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The growth dynamics of submonolayer coverages of cobalt during buffer-layer assisted growth on Ag(111) and Pt(111) substrates is investigated by variable temperature scanning tunneling microscopy in the temperature range between 80 and 150 K. It is found that attractive cluster-substrate interactions can govern the cluster formation on the Xe buffer layer if the Xe layer is sufficiently thin. The interpretation of the microscopy results is supported by x-ray magnetic circular dichroism data which monitor the effect of cluster-substrate interactions on the formation of magnetic moments and magnetic anisotropy of Co nanocluster during the different stages of growth. Ab initio calculations show that the cluster magnetism is controlled by the interface anisotropy, leading to perpendicular magnetization for Co on Pt(111). Limits of and new potential for nanocluster fabrication by buffer-layer-assisted growth are discussed.