Nanosized CoO Films on the α-Al2O3 (0001) Surface: A Density Functional Study

The structural and magnetic properties of ultrathin CoO films supported on alpha-Al2O3(0001) surface are investigated on the basis of density functional theory calculations. By examining both stoichiometric and nonstoichiometric submonolayer structures at an atomic level we establish that the preferred surface arrangements are stabilized by the presence of a sublayer of O adatoms, even at the expense of creating O vacancies at the subsurface layer. Further addition of oxygen to this thin COO layer fills the vacancies resulting in the formation of nondefective, atomically smooth oxygen-terminated surfaces as the preferred structural arrangement. Later, we examine the possible coexistence of rock-salt, wurtzite, or blende-type structures and different arrangements of the Co atoms magnetic moments at higher coverages. The observed structures and magnetic arrangements are rationalized on the basis of the surface dipole moment reduction induced by the O-termination of the overlayers and on the observed intra- and interlayer distances that favor, at these coverages, an intralayer antiferromagnetic arrangement of the Co magnetic moments.