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 Fe” ions with short-range order, and a charge fluctuation time greater than lo3 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-poIarized minority-spin band derived from d,,= orbitals is stabilized and narrowed by the absence of an apicial oxygen from the B-site octahedron. This leads to surface anisotropy where the Fe?-+ spins are pinned normal to the {lOO} surfaces. Surface anisotropy is expected to outweigh bulk anisotropy in submicron particles,