Effect of C replacement on magnetic properties and nanostructure of FePt-nitride granular film for HAMR media

The effect of adding carbon (C) grain boundary material (GBM) on magnetic properties and nanostructure of FePt-nitride granular films with various nitrides is reported. Saturation magnetization of FePt-nitride-C granular films shows similar tendency with that of FePt-nitride granular films which varies almost linearly against volume averaged melting point (${T_{\text{m}}}^{{\text{ave}}}$) of the GBM. The value of perpendicular anisotropy for all FePt-nitride granular films is enhanced when nitride is partly replaced with C. By replacing nitride with C, the grain to grain separation is promoted significantly. When ${T_{\text{m}}}^{{\text{ave}}}$ of the GBM increases from around 2500 to 3500°C, in general, the average cluster size and cluster boundary decreases from around 13 to 9 nm and from 12 to 5 nm, respectively. Considering of ${T_{\text{m}}}^{{\text{ave}}}$ of the GBM, these results suggest that the GBM with high ${T_{\text{m}}}^{{\text{ave}}}$ will solidify prior to FePt which acts as the nucleation site to reduce the CD, which is similar to CoPt-based alloy granular films. In conclusion, the combination of nitride with C is more effective than single nitride to promote the grain isolation, while keeping higher degree of phase separation than C GBM.