Microstructure Engineering and Thermal Properties of FePt-based Media for Heat-Assisted Magnetic Recording (HAMR) Technology

 Heat-assisted magnetic recording (HAMR) is a next-generation technology enabling further increases in areal densities of hard disk drives beyond 1 Tb/in². HAMR relies on a novel recording medium composed of chemically ordered L1₀-FePt magnetic grains separated by insulating, thermally resistive grain boundary materials (GBMs). Achieving areal densities up to 4 Tb/in² requires optimal media design with FePt grains of 6-8 nm diameter and high aspect ratios (>2.0) fully encircled by GBMs. This research aims to develop L1₀-FePt based HAMR media exhibiting the desired granular microstructure using innovative GBM materials and optimized deposition processes. Initially, a bilayer media design of FePt-BN/FePt-SiOx was systematically studied by varying SiOx compositions and deposition temperatures. An optimized graded deposition enabled growth of 11.5 nm tall columnar FePt grains (7 nm diameter) encircled by SiOx GBMs. Crystalline hexagonal boron nitride (h-BN) was then explored as a high thermal stability GBM, forming a unique FePt-(h-BN) core-shell nanostructure at temperatures >600℃. The substrate bias facilitates the formation of h-BN nanosheets in grain boundary regions, which promote columnar and coherent FePt grain growth up to 16 nm thickness with a record-high grain aspect ratio of 2.5. Varying h-BN content allowed tuning FePt grain sizes from 4-7 nm with high areal densities up to 2.96×10⁴ μm^-2. To investigate thermal transport in this FePt-(h-BN) material system, a model structure of [h-BN/FePt] multilayer thin films were fabricated and characterized. Thermal measurements using the three-omega method revealed an ultralow cross-plane thermal conductivity of 0.6 W/(m∙K), dominated by the low thermal boundary conductance across van der Waals interfaces between h-BN basal planes and ordered FePt. The thermal boundary conductance for [FePt/h-BN/FePt] contact was estimated as 67.9 MW/(m²∙K). Other 2D materials like graphene, WS₂, and MoS₂ were also explored as potential GBMs, exhibiting distinct growth behaviors when incorporated into the FePt films. This research provides comprehensive insights into microstructure engineering and thermal properties of FePt-based HAMR media using novel GBM materials and deposition processes. The findings establish guidelines for further optimization towards ultrahigh-density magnetic recording up to 4 Tb/in² areal densities by elucidating microstructure-property relationships and growth mechanisms.