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Structural and magnetic properties of FePt thin films were affected strongly by capped MnOx layers prepared by ion-beam bom-bardment and post-annealing. As-deposited FePt/MnOx bilayer exhibited a magnetically soft fee phase, and it turned to an ordered fct FePt phase with large coercivity (~8 kOe) after annealing at 550°C. Increasing the %02/Ar in capped MnOx layer during de position resulted in smaller ordered FePt grains separated by grain boundaries of MnOx. We found that the superlattice (001) peak is broadened considerably with larger amount of MnOx incorporated into FePt, likely due to the hindered formation of hard phase. Our results indicate that FePt/MnOx films deposited with lower %02/Ar, the oxygen atoms may occupy the interstitial positions in the FePt lattice to induce a local strain thus enhancing the FePt ordering. Further increased %02/Ar in capped MnOx layer, the excess oxygen atoms act a diffusion barrier effectively to inhibit the FePt grain growth and ordering.