Surface alloy formation by interdiffusion across a linear interface

Interdiffusion profiles across a linear interface separating two metals that are miscible as monolayers [Co and Cu on a Ru(0001) surface] have been measured as a function of time. The interface remains sharp even after substantial intermixing, in dramatic contrast to bulk interdiffusion experiments between miscible metals. The unusual concentration profiles are explained by a combination of adatom surface diffusion and exchange of adatoms with atoms in the monolayer films. A simple Monte Carlo simulation of the adatom exchange mechanism quantitatively agrees with observations.