Two-dimensional surface ordered Mn-Co alloys on Co(001)

Abstract

The spin polarization of various surface alloys of manganese and cobalt on cobalt (001) is determined by the tight-binding linear muffin-tin-orbitals method. An ordered surface alloy restricted to the surface plane i.e., ${\mathrm{Mn}}_{0.5}{\mathrm{Co}}_{0.5}/\mathrm{Co}(001),$ is shown to be more stable as compared to large islands of Mn or Co in the surface plane. The coupling between Mn and Co is found to be antiferromagnetic in the ground state with a metastable ferromagnetic configuration a few mRy higher in energy. This ferromagnetic coupling between Mn and Co is in agreement with the recent magneto-optical Kerr effect and x-ray magnetic-circular dichroism results. The two-dimensional (and two-layer thick) surface-ordered alloy $({\mathrm{Mn}}_{0.5}{\mathrm{Co}}_{0.5}{)}_{2}/\mathrm{Co}(001)$ is more stable as compared to the perfect Mn monolayer on Co(001) and only solutions with antiferromagnetic couplings between Mn atoms are obtained. A ferromagnetic coupling between the Mn atoms at the surface and the Co atoms in the substrate is the ground state. A buried monolayer of Mn, i.e., Co/Mn/Co(001), is also found more stable as compared to Mn/Co(001) so that a complete exchange between the surface Mn monolayer and the Co subsurface layer is energetically favorable.