Growth and atomic structure of ordered Mn surface alloys on Au(001)

The atomic structures of two different, ordered Mn surface alloys on Au(001) are studied by the low-energy electron diffraction $\mathrm{IV}$ (spot intensity vs incident electron energy) analysis. The first one is formed of a subsurface $c(2\mathrm{}\times 2)$ alloy layer beneath 1-monolayer- (ML) thick Au capping layer, 1-ML Au/1-ML $c(2\mathrm{}\times 2)$ Au-Mn/Au(001). The atomic structure is just like the bulk terminated ${\mathrm{Au}}_3\mathrm{Mn}\mathrm{}\left(001\right)\mathrm{}$ with small surface relaxation. The second alloy film is largely formed of trilayer, ${\mathrm{AuMn}}_3\mathrm{}\left(001\right)\mathrm{}$-like structure, $c(2\mathrm{}\times 2)$ 1-ML Au-Mn/1-ML Mn/1-ML $c(2\mathrm{}\times 2)\mathrm{Mn}-\mathrm{Au}/\mathrm{Au}\mathrm{}\left(001\right),$ where the position of Mn and Au in the third layer is exchanged with respect to those of the first layer to reduce the strain normal to the surface. Contrary to the first-ordered alloy, we find large contraction of the layer spacing and the strong buckling of Mn atoms in the surface layer for the second one. We discuss the energetics relevant to the formation of the observed alloy structures.