We have studied the effect of depositing Cu overlayers onto CO gas dosed Co͞Cu(110) ultrathin films ͑6 , d Co , 30 monolayers͒. We find that submonolayer Cu coverages can completely reverse the in-plane 90 ± easy axis switch caused by the CO adsorption for all Co thicknesses studied and for sufficiently thick Co films ͑d Co . 15 monolayers͒. This enables us to "controllably engineer" the direction of the easy axis at a constant Co thickness. [S0031-9007(98)

We have studied the magnetic moment and in-plane strain in epitaxial Cu/Ni/Cu/Si͑100͒ structures by varying both the Ni and Cu buffer layer thickness. We find a sharp reduction in magnetic moment with increasing Ni lattice strain. Our structural and temperature-dependent studies exclude interdiffusion, interface roughness, and a decreased Curie temperature as possible causes of the reduced moment, but reveal a strong correlation between the strain and magnetic moment in Cu/Ni/Cu structures.

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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Near the critical region for the onset of ferromagnetic order, the magnetic susceptibility χ follows a power law with a critical exponent γ=2.39±0.08, in excellent agreement with the theoretical value for a perfect two-dimensional geometric percolation phase transition. Scanning tunneling microscopy studies reveal that the Co grows via the formation of three-dimensional elongated island structures, forming an array of Co “nanomagnets.” The ordering transition is a consequence of direct exchange developing between Co islands. The in-plane uniaxial anisotropy in the ferromagnetic phase is predominantly attributed to the internal dipolar field of the Co islands.