Mesoscopic organization of cobalt thin films on clean and oxygen-saturated Fe(001) surfaces

The different morphologies of Co films grown on either the clean Fe(001) surface and the oxygen-saturated Fe(001)-p(1 71)O substrate are investigated by means of scanning tunneling microscopy, Auger electron spectroscopy, and density functional theory. The considered Co coverage range extends beyond the thickness at which layer-by-layer growth is destabilized by plastic deformations induced by the relaxation of the strain accumulated in the film. Our findings indicate that the oxygen overlayer of the Fe(001)-p(1 71)O surface floats on top of the growing Co film and strongly influences both the Co nucleation process and the film structural evolution. The layer-dependent islands nucleation of Co films grown on clean Fe(001) substrates, recently associated with a thickness-dependent adatom mobility [A. Picone et al., Phys. Rev. Lett. 113, 046102 (2014)], is found to be suppressed by the oxygen overlayer. The latter also significantly delays the layer-by-layer instability with respect to the oxygen-free growth. Furthermore, the body-centered-tetragonal/hexagonal-close-packed transition is not observed in the case of Co/Fe(001)-p(1 71)O sample, replaced by the development of highly ordered surface undulations. These form a mesoscopic square pattern with the sides aligned to the Fe\u27e8110\u27e9 directions, while the surface atomic structure retains the square p(1 71) symmetry in registry with the substrate. Such undulations are likely generated by a highly ordered array of interfacial misfit dislocations running along the Fe\u27e8110\u27e9 directions