  We have calculated surface energies and surface magnetic order of various
low-indexed surfaces of monoatomic Fe, Co, and Pt, and binary, ordered FePt,
CoPt, and MnPt using density functional theory. Our results for the binary
systems indicate that elemental, Pt-covered surfaces are preferred over Fe- and
Co-covered and mixed surfaces of the same orientation. The lowest energy
orientation for mixed surfaces is the highly coordinated (111) surface. We find
Pt-covered (111) surfaces, which can be realized in the L11 structure only, to
be lower in energy by about 400 meV/atom compared to the mixed L10 (111)
surface. We conclude that this low surface energy stabilizes the L11 structure
in small nanoparticles, which is suppressed in bulk alloys, but has been
recently synthesized as thin film for CoPt. From the interplay of surface and
bulk energies, equilibrium shapes of single-crystalline ordered nanoparticles
and crossover sizes between the different orderings can be estimated.
