Spin-dependent properties and images of MnO, FeO, CoO, and NiO(001) surfaces

Abstract

We investigate the nonpolar, cleavage $(001)(2\ifmmode\times\else\texttimes\fi{}1)$ surfaces of antiferromagnetic late transition-metal oxides by means of the density functional theory for non-collinear spins and including the spin-orbit interaction. The effects of strong Coulomb interaction among the localized transition-metal $3d$ electrons are described by an on-site Hubbard $U$ added to the exchange-correlation functional in the local-density approximation. This approach guarantees finite energy gaps also for FeO and CoO, although they are still underestimated. Besides a change in the geometry due to rumpling and relaxation, we find that the size and orientation of the surface magnetic moments are also influenced by the surface. Electronic surface states appear in the fundamental gap close to the edges of the projected bulk bands. Only in the NiO case does an empty, well-separated band of ${e}_{g}$-derived surface states occur in the fundamental gap. As a consequence the scanning tunneling microscopy (STM) images exhibit almost the bulk $1\ifmmode\times\else\texttimes\fi{}1$ translational symmetry. For ferromagnetic tips strong spin contrasts are observed in spin-polarized STM. As a consequence of the bulk antiferromagnetic ordering, mainly chain structures become visible, independent of the tunneling into empty states or out from filled ones. Clear chemical trends with the occupation of the ${t}_{2g}$-minority-spin channel states along the row $\mathrm{MnO}\ensuremath{\rightarrow}\mathrm{NiO}$ are observed and discussed.