Growth of ultrathin Mn films on Ag(100): Evidence of a buried Mn monolayer

Mn films with a thickness of one monolayer have been deposited, in ultrahigh vacuum, on a Ag(100) single crystal at room temperature, and analyzed by X-ray photoelectron diffraction (XPD) and surface extended X-ray absorption fine structure (SEXAFS). The strong enhancement of the X-ray photoelectron Mn2P(3/2) signal along the Ag[110] direction indicates that a substantial fraction of the Mn atoms is located within the second topmost atomic layer. The SEXAFS data demonstrate that the Mn-Ag and Mn-Mn first neighbor distances are the same as the Ag-Ag distances in the Ag lattice; as a result, the Mn atoms occupy Ag sites on the fee lattice. Comparison of the experimental data with FEFF calculations indicates that a superficial Mn-Ag alloy is formed. If the film is mildly (330 K) annealed, the contrast in the XPD Mn 2p(3/2) modulations versus polar angle increases and reaches a maximum value of 60%. As confirmed by single scattering simulations, these results mean that atomic place exchange occurs between Mn and Ag atoms, and eventually the second atomic plane of the sample is constituted mainly by Mn atoms, whereas the first atomic layer is a pure Ag plane, i.e. a buried atomic Mn monolayer is formed.