Opposite spin reorientation transitions driven by a magnetic orbital moment: Ultrathin Ni films on Cu surfaces

We observed two opposite spin reorientation transitions (SRT), perpendicular to in-plane magnetization and vice versa, in ultrathin Ni films grown on clean and preoxidized Cu(001) surfaces covered with Cu overlayers. The magneto-optical Kerr effect measurement shows that the Cu capping stabilizes perpendicular magnetization for the clean surface and in-plane magnetization for the preoxidized surface. Correspondingly, the x-ray magnetic circular dichroism measurement elucidates that upon Cu capping the Ni orbital moment is suppressed for clean Cu(001) and enhanced for preoxidized Cu(001). These clearly contrasting findings can be explained by the fact that oxygen atoms act as a surfactant; oxygen always locates at the surface even after Ni and Cu deposition. The present results clearly demonstrate that the modification of the in-plane orbital moment drives the SRT, which is sensitive to interfacial interaction.