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

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) over bar 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%+/-5% of the Mn are at a z position of 0.57+/-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 alpha-Mn, gamma-Mn (fcc), or delta-Mn (bcc). However, the trends are entirely consistent with the Mn overlayer being a Laves phase similar in structure to Zn2Mg.