Structure, growth, and magnetism of Mn on Cu(110)

Abstract

We found a two-dimensional, ordered surface alloy $\mathrm{Cu}(110)\ensuremath{-}c(2\ifmmode\times\else\texttimes\fi{}2)\ensuremath{-}\mathrm{Mn}.$ The structure and composition of this surface compound were determined by quantitative low-energy electron-diffraction (LEED) analysis, which shows a large buckling in the surface alloy layer. The Mn atoms buckle outwards, and the Cu atoms inwards with a total buckling amplitude of 0.22 \AA{} [17.2% of the ideal interlayer distance of Cu(110)]. The results are compared to ab initio total-energy and force calculations. The first-principles structure optimizations are restricted to structural relaxations normal to surface, which is consistent with our LEED analysis. The theoretically determined buckling of 16.3% reproduces the experimental situation. The calculations predict a large magnetic moment for Mn of $M=3.82{\ensuremath{\mu}}_{B}.$ A hypothetical nonmagnetic $\mathrm{Cu}(110)\ensuremath{-}c(2\ifmmode\times\else\texttimes\fi{}2)\ensuremath{-}\mathrm{Mn}$ surface alloy shows no buckling $(<1%),$ proving that the buckling is due to the magnetovolume effect of Mn. Investigation of the growth shows that, for substrate temperatures above 180 K, deposition of submonolayer amount of Mn leads to the formation of a $c(2\ifmmode\times\else\texttimes\fi{}2)$ superstructure. A well-ordered structure at 0.5 ML was observed in the temperature range between 270 and 350 K. For films above 1 ML, a $16\ifmmode\times\else\texttimes\fi{}1$ superstructure was observed giving evidence of a buckled, Mn-rich top layer. We also investigated the work-function change upon surface alloy formation. The ab initio calculations predict a work-function lowering of about 0.5 eV, and we identified the magnetism of Mn as the basic origin of the work-function change. The results are compared to the $\mathrm{Cu}(100)\ensuremath{-}c(2\ifmmode\times\else\texttimes\fi{}2)\ensuremath{-}\mathrm{Mn}$ surface alloy.