Nonepitaxially grown nanopatterned Co–Pt alloys with out-of-plane magnetic anisotropy

A study on the structural and magnetic properties of 5-nm-thick Co–Pt alloy films grown on thermally oxidized $SiO2/Si(100)$ substrates as well as on self-assemblies of spherical $SiO2$ particles with sizes down to 10 nm is presented. An out-of-plane easy axis of magnetization was stabilized at deposition temperatures as low as $250°C$ in a broad composition range between 40 and $70at.%$ of Pt. Owing to the low deposition temperatures, no chemical long-range order is found. Thus, the strong out-of-plane magnetic anisotropy is expected to be caused by anisotropic short-range order effects. The magnetic behavior of CoPt alloys with an equiatomic composition grown on arrays of $SiO2$ particles was found to be similar to those on planar substrates. Structural investigations using high-resolution transmission electron microscopy revealed that a continuous CoPt layer has been formed, covering the particle tops and connecting them. The magnetic CoPt caps exhibit an out-of-plane easy axis for all particle sizes; however, no pronounced difference in coercive field with particle size was observed, which is associated with the specific morphology of the film structure.