Electronic structure of two-dimensional magnetic alloys: c(2x2) Mn on Cu(100) and Ni(100)

Half a monolayer (ML) of Mn deposited above 270 K on the (100) surfaces of Cu and Ni form ordered surface alloys of c(2x2) structure. Their electronic structure is studied in a combined experimental and theoretical work. The experimental approach, comprising angle-resolved photoemission and inverse photoemission, characterizes both systems as ideal cases of well-ordered magnetic surface alloys. A large atomiclike splitting between majority- and minority-spin Mn-3d states is measured: 5.5 eV for the Cu-based system and 5.25 eV for the Ni-based system. The large splittings are direct evidence that Mn develops a high local magnetic moment in these systems. Our first-principles band-structure calculations of c(2x2) CuMn/Cu(100) and c(2x2) NiMn/Ni(100) corroborate this finding and give values of 3.75 mu(B) and 3.5 mu(B), respectively, for the Mn moments. We find that the measured splittings are even larger than the ones calculated from first principles. The difference amounts to 2.7 eV and 1.7 eV for the Cu-based and Ni-based system, respectively. We suppose that the splitting measured in photoemission and inverse photoemission is increased by a Coulomb energy U due to the enhanced localization of the Mn-3d states in the surface alloy. This high localization can be quantified by the small band dispersion of 110+/-60 meV measured for the Mn minority d band in the Cu-based system. We also investigated the work-function change upon surface-alloy formation. By comparing our results with our ab initio calculations we identified the magnetism as the source of the work-function change.