Impact of carbon segregant on microstructure and magnetic properties of FePt-C nanogranular films on MgO (001) substrate

Abstract 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 L10 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.