Understanding of the magnetocrystalline anisotropy in magnetic materials (the influence of different elemental components on the direction of easy magnetization) can be greatly enhanced by measuring the orbital moment anisotropy of the elemental constituents. A circular x-ray dichroism technique is presented that allows the determination of the microscopic origin of the spin reorientation transition in ultrathin single-crystalline cobalt/nickel films. The stronger anisotropy contribution of a much thinner cobalt layer redirects the easy magnetization direction of the entire film.

Systematic measurements of the magnetic moment per Ni atom in Cu/Ni/Cu/Si͑001͒ structures have been made using polarized neutron reflection ͑PNR͒ for Ni thicknesses in the range 30 ÅϽtϽ400 Å at room temperature. We find a dramatic reduction in the magnetic moment per atom for tϽ100 Å and near bulk values above 100 Å. These results are corroborated by alternating gradient magnetometer measurements on the same samples. A Cu/Ni-wedge/Cu/Si͑001͒ structure with 30 ÅϽtϽ150 Å was studied using magnetic circular x-ray dichroism ͑MCXD͒, polar magneto-optical Kerr effect ͑MOKE͒, and reflection high-energy electron diffraction ͑RHEED͒ in order to estimate the variation in the values of ͗L z ͘, ͗S z ͘, perpendicular anisotropy strength, and surface in-plane Ni lattice constant, respectively, during epitaxial growth. RHEED measurements show that the in-plane lattice constant falls by 1.7% in the Ni thickness range 30 ÅϽtϽ90 Å. The MCXD measurements reveal the same trend for ͗L z ͘, ͗S z ͘, and total moment per atom versus Ni thickness as found for the total moment by PNR. Polar MOKE measurements confirmed the transition from a perpendicular easy axis towards an in-plane magnetic easy axis as has already been extensively studied in the literature. Comparison of the PNR results with RHEED measurements reveal a striking correlation between the increase of in-plane strain and reduction in magnetic moment per atom with decreasing Ni thickness. While a direct strain-induced variation of the moment based on bulk phase calculations cannot account for the magnitude of the moment variations we observe, we show that the results cannot be attributed to sample contamination, interdiffusion, or a reduction of the Curie temperature with decreasing Ni thickness. Furthermore, the presence of a magnetically dead layer in the samples is not consistent with the PNR results. The strong moment variation partially explains the large thickness range for which perpendicular anisotropy is observed in this system.

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