Cu Adsorption on Co Films: Edge Decoration versus Intermixing

Heteroepitaxial growth of the Co-Cu system is investigated by scanning tunneling microscopy. Cu deposited on Co/Cu(001) decorates the step edges of Co islands and forms nanometer-sized rings. Subsequent deposition of Co transforms the film morphology from compact to heterogeneously intermixed. We relate this growth asymmetry to the largely different surface free energies of Co and Cu, which imposes a diffusion barrier at Co-Cu boundaries, and to atomic exchange processes that occur in the presence of Co adatom gas.