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 Å 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 Å are imaged. The periodicity between Fe cations of the same kind is about 12 Å. 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.