Structure of ultrathin films of Mn on Cu(111) investigated by the normal-incidence x-ray standing-wave method

Abstract

The growth of Mn films on Cu(111) has been investigated in the range of 0.5--4.5 monolayer equivalents using the normal-incidence x-ray standing-wave method. The films have been found to be incommensurate at all coverages as determined by the coherent fraction of the Mn in the substrate $(1\ifmmode\bar\else\textasciimacron\fi{}11)$ reflection. The (111) reflection gives a coherent position that increases with coverage, but a coherent fraction that decreases. At submonolayer exposure, the low coherent position reveals that the majority of the Mn atoms are at a z position near to, but slightly expanded from, the Cu(111) d spacing, but that $15%\ifmmode\pm\else\textpm\fi{}5%$ of the Mn are at a z position of $0.57\ifmmode\pm\else\textpm\fi{}0.18$ of the substrate d spacing. Modeling of the trends in coherent position and fraction with Mn exposure eliminates the possibility of the overlayer being \ensuremath{\alpha}-Mn, \ensuremath{\gamma}-Mn (fcc), or \ensuremath{\delta}-Mn (bcc). However, the trends are entirely consistent with the Mn overlayer being a Laves phase similar in structure to ${\mathrm{Zn}}_{2}\mathrm{Mg}.$