Surface alloying and pinhole formation in ultra-thin FeCu(100) films
ABSTRACT Relatively mild annealing sometimes produces unexpectedly large changes in surface composition of ultra-thin films. We report results of a scanning tunnelling microscopy study of such annealing in an system. Copper appears in the surface region after a short 490 K anneal. We specifically exclude the possibility of bulk interdiffusion and conclude that this surface copper comes from the substrate through microscopic pinholes. Our primary finding is that the balance of surface free energies which strongly prefers Cu/Fe/Cu over is the main driving force for pinhole formation and copper segregation. Furthermore, we report a significant surface alloying in this otherwise almost immiscible system.