Stacking reversal as a source of perpendicular magnetic anisotropy in Ni-Pt multilayers

Abstract

Based on surface x-ray diffraction and resonant magnetic surface x-ray diffraction measurements on ultrathin films of Pt on Ni(111) and Ni on Pt(111), we propose a simple explanation for the perpendicular magnetic anisotropy (PMA) observed at room temperature in Ni-Pt multilayers. The stacking sequence of Ni grown on Pt(111) is the same as that of the Pt substrate (normal stacking: ABCABC) whereas that of Pt grown on Ni(111) is reversed (ACBACB). As a consequence the Ni layers in Ni-Pt multilayers alternate between normal and reversed stacking. The [111] direction, which is normal to the interface planes, is invariant under the two types of stacking whereas the other equivalent crystallographic directions are not. This specific symmetry selects the [111] direction as the preferred magnetic easy axis over all the multilayer stack, causing PMA. The influence of the Pt layer thickness on the PMA is also discussed.