Surface reconstruction of Fe3O4(001)

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 similar to 5000 Angstrom thick epitaxial films of Fe3O4(001) grown by oxygen-plasma-assisted molecular-beam epitaxy (OPA-MBE) on MgO(001) substrates. The (root 2 x root 2)R45 degrees 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 420 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 iron and tetrahedrally coordinated oxygen, along with one oxygen vacancy per unit cell. Evidence for a vacancy-induced lateral relaxation of the adjacent octahedral iron ions is presented. Further annealing in ultrahigh vacuum causes a transformation to either a (1 x n) or a (2 root 2 x root 2)R45 degrees structure. These surfaces can be reproducibly transformed back to the (root 2 x root 2)R45 degrees reconstruction by annealing in oxygen. Interestingly, at no time do we observe the other autocompensated termination, which consists of one-half of a 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. (C) 2000 Elsevier Science B.V. All rights reserved.