We have investigated the surface termination, structure, morphology, and composition of Fe3O4(001) using scanning tunneling microscopy (STM), low-energy electron diffraction (LEED), low-energy He+ ion scattering (LEIS), and X-ray photoelectron spectroscopy (XPS). The samples consisted of ~5000Å-thick epitaxial films of Fe3O4(001) grown by oxygen-plasma-assisted molecular beam epitaxy (OPA-MBE) on MgO(001) substrates. The (√2 × √2)R45° surface reconstruction that is present on the as-grown surface is recovered by heating the sample in oxygen (10-6 - 10-7 mbar) at temperatures between 520 K and 770 K after a through-air transfer from the MBE chamber. STM results are best interpreted by assuming an autocompensated B-layer termination, which consists of a layer of octahedrally-coordinated Fe and tetrahedrally-coordinated O, along with one O vacancy per unit cell. Evidence for a vacancy-induced lateral relaxation of the adjacent octahedral Fe ions is presented. Further annealing in UHV causes a transformation to either a (1 × n) or a (2√2 × √2)R45° structure. These surfaces can be reproducibly transformed back to the (√2 × √2)R45° reconstruction by annealing in oxygen. Interestingly, at no time do we observe the other autocompensated termination, which consists of one-half monolayer of tetrahedrally coordinated Fe(III), despite its observation on the as-grown surface. Thus, it appears that the surface termination is critically dependent on the method of surface preparation.
Correspondence to: Ulrike Diebold (Tulane University).
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