Charge ordering on the surface of Fe3O4(001)

Abstract

We have studied the (001) surface of a Fe3O4 single crystal using scanning tunneling microscopy (STM), low-energy electron diffraction (LEED), and Auger electron spectroscopy. A clean surface was obtained by a combination of Ar+ ion sputtering, in situ annealing in O2 atmosphere, and further annealing in ultra high vacuum conditions. A sharp (?2??2)R45? reconstruction was observed by LEED and STM analysis at room temperature. The STM measurements were performed using a tip of antiferromagnetic MnNi alloy. Atomically resolved STM images provide evidence of a surface terminated at the octahedral plane, with rows of Fe cations running along the [110] and [11?0] crystallographic axes. The 3 ? periodicity of the Fe rows expected for a bulk-terminated B plane is not observed. Instead, two different kinds of Fe cations with a separation of 6 ? are imaged. The periodicity between Fe cations of the same kind is about 12 ?. We propose a model to explain the observed symmetry in terms of charge ordering of the Fe cations on the octahedral sites of the inverse spinel lattice. We also explain the (?2??2)R45? LEED pattern in terms of charge ordering, as opposed to a structural rearrangement of the atoms on the surface. We further suggest the possibility of a spin polarized effect, in view of the anomalous corrugation observed along the [110] Fe rows on the octahedral plane, in agreement with the different spin configuration of Fe2+ and Fe3+ ions.