Atomic and electronic structure of polar Fe2O3(0001)/MgO(111) interfaces

We present a first-principles investigation of the structural, electronic, and magnetic properties of ultrathin Fe2O3(0001) films on a polar MgO(111) substrate. The results imply that the heterointerface is atomically abrupt with oxidelike stacking for film thicknesses between similar to 1.5 and 8.5 angstrom. The Fe-Fe bilayer (nominal separation of 0.59 angstrom in Fe2O3) at the interface collapses into an "Fe-2" monolayer. Both electronic polarization and structural relaxations effectively screen the dipole field of the polar interface system. The structural relaxations-consisting of interpenetration, separation, and merger of Fe and oxygen planes-are particularly drastic in the three-and four-bilayers-thick films, giving rise to barrierless movement of oxygen towards the surface and the formation of an "Fe-2 vertical bar FeO3" layer structure not seen in hematite. Comparisons to calculations of unsupported polar Fe2O3(0001) slabs demonstrate that these unusual changes in stacking sequence and electronic structure are associated with the polar nature of this oxide heterointerface.