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Both Cu-FePt and Au-FePt bilayer thin films were prepared by depositing a Cu or Au cap layer on a fully ordered FePt film at room temperature. A heat treatment at 300°C-600°C was then applied to the bilayer samples to facilitate interfacial diffusion. For the Cu-FePt system, extensive volume diffusion between L10 FePt and Cu layers occurred as the samples were annealed at 500°C and higher temperatures, forming a ternary Fe-Pt-Cu phase with face centered cubic (fcc) structure. As the annealing temperature was lowered to 400°C, the diffusion was dominated by grain-boundary mechanism. The best coercivity thus obtained was 14.0 kOe, being 22% larger than that of FePt film without cap layer. For the Au-FePt system, grain-boundary diffusion remains dominant even the annealing temperature is as high as 600°C. In both Cu-FePt and Au-FePt bilayer systems, grain-boundary diffusion was found to successfully enhance the coercivity. The ΔM plot data indicate that incoherent rotation among magnetic grains can be responsible for the high coercivity.