Core-shell morphologies of FePt and CoPt nanoparticles: An ab initio comparison

Large-scale first principles calculations for near-stoichiometric FePt and CoPt clusters with up to 923 atoms are presented including unconstrained structural relaxations. The energetic order and magnetic properties of 561 atom FePt and CoPt nanoparticles are evaluated with emphasis on segregated morphologies with Pt covered surfaces. The results imply that for CoPt particles segregation of Pt to the surface and the formation of a Pt depleted subsurface layer is dominant also for nanometer-sized single crystalline particles and may help to stabilize particles with partial L10 order, while for FePt multiple twinning is the most important mechanism at small particle sizes. The mixed (100) surfaces of the fully L10 ordered FePt and CoPt isomers exhibit a characteristic reconstruction.