  The growth of binary alloy clusters on a weakly interacting substrate through
codeposition of two atomic species is studied by kinetic Monte Carlo
simulation. Our model describes salient features of CoPt$_3$-nanoclusters, as
obtained recently by the molecular-beam epitaxy technique. The clusters display
perpendicular magnetic anisotropy (PMA) in a temperature window of growth
favorable for applications. This temperature window is found to arise from the
interplay of Pt surface segregation and aspect ratio for cluster shapes.
Conclusions are drawn how to optimize growth parameters with respect to PMA.
