We report the mechanisms of atomic ordering in Fe-Pt bimetallic alloys
using density functional theory (DFT) and machine-learning interatomic potential
Monte Carlo (MLIP-MC) simulations. We clarified that the formation enthalpy of
the ordered phase was significantly enhanced by spin polarization compared to that
of the disordered phase. Analysis of the density of states indicated that coherence
in local potentials in the ordered phase brings energy gain over the disordered phase,
when spin is considered. MLIP-MC simulations were performed to investigate the
phase transition of atomic ordering at finite temperatures. The model trained using
the DFT dataset with spin polarization exhibited quantitatively good agreement with
previous experiments and thermodynamic calculations across a wide range of Pt
compositions. In contrast, the model without spin significantly underestimated the
transition temperature. Through this study, we clarified that spin polarization is
essential for accurately accounting for the ordered phase in Fe-Pt bimetallic alloys,
even above the Curie temperature, possibly because of the remaining short-range spin
order.