Angle-, field-, temperature-, and size-dependent magnetic circular X-ray dichroism in Au/Co nanoclusters/Au(111)

We have studied the magnetic states of Au/2-monolayer Co clusters/Au(1 1 1) by angle-, field-, temperature-, and cluster-size-dependent magnetic circular X-ray dichroism (MCXD) measurements in the longitudinal geometry with full use of a magic angle of 54.7°. The absolute magnitude of the normal-incidence MCXD provides evidence for a single domain of clusters and for a phase transition from ferromagnetism (FM) to superparamagnetism (SPM) with decreasing cluster size. The spin, in-plane and out-of-plane orbital, and in-plane and out-of-plane magnetic dipole moments of almost purely interfacial Co atoms were directly determined. The interfacial spin moment is strongly enhanced and the interfacial orbital and magnetic dipole moments are highly anisotropic and enhanced as compared with those in bulk Co, verifying theoretical predictions. We argue the implications of the results in particular relation to their physical origin and to a potential technological application of the nanoclusters.