A simple relationship between the ratio of atomic transport induced by
ion mixing and the activation energies for the impurity diffusion of
constituents in a bilayer is presented to describe quantitatively the
symmetric and asymmetric atomic transport in the thermal spike induced
ion mixing. The model predicts fairly satisfactorily the trend of exp
erimental observations in the bilayer systems which have near zero hea
ts of mixing and relatively high spike activation energies. For instan
ce, the Pd/Co bilayer system shows nearly symmetric atomic transport,
since its constituents have similar activation energies for the impuri
ty diffusion.