As we are stepping into the “Digital Society”, the exploding digital universe is causing serious storage capacity crisis . L10-ordered FePt-X nanogranular films have been developed as heat-assisted magnetic recording (HAMR) media with ultrahigh recording density about 4 Tbit/in2 . One main challenge in realizing FePt media for hard disk drives with areal density of 4Tb/in2 is the in-plane component of magnetization (media noise) which seriously degrades the signal-to-noise ratio and hinders further improvements in the media capacity and performance. In order to minimize the in-plane component of magnetization, fundamental understanding on its origin is needed.
Additionally, a high signal-to-noise ratio in high-density recording has been implemented by aligning the magnetic easy axis of the FePt grains perpendicular to the film plane. However, this requires epitaxial growth on the single crystal-MgO substrate that is very costly. To this end, alternative methods for growing of (001)-textured FePt films on a low-cost amorphous substrate like glass are highly desirable.
To accommodate the increasing demand for high storage density, magnetic in-plane components of the FePt nanogranular film have been recently adopted to help in preventing the medium noise effects. These films originate from misoriented FePt grains with magnetic easy axis tilted away from the film normal distribution. Unfortunately, the past review has shown that despite the improvement in the recording density, physical limits in refining the grains have been reached. To ensure continued improvement in the magnetic recording performance, it would be a good idea to eliminate the in-plane magnetic components taking into account their potential sources i.e. the polycrystalline underlayers.
Herein, scientists from the National Institute for Materials Science in Japan: Dr. Jian Wang, Dr. Hossein Sepehri-Amin, Professor Y.K. Takahashi, Professor Tadakatsu Ohkubo and Professor Kazuhiro Hono investigated the impacts of polycrystalline MgO underlayer (surface roughness, grain boundary, interfacial lattice and crystallographic texture distribution) on the magnetic in-plane components of granular films. The main objective was to reveal the origin of the underlying microstructural components and suitable methods for their suppression. The work is published in the journal, Acta Materialia.
Transmission electron microscope observations revealed the possibility of developing good (001) texture on FePt nanogranular films whether grown on the rough single crystal-MgO (001) substrate surface or across the boundary of the polycrystalline-MgO underlayer. Well, this observation was attributed to the similar orientation of both MgO and FePt thus the former exhibited insignificant influence on the surface roughness of MgO substrate on the texture of the FePt magnetic layers.
On the other hand, the epitaxial growth on the initially misoriented MgO underlayer grains was produced from the misoriented FePt grains with magnetic easy axes tilted away from film normal distribution. It was noteworthy that magnetic easy axis had limited influence on magnetic in-plane components despite possible small deviation. As such, magnetic in-plane components were detected with FePt grains with misorientation angles great than 20°. Furthermore, grain formation could be induced by the release of the stored grain energy during the growth of large FePt grains.
In summary, the findings by researchers at National Institute for Materials Science provided a comprehensive understanding of the effects of magnetic in-plane components on polycrystalline MgO underlayer.
It is possible to suppress the magnetic in-plane component of the FePt nanogranular media by improving the key parameters (the texture quality of the poly-MgO underlayer). With this purpose, the authors went further introduce a texture template layer for poly-MgO underlayer (Reference 3 Japanese Patent) which proved can improve the texture of poly-MgO underlayer then further suppress the in-plane component for FePt nanogranular film-based HAMR media.
Therefore, as stated by Dr. Wang, first author in a statement to Advances in Engineering, the study will pave the guideline for the development of high-performance FePt-based-heat-assisted magnetic recording media for numerous applications.
1) Wang, J., Sepehri-Amin, H., Takahashi, Y., Ohkubo, T., & Hono, K. (2019). Magnetic in-plane components of FePt nanogranular film on polycrystalline MgO underlayer for heat-assisted magnetic recording media. Acta Materialia, 177, 1-8.
2) J. Wang, S. Hata, Y.K. Takahashi, H. Sepehri-Amin, B.S.D.C.S. Varaprasad, T. Shiroyama, T. Schrefl, K. Hono, (2015). Effect of MgO underlayer misorientation on the texture and magnetic property of FePteC granular film, Acta Materialia,91 41-49.
3) J. Wang, H. Pandey, Y. K. Takahashi, K. Hono, K. Yakushiji, H. Kubota, Texture control of MgO underlayer for highly (001) textured FePt based magnetic recording media, JP2017157265A, JPO, Japan (2017) (in Japanese).