Structural and magnetic properties of FePt thin films were affected strongly by capped MnO<sub>x</sub> layers prepared by ion-beam bom-bardment and post-annealing. As-deposited FePt/MnO<sub>x</sub> 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 MnO<sub>x</sub> layer during de position resulted in smaller ordered FePt grains separated by grain boundaries of MnO<sub>x</sub>. We found that the superlattice (001) peak is broadened considerably with larger amount of MnO<sub>x</sub> incorporated into FePt, likely due to the hindered formation of hard phase. Our results indicate that FePt/MnO<sub>x</sub> 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 MnO<sub>x</sub> layer, the excess oxygen atoms act a diffusion barrier effectively to inhibit the FePt grain growth and ordering.
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