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Understanding the L10 phase transformation of FePt nanocrystals in FePt:X (X = nonmagnetic materials) nanocomposite thin films is of particular importance for controlling and designing these films as ultrahigh density magnetic recording media. In this study, by employing temperature- and pressure-dependence of the rate constant of phase transition, the nucleation (En = 0.3 eV) and growth (Eg = 0.9 eV) activation energies of L10 ordered domains, as well as an activation volume ΔV* ≈ 0.96 Ω (Ω = average atomic volume) for L10 ordering transition in FePt:Ag nanocomposite thin films have been successfully determined. These data suggest that the growth of L10 ordered domains is predominantly dependent on the diffusion of Fe atoms in FePt crystals, and that the diffusion of Ag atoms out of FePt lattice prompts the nucleation of the L10 ordered domains. These findings have a direct implication for designing FePt:Ag nanocomposite thin films with high pack fraction of L10-FePt nanocrystals. [ABSTRACT FROM AUTHOR]