Thin films of Co on Pt(111): Strain relaxation and growth

The growth, structure and morphology of thin Co layers with a thickness ranging from 1 to 15 monolayers deposited at room temperature on Pt(111) have been studied by the use of scanning tunneling microscopy. We demonstrate that the first Co layer grows preferably in the Pt fcc lattice sites, with a high density of defects due to the lattice mismatch. The second Co layer is found to exhibit a moiré structure, with the Co in-plane lattice distance close to that of bulk Co. The growth of thin Co films is observed to be mostly in terms of flat layers (two dimensional) up to a Co coverage of about 3.5 ML. At higher coverages, we find that the Co grows in (three dimensional) islands and we show that the growth is characterized by a mainly twinned fcc-like stacking. We argue that the reason for the two dimensional growth mode at lower Co coverages is due to the strained interface between the Co overlayers and the Pt(111) surface resulting in a large number of kinks and corners which facilitate interlayer diffusion. For higher coverage such sites become less common, due to the decreasing influence of the strained interface, resulting in no interlayer diffusion leading to a three dimensional growth mode. The implications by these observations on the magnetic properties of the Co/Pt(111) interface system are discussed.