Carbon is the essential segregant to achieve the nanogranular structure of FePt-based heat-assisted magnetic recording media. However, a fundamental understanding of its effect on the microstructure and magnetic properties of the medium is yet to be elucidated. Here, a systematic investigation of FePt-C nanogranular films deposited on the single-crystalline MgO substrate with various C concentrations was performed. While the averaged FePt grain size can be successfully reduced to 5.8 nm, introducing excess C significantly degrades the degree of L1 0 ordering, the magnetic anisotropy, and the effective total magnetic moments of FePt films. Such degradation is mainly due to the finite size effects. Besides, the possible contribution form the dissolution of C atoms in the interstitial sites of the FePt lattice for the resulting magnetic properties was also studied via the first-principles calculations. More interestingly, fully in-plane c-axis-oriented FePt grains were observed even for growth on the MgO (001) substrate with a C concentration beyond 34.6 vol.%. Their formation is attributed to the reduced epitaxial strain energy due to the suppressed contact area at the FePt/MgO interface with excess C. Such grains were then identified as the main source of the in-plane component of the medium through micromagnetic simulation.
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