Electronic structure of the (001) surface of half-metallic manganites

A good understanding of the physical properties of surfaces and interfaces of in colossal magneto-resistant hole-doped manganese oxides is highly desirable for future applications of these promising materials in magnetoresistive devices for spintronics. We present results for a fully relaxed (001) surface in the optimally doped half-metallic manganites La$_{0.7}$Sr$_{0.3}$MnO$_3$ in its distorted orthorhombic phase, based on density functional calculations with explicit doping. The crystal termination gives rise to a splitting in the degenerate $t_{2g}$ and $e_g$ levels, and a surface state with $d_{z^2}+d_{xy}$ character develops. The calculations for the relaxed structure show that the tilting of MnO$_6$ octahedra is reduced near the surface, and there is a layer buckling, with a Mn off-centering in the top octahedral layer similar to what observed for Ti in bulk BaTiO$_3$, which decreases rapidly inside the material. As a result of this distortions, the surface state shifts to lower energies, although it is still very localized in the first layer. The Mn off-centering favours the occupation of the conduction $d_{z^2}$ states, which become more localized and affects the magnetic properties at the surface.