B.Stanka, W.Hebenstreit, U.Diebold, S.A.Chambers: Surface Science, 2000, 448[1], 49-63