Mesoscopic 'volcano' structures formed in the epitaxial growth of Co on Cu

Co has been grown by molecular beam epitaxy on Cu (111), with the Cu made pinhole-free by use of Au surfactant. We find that 'volcano'-like structures of metastable Cu-Co alloy self-assemble during Co growth at 500°C. This happens because the 'crater' affords a pathway for free energy reduction as the Cu migrates to the surface by a surface pump mechanism discussed earlier by the present authors.