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 arra ys of pairs of Fe2+ ions with short-range order, and a charge fluctuat ion time greater than 10(3) s. The surface appears to be a Wigner glas s with electron pairs localized on adjacent ions as the basic unit. Th e explanation of Wigner localization at room temperature on the surfac e only is that the spin-polarized minority-spin band derived from d(yz ) orbitals is stabilized and narrowed by the absence of an apicial oxy gen from the B-site octahedron. This leads to surface anisotropy where the Fe2+ spins are pinned normal to the {100} surfaces. Surface aniso tropy is expected to outweigh bulk anisotropy in submicron particles.