Formation of c(2×2) Mn–Ag superficial bilayer alloys on Ag(001): role of thermally activated surface atomic exchange and ordering

Ultra-thin Mn films have been evaporated in ultra-high vacuum on a Ag(001) substrate kept at temperatures between 210 and 340 K. A c(2×2) LEED (low-energy electron diffraction) diagram is obtained due to the formation of a two-layer-thick superficial Mn–Ag alloy. The intensity and width of the half integer spots are investigated as a function of coverage, deposition temperature, and time elapsed after Mn deposition. The brightest and sharpest diagram is obtained for ∼1.5 monolayers deposited at room temperature, which corresponds to a surface plane made of c(2×2) domains formed by Mn and Ag atoms in a checkerboard arrangement, while the subsurface plane is saturated in Mn. Depending on the preparation conditions, various c(2×2) surface alloys may be obtained that mainly differ in the MnxAg1−x composition of the second atomic plane. A comparison of the time evolution of the LEED spot intensity with ion scattering spectroscopy data reveals that two different thermally activated phenomena determine the formation and nature of the c(2×2) superstructure: on the one hand, part of the deposited Mn atoms must exchange with Ag atoms from the substrate first plane, and on the other hand, the Mn and Ag atoms of the film surface plane diffuse laterally so that nucleation and growth of the c(2×2) domains take place.