Real-space imaging of the Verwey transition at the (100) surface of magnetite

Effects of the Verwey transition on the (100) surface of magnetite were studied using scanning tunneling microscopy and spin polarized low-energy electron microscopy. On cooling through the transition temperature T _V, the initially flat surface undergoes a rooflike distortion with a periodicity of ∼0.5 μm due to ferroelastic twinning within monoclinic domains of the low-temperature monoclinic structure. The monoclinic c axis orients in the surface plane, along the [_001]c directions. At the atomic scale, the charge-ordered (√2×√2)R45 ^â̂̃ reconstruction of the (100) surface is unperturbed by the bulk transition, and is continuous over the twin boundaries. Time resolved low-energy electron microscopy movies reveal the structural transition to be first order at the surface, indicating that the bulk transition is not an extension of the Verwey-like (√2×√2)R45 ^â̂̃ reconstruction. Although conceptually similar, the charge-ordered phases of the (100) surface and sub-T_V bulk of magnetite are unrelated phenomena.