Grain density control in FePt granular films for heat-assisted magnetic recording media

To realize high areal density, hard disk drives larger than 4 Tbit in⁻², ultrafine FePt grains of less than 5 nm and grain density larger than 24 T in⁻² are required. Although there have been many investigations to reduce the grain size of FePt, there are only a few reports on the control of grain density. To increase the grain density, we focused on three aspects of the surface morphology and grain density: nucleation sites on the substrate surface, surface free energy, and lattice mismatch. We achieved 14 T in⁻² by maximizing the number of nucleation sites in the FePt-C granular film and found that the surface free energy and lattice mismatch are crucial parameters for controlling the grain density.