LOCAL MAGNETIC-PROPERTIES AND MAGNETIC-ORDERING OF THE 2-DIMENSIONAL SURFACE ALLOY C(2X2) MN/CU(001)

At a coverage of 1/2 monolayer, Mn forms an ordered c(2x2) structure on Cu(100). This two-dimensional surface alloy exhibits a large outward relaxation of the Mn atoms, which cannot be explained simply by atomic-radius arguments. A recent total-energy calculation by Wuttig and Gauthier finds that this unusual two-dimensional alloy, for which there is no bulk analogue, is stabilized by magnetism of the Mn atoms. We investigate the magnetism of this surface alloy by soft x-ray absorption, photoemission and magnetic circular dichroism. Our soft x-ray absorption results conclusively show that the Mn atoms are in a high-spin ground state, which accounts for the large apparent atomic radius in the c(2x2) phase. Large binding-energy shifts, 0.8 eV, are found in the Mn 3p, 3s and 2p photoemission peaks, which we attribute to the change in screening that accompanies the large outward relaxation of the Mn atoms. Magnetic circular dichroism shows that the c(2x2) phase is stable above the Curie temperature for long-range ordering of the surface alloy. If the surface-alloy formation is stabilized by magnetic properties it is driven by the local high-spin character of the Mn atoms and not by long-range ferromagnetic coupling.