High magnetic moments and anisotropies for Fe(x)Co(1-x) monolayers on Pt(111)

The magnetism of 1-ML-thick films of Fe(x)Co(1-x) on Pt(111) was investigated both experimentally, by x-ray magnetic circular dichroism and magneto-optical Kerr effect measurements, and theoretically, by first-principles electronic structure calculations, as a function of the film chemical composition. The calculated Fe and Co spin moments are only weakly dependent on the composition and close to 3 mu(B)/atom and 2 mu(B)/atom, respectively. This trend is also seen in the experimental data, except for pure Fe, where an effective spin moment of only S(eff)=(1.2 +/- 0.2)mu(B)/atom was measured. On the other hand, both the orbital moment and the magnetic anisotropy energy show a strong composition dependence with maxima close to the Fe(0.5)Co(0.5) stoichiometry. The experiment, in agreement with theory, gives a maximum magnetic anisotropy energy of 0.5 meV/atom, which is more than 2 orders of magnitude larger than the value observed in bulk bcc FeCo and close to that observed for the L1(0) phase of FePt. The calculations clearly demonstrate that this composition dependence is the result of a fine tuning in the occupation number of the d(x)(2)-y(2) and d(xy) orbitals due to the Fe-Co electronic hybridization.