The key to realizing high area recording density capability (ADC) for heat assisted magnetic recording (HAMR) is to achieve small grain size (and pitch) in the granular L1₀ FePt media with a sufficiently high grain aspect ratio 1. Media such as FePt-C, FePt-SiO_x, and FePt-TaO_x, or multilayers made of a combination of them, appear not able to reach the desired goal because of their failure to achieve either high grain aspect ratio, small grain size, or high L1₀ ordering, respectively 2, 3.
In this study, using the co-sputtering technique, we demonstrate the fabrication of FePt-BN granular media with much reduced FePt grain size and high grain aspect ratio. In particular, an underlayer stack of Ta(5nm)/Cr(50)/MgO(9) is first deposited on Corning Lotus^TM NXT glass followed by co-sputtering of FePt and BN at an elevated substrate temperature along. A RF bias is applied to the substrate during the FePt-BN deposition with a short and optimized delay. Figure 1C shows the order parameter as a function of substrate temperature for the FePt-BN granular film media, calculated from XRD measurements. The volumetric concentration of BN is at 38%. Figure 1A on the left of the figure shows a cross-section TEM micrograph of the FePt-BN layer formed at a substrate temperature of 725 ^oC. Figure 1B shows a cross-section TEM picture of FePt-BN/FePt-SiO_x multilayer granular film with temperature and composition graded over the grain height for producing tall grains 4 for comparison. The grain size of single layer FePt-BN film (Figure1 A) is around 4 nm with an aspect ratio of 2 while the grain size of the FePt-BN/FePt-SiO_x multilayer film is around 7nm and grain height 11nm. The small grain size with a relatively high grain aspect ratio in the single layer FePt-BN film is promising for enabling high area recording density capability.
References
1Y. Kubota et al., “Heat-Assisted Magnetic Recording’s Extensibility to High Linear and Areal Density,” IEEE Transactions on Magnetics, vol. 54, no. 11, Nov. 2018.
2 A. Perumal, Y.K. Takahashi, K. Hono, “L10 FePt-C Nanogranular Perpendicular Anisotropy Films with Narrow Size Distribution”, Appl. Phys. Express 1 101301 (2008).
3 B. S. D. C. S. Varaprasad, M Chen, YK Takahashi, K Hono “L1-Ordered FePt-Based Perpendicular Magnetic Recording Media for Heat-Assisted Magnetic Recording” IEEE transactions on magnetics 49 (2), 718-722 (2013)
4 C. Xu, B. Zhou, T. Du, B.S.D.C.S. Varaprasad, D.E. Laughlin, J.-G. Zhu, Understanding the growth of high-aspect-ratio grains in granular L10-FePt thin-film magnetic media, APL Materials. 10 (2022).

Figure 1 (A) Out of plane micrographs of FePt-BN (8 nm) (B) Out of plane micrographs of FePt-BN/FePt-SiO 2 (11 nm) and (C) Order parameter Vs set temperature of FePt-BN sample