Structure of ultrathin films of Co on Cu(111) from normal-incidence x-ray standing wave and medium-energy ion scattering measurements

Applications of the techniques of normal-incidence x-ray standing wave (NIX SW) and medium-energy ion scattering (MEIS) to the elucidation of the struc ture of an ultrathin metallic film, Co on Cu(lll), are reported. NIXSW and MEIS are shown to yield valuable and complementary information on the struc ture of such systems, yielding both the local stacking sequence and the glo bal site distribution. For the thinnest films of nominally two layers, the first layer is of entirely fee registry with respect to the substrate, but in the outermost layer there is significant occupation of hcp local sites. For films up to 8 monolayers (ML) thick, the interlayer spacing of the Co l ayers is 0.058+/-0.006 Angstrom smaller than the Cu substrate (111) layer s pacing. With increasing coverage, the coherent fraction of the ((1) over ba r 11) NIXSW decreases rapidly, indicating that the film does not grow in a fee continuation beyond two layers. For films in this thickness range, hcp- type stacking dominates fee twinning by a ratio of 2:1. The variation of th e ((1) over bar 11) NIXSW coherent fraction with thickness shows that the t winning occurs close to the Co/Cu interface. For thicker films of around 20 ML deposited at room temperature, medium-energy ion scattering measurement s reveal a largely disordered structure. Upon annealing to 300 degreesC the 20-ML films order into a hcp structure.