CHARGE FREEZING AND SURFACE ANISOTROPY ON MAGNETITE(100)

Scanning tunneling microscope images of the (100) surface of slightly nonstoichiometric magnetite taken at room temperature show static arrays of pairs of Fe2+ ions with short-range order, and a charge fluctuation time greater than 10(3) s. The surface appears to be a Wigner glass with electron pairs localized on adjacent ions as the basic unit. The explanation of Wigner localization at room temperature on the surface only is that the spin-polarized minority-spin band derived from d(yz) orbitals is stabilized and narrowed by the absence of an apicial oxygen from the B-site octahedron. This leads to surface anisotropy where the Fe2+ spins are pinned normal to the {100} surfaces. Surface anisotropy is expected to outweigh bulk anisotropy in submicron particles.