Room-temperature instability of the Mn/Ag(100) interface in the monolayer range

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Abstract
Ultrathin Mn films deposited at room temperature (RT) in ultrahigh vacuum on Ag(100) were investigated using ion scattering spectroscopy (ISS), angle-resolved ultraviolet photoemission spectroscopy (ARUPS) and low-energy electron diffraction (LEED). Up to ∼1 ML, ISS and ARUPS definitely indicate that the Mn/Ag(100) interface is unstable at RT. We find that with time an Ag enrichment of the film surface takes place with eventually formation of a two-dimensional (2D) p(1×1) Ag layer on top of a mixed Ag-Mn subsurface monolayer. Moreover a c(2×2) LEED superstructure is observed in the 0.5–1-ML range immediately after deposition that also undergoes an evolution with increasing time and eventually disappears, leaving a p(1×1) pattern, i.e., long-range c(2×2) order is broken. Our observations clearly demonstrate a structural evolution of the film at RT over a period of several hours. The data suggest that in the initial stages of growth the instability of the interface is essentially due to an atomic place exchange of Mn atoms with substrate Ag atoms, both during and after deposition. Concomitantly, surface diffusion of Ag and Mn adatoms results in nucleation of 2D mixed c(2×2) Ag-Mn and pure p(1×1) Ag surface islands in relative amounts depending on Mn coverage and time of measurement. In particular, it is found that the c(2×2) atomic order corresponds to a metastable surface structure of this kinetically hindered system formed during its evolution toward the more stable inverted layer configuration. This shows that kinetic factors play a major role in the growth of this metal on metal system near RT.