We present the results of a systematic investigation of the coverage-dependent magnetic response and anisotropy of ultrathin Co75Fe25∕Cu(110) films using the magneto-optic Kerr effect. A paramagnetic-ferromagnetic phase transition is found to occur at a critical thickness dc=4.0±0.25 ML. In the vicinity of dc, the paramagnetic susceptibility χ follows a power law with a critical exponent of γ=2.385±0.069 in excellent agreement with the theoretical value for a perfect two-dimensional geometric percolation phase transition. At the very onset of ferromagnetic order, a mixture of cubic and uniaxial anisotropy was observed.

The influence of submonolayer quantities of O and N adsorbed on ultrathin Co∕Cu(001) films as a function of Co thickness has been studied using spin polarized secondary electron spectroscopy. The gaseous adsorbate was prepared by depositing Co on (2×22)R45°-O and c(2×2)-N reconstructed Cu(001) utilizing surfactant effects to reproducibly control quantity. Adsorbed quantities were monitored by Auger electron spectroscopy and surface reconstructions by low energy electron diffraction. The secondary electron spin polarization increases with the Co film thickness, following an exponential law, and the chemical interaction between the adsorbate and the Co reduces polarization to (98±2)% in the case of O and (84±3)% in the case of N compared to the uncovered substrate. For both, the onset of ferromagnetism is suppressed by approximately 1 ML. The effects on polarization and the onset of ferromagnetism are attributed to the partial cancellation of the magnetic moment in the Co layer adjacent to the adsorbate. The estimated reduction in moment is comparable to the results of theoretical predictions.

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We report on the effects of O2 and H2 dosing on the magnetism of Co75Fe25 films, highlighting the extreme sensitivity of the magnetic behavior of ultrathin films to specific chemical interactions at the surface. We have performed a systematic in situ study of the magnetic behavior as a function of O2 and H2 dosage on ultrathin CoFe and Co films grown on Cu(110) by means of the magneto-optical Kerr effect and low energy electron diffraction. Oxygen and hydrogen dosing are found to have distinctly different influences on the evolution of the coercive field Hc of the CoFe and Co films. Significant differences become also apparent for the evolution of the M-H loop squareness upon gas adsorption. Oxygen dosing leads to a rapid decrease in squareness above 10L (1L≡10−6Torrs) for CoFe films. For Co films, however, the squareness increases initially and then stabilizes at a higher dosage level. Furthermore, a striking sensitivity of the easy axis magnetization in the CoFe films was observed; 50L of O2 caused a rotation of the easy axis by 60° within the plane. The observed differences between the different gases and between the different ferromagnetic films are briefly discussed in terms of d band occupation and details of chemical bonding.