Formation of c(2 x 2) Mn-Ag superficial bilayer alloys on Ag(001): role ofthermally 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 x 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 hal f integer spots are investigated as a function of coverage, deposition temp erature, and time elapsed after Mn deposition. The brightest and sharpest d iagram is obtained for similar to 1.5 monolayers deposited at room temperat ure, which corresponds to a surface plane made of c(2 x 2) domains formed b y Mn and Ag atoms in a checkerboard arrangement, while the subsurface plane is saturated in Mn. Depending on the preparation conditions, various c(2 x 2) surface alloys may be obtained that mainly differ in the MnxAg1-x compo sition of the second atomic plane. A comparison of the time evolution of th e LEED spot intensity with ion scattering spectroscopy data reveals that tw o different thermally activated phenomena determine the formation and natur e of the c(2 x 2) superstructure: on the one hand, part of the deposited Mn atoms must exchange with Ag atoms from the substrate first plane, and on t he other hand, the Mn and Ag atoms of the film surface plane diffuse latera lly so that nucleation and growth of the c(2 x 2) domains take place. (C) 2 000 Elsevier Science B.V. All rights reserved.