Sputtering technique has been the choice for fabricating L1₀ FePt granular thin-film media for heat-assisted magnetic recording (HAMR). A variety of amorphous grain boundary materials have been studied extensively 1,2. Among them, SiOx has been shown to facilitate columnar growth of FePt grains; however, its limitation on deposition temperature prevents high ordering. Amorphous carbon can yield high-temperature-enduring microstructure, however, with a limitation on grain height leveling to an aspect ratio ~1. Amorphous BN was also reported to affect the ordering of FePt 3.
In this paper, we report a study for creating a granular FePt-L1₀ film with crystalline BN grain boundaries that facilitate small grains with high aspect ratios and high ordering. By applying RF substrate bias with a substrate temperature of 700 ^oC, a microstructure of columnar FePt grains separated and wrapped around by hexagonal boron nitride(h-BN) layers is produced, as shown in Fig. 1(a). The spacing of the lattice fringes in grain boundaries is ~0.34 nm, close to that of h-BN basal planes (0.331 nm). The electron energy loss spectrum (EELS) shows the π* peak at 191.8 eV in the B-K edge, confirming the formation of turbostratic h-BN. Fig.1(e) shows columnar grains with an aspect ratio of ~1.85. A similar nanostructure was observed in FePt-Carbon film deposited with RF substrate bias, in which the graphite phase was likely formed in grain boundary regions. The covalent binding within the honeycomb structured nanosheets imparts the grain boundary materials high stability at high temperatures. This study provides an extendibility of FePt-based granular media to utilize partial crystalline grain boundary materials to improve microstructure. It also presents more understanding of why carbon and BN can maintain good grain isolation at quite high temperatures.
References
1 A. Perumal, Y.K. Takahashi, K. Hono, L10 FePt-C Nanogranular Perpendicular Anisotropy Films with Narrow Size Distribution, 1 (2008) 101301. https://doi.org/10.1143/apex.1.101301.
2 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). https://doi.org/10.1063/5.0089009.
3 B.H. Li, C. Feng, X. Gao, J. Teng, G.H. Yu, X. Xing, Z.Y. Liu, Magnetic properties and microstructure of FePt/BN nanocomposite films with perpendicular magnetic anisotropy, Applied Physics Letters. 91 (2007). https://doi.org/10.1063/1.2798584.

Fig.1 (a) Plane-view HRTEM of FePt-hBN film, (b) FFT shows a ring pattern of h-BN (002), corresponding to a d-spacing of 0.33~ 0.35 nm; (c) and (d) grain size and center-to-center pitch distance distributions; (e) cross-sectional TEM image of the 8-nm-thick FePt-hBN film grown on MgO seed layer. An order parameter of 0.84 was achieved with substrate temperature around 700^oC.