The initial growth of Co on Cu͑001͒ is atomically identified by scanning tunneling microscopy using CO titration and density-functional theory total-energy calculations. Both reveal that at low coverage Co adatoms occupy substitutional sites in the Cu substrate surface that act as pinning centers for subsequent island nucleation. The interaction with diffusing adatoms is found to be attractive and stronger for nearest-neighbor Co compared to Cu. The atomic substitution process actuates a

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... growth mode as experimentally found in the island size distributions. This gives rise to a high density of small Co islands and large Co-decorated Cu islands. ͓S0163-1829͑99͒08943-2͔