Stress and structure ofc(2×2)andp2gg(4×2)Mn∕Cu(001)surface alloys

Abstract

We present a combined in situ surface stress and structural study of MnCu surface alloys formed by deposition of Mn on Cu(001) at 300 and $420\phantom{\rule{0.3em}{0ex}}\mathrm{K}$. Mn-induced surface alloying induces a compressive stress change, which grows in proportion to the Mn coverage up to 0.5 monolayers (ML), where it reaches $\ensuremath{-}1.2\phantom{\rule{0.3em}{0ex}}\mathrm{N}∕\mathrm{m}$ (1 ML: $1.5\ifmmode\times\else\texttimes\fi{}{10}^{15}\phantom{\rule{0.3em}{0ex}}\text{atoms}\phantom{\rule{0.2em}{0ex}}{\mathrm{cm}}^{\ensuremath{-}2}$). This stress is related to the formation of the $c(2\ifmmode\times\else\texttimes\fi{}2)$ surface alloy. No further alloying is observed upon subsequent Mn deposition at $300\phantom{\rule{0.3em}{0ex}}\mathrm{K}$, at $420\phantom{\rule{0.3em}{0ex}}\mathrm{K}$ the formation of the $p2gg(4\ifmmode\times\else\texttimes\fi{}2)$ $\mathrm{Mn}\text{\ensuremath{-}}\mathrm{Cu}$ alloy occurs, and a compressive stress change of $\ensuremath{-}2.3\phantom{\rule{0.3em}{0ex}}\mathrm{N}∕\mathrm{m}$ at 1.3 ML is found. Surface x-ray-diffraction analysis of the two-layer alloy $p2gg\text{\ensuremath{-}}(4\ifmmode\times\else\texttimes\fi{}2)$-phase indicates an amplitude of $0.9\phantom{\rule{0.3em}{0ex}}\mathrm{\AA{}}$ for vertical buckling and lateral modulations of the atomic positions, the latter leading to the doubling of the lattice constant as compared to the $c(2\ifmmode\times\else\texttimes\fi{}2)$ structure. Evidence for a compositional gradient within the alloy structure is given, where the Mn concentration is above and below 50% in the topmost and second alloy layer, respectively. The importance of surface stress relief and $\mathrm{Mn}∕\mathrm{Cu}$ atomic size mismatch for the Mn-induced surface stress change is discussed.