Magnetic Properties of Co Clusters on Alumina on Ni3Al(111)

Full text: The structure of the ∼0.5 nm thick oxide film on Ni3Al(111) exhibits holes reaching down to the metal substrate at the corners of the (√67 x √67)R12.2o unit cell. Pd atoms trapped in these corner holes create metallic nucleation sites where Co clusters can nucleate and form a well-ordered hexagonal arrangement on the oxide nanomesh. The crystal structure of these Co clusters has been well characterized by scanning tunneling microscopy (STM), surface x-ray diffraction (SXRD) and grazing-incidence small-angle x-ray scattering (GISAXS). They show random stacking of closed packed Co planes parallel to the substrate surface. Magnetic properties of Co clusters with ∼350 atoms/cluster have been investigated by x-ray magnetic circular dichroism (XMCD) assisted by STM. This technique allowed us to measure the orbital and spin contribution to the magnetic moment per Fe or Co atom, as well as magnetization curves down to temperatures of 7 K and up to fields of 5 T and correlate the measured magnetic properties to the structure of the clusters. The Co clusters are super-paramagnetic down to 7K and show bulk like magnetic moments of ∼2 μB. From fitting the magnetization curves the magnetic anisotropy energy (MAE) and the direction of the easy axis can be determined. The MAE of the Co clusters is enhanced by a factor of . 4 compared to hcp bulk Co and the easy axis is not perpendicular to the surface but tilted. The magnetization curves also suggest magnetic coupling to the substrate, which depends on the number of Pd atoms in the corner hole below the clusters. (author)