Initial Stages of the Growth of Mixed Iron-cobalt Oxides on Ru(0001)

Mixed iron-cobalt oxides have been grown on a Ru(0001) single crystal substrate by reactive molecular beam epitaxy. The growth has been followed by low-energy electron microscopy and diffraction. Chemical characterization has been performed by selected area x-ray absorption spectroscopy. As previously known, iron grows into a wetting layer of FeO. In contrast, cobalt grows into three-dimensional islands of CoO, of either with a (111) -most common- or a (100) orientation. For mixed compositions, flat 2D growth is regained. Depending on temperature, either segregation into two FeCo compositions or a single phase is detected. In all cases the structure corresponds to an in-plane expanded (111)-oriented halite one. When only one phase is observed or for the Co-rich phase in the two phase film, its crystal structure is rotated by 30° relative to the Ru substrate, unlike the Co-poor phase which appears aligned with the substrate.