Surface x-ray-diffraction measurements have been done in cobalt ultrathin films (2–12 atomic layers) grown on Pt(111). Cobalt grows, at room temperature, in an imperfect layer-by-layer mode with its own lateral lattice spacing. The packing of the atomic planes is predominantly fcc but it changes to hcp upon annealing to $\sim 450\mathrm{K}.$ Further annealing to $\sim 550\mathrm{K}$ results in alloying. Several alloys have been identified with stoichiometries close to the bulk stable phases. Resonant magnetic diffraction experiments in the $L_{\mathrm{III}}$ absorption edge of Pt allow one to determine the magnetism of the Pt atoms at the interface. By measuring magnetic crystal truncation rods, it has been found that only the Pt atoms in contact with the Co overlayer are magnetized $\left(\sim 0.2{\mu }_B\right)$ being the magnetization of the second Pt layer about ten times smaller. The induced magnetic moment of the Pt interface has been found to depend rather linearly on the thickness of the Co film, similarly to the magnetization of the overlayer. The transformation fcc to hcp of the cobalt stacking results in a ∼25% enhancement of the magnetization at the interface.

• Received 7 April 1997

DOI:https://doi.org/10.1103/PhysRevB.56.9848