  FePt nanoparticles are known to exhibit reduced L1$_0$ order with decreasing
particle size. The reduction in order reduces the magnetic anisotropy and the
thermal stability of the direction of magnetization of the particle. The
phenomenon is addressed by investigating the thermodynamic driving forces for
surface segregation using a local (inhomogeneous) cluster expansion fitted to
ab initio data which accurately represents interatomic interactions in both the
bulk and surface regions. Subsequent Monte Carlo simulations reveal that first
surface layer Pt segregation is compensated by Pt depletion in the second
subsurface layer. This indicates that the core's ordered state is not affected
by surface thermodynamics as much as previously thought. Thus, the weak
ordering experimentally observed is likely not due to fundamental thermodynamic
limitations but rather to kinetic effects.
