The upper figure shows an overview of the microstructure formation simulation of FePt-C nanogranular films using the phase-field model developed in this study. The upper left shows the TEM image of an experimentally fabricated FePt-C film, which was used as a reference for determining the parameters used in the simulation. The upper right shows the three-dimensional simulation results (red: L10-FePt, blue: C, green: MgO), in which isolated island-like structures equivalent to those observed experimentally are reproduced. The lower row shows the stress field distribution (σ11; blue: compressive, red: tensile) on the film cross section (the (010) plane).

Owing to the lattice mismatch between FePt and MgO, a large tensile stress exceeding 10 GPa arises within the FePt grains. Because FePt grains prioritize the release of this strain energy, they tend to grow preferentially in the vertical direction. In addition, because tensile stress also develops on the MgO surface away from the FePt grains, the formation of new FePt/MgO interfaces is elastically unfavorable, and the coalescence of adjacent grains is suppressed. It was revealed that the combination of these two effects leads to the formation of uniform, well-isolated island-like structures. Furthermore, when the C segregant is added, the Fe and Pt atoms supplied by sputtering are captured on the C surface, shortening the diffusion distance to the top of the FePt grains, which further promotes vertical growth and facilitates the refinement of the FePt grains. From these findings, the following practical design guidelines for FePt-C films for high-recording-density HDDs were derived: (1) select a substrate that enables epitaxial growth while maintaining a sufficient lattice mismatch, (2) select a segregant material that does not exhibit interfacial coherence with FePt and the substrate, and (3) perform deposition at the highest sputtering rate that does not induce grain coalescence.