High magnetic moments and anisotropies for Fe_xCo_1−x monolayers on Pt(111)

The magnetism of 1-ML-thick films of Fe_xCo_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 3μ_B/atom and 2μ_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)μ_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_0.5Co_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 L1₀ 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_x²−y² and d_xy orbitals due to the Fe-Co electronic hybridization.