Heat assisted magnetic recording (HAMR) is a new hard disk drive (HDD) recording technology which uses a temporary near field heating of the media during write to increase hard disk drive storage density. It has been under development in HDD industry for more than 10 years. A phenomenon totally overlooked during the past development of this technology has been revealed in this study: The existence of effective coupling between grains in the recording media due to thermoemission of spin-polarized electrons. It has important consequences on recording performance.
In HAMR, extremely large thermal gradients are created in the recording media (up to 10K/nm) due to the combination of local heating achieved by a plasmonic antenna and media velocity as the hard disk rotates (20m/s). State of the art HAMR magnetic media consists of grains of FePt alloys exhibiting high perpendicular anisotropy separated by 1 to 2 nm thick carbon segregant. Next to the plasmonic antenna, the difference of temperature between two nanosized FePt grains in the media can reach 80K across the 1 to 2 nm thick grain boundary. This represents a gigantic local thermal gradient of 40 to 80K/nm across a carbon tunnel barrier. For comparison, in magnetic tunnel junctions, much weaker thermal gradients of the order of 1K/nm across the tunnel barrier were shown to cause an effective magnetic coupling due to thermoemission of spin-polarized electrons capable of inducing magnetization switching in the magnetic electrodes. Considering that the thermal gradients in HAMR are one to two orders of magnitude larger than those used in magnetic tunnel junctions, one may expect a strong impact from these thermal spin-transfer torques on magnetization switching dynamics in HAMR recording. This issue has been totally overlooked in the earlier development of HAMR technology. Our study carried out in collaboration between SPINTEC , Headway Technologies and NIMS Japan combined theory, experiments aiming at determining the polarization of tunneling electrons across the media grain boundaries and micromagnetic simulations of recording process taking into account these thermal gradients. It was shown that the magnetic coupling due to the huge thermal gradient in the media can have a detrimental impact on the recording performances by favoring antiparallel magnetic alignment between neighboring grains during the media cooling. Implications on recording media design were made in order to overcome the influence of these thermally induced coupling
Funding: ERC MAGICAL n°669204
Further reading: Impact of Intergrain Spin-Transfer Torques Due to Huge Thermal Gradients in Heat-Assisted Magnetic Recording, Bernard Dieny, Mair Chshiev, Brian Charles, Nikita Strelkov, Alain Truong, Olivier Fruchart , Ali Hallal, Jian Wang, Yukiko K. Takahashi, Tomohito Mizuno, and Kazuhiro Hono, Advances in Magnetics, IEEE Trans.Mag. 54(12), 0800111 (2018); DOI: 10.1109/TMAG.2018.2863225