Surface composition changes caused by Ar ion bombardment of Au-Ni alloys

Surface composition changes caused by 1 keV Ar ion bombardment of a series of Au-Ni alloys at room temperature were studied. Using 11 combinations of different energy Auger spectra, the variation of the composition with depth from 0.4 to 1.5 nm in a bombarded Au-Ni alloy was obtained. We found that a drastic variation in concentration took place within the first few layers. This is evidently indicating that a bombardment-induced Gibbsian segregation exists in the Au-Ni system. We also found that the degree of gold depletion at the second layer depended strongly on the bulk composition, thus this depletion became slight as the gold content in alloy was high. In addition, we have roughly estimated the concentration of Au at the topmost layer of a bombarded Au-Ni alloy and found that this value tentatively might be smaller than the bulk one. This result differs from two known examples: C(Au, top) > C(Au, bulk) in a bombarded Au-Cu alloy and C(Cu, top) = C(Cu, bulk) in a bombarded Cu-Ni alloy. Comparing our experimental results with a recent theory presented by Kelly, we suggest that a more sophisticated theory should take this new phenomenon into account.