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 2nm 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.

Schematic principle of Heat Assisted Magnetic Recording.
Simulations illustrating the degradation of the written patterns on the magnetic media.
Each bit of information (0 or 1) appears as a bunch of magnetic grains magnetized up (black) or down (white). The grain size is 8nm.