Impact of Intergrain Spin-Transfer Torques Due to Huge Thermal Gradients in Heat-Assisted Magnetic Recording

Heat-assisted magnetic recording (HAMR) is a new technology which uses temporary near-field heating of the media during write to increase hard disk drive storage density. By using a plasmonic antenna embedded in the write head, an extremely high thermal gradient is created in the recording media (up to 10 K/nm). State-of-the-art HAMR media consist of grains of L10-FePt exhibiting high perpendicular anisotropy separated by 1-2 nm-thick carbon segregant. Next to the plasmonic antenna, the difference of temperature between two nanosized FePt grains in the media can reach 80 K across the 2 nm-thick grain boundary. This represents a gigantic local thermal gradient of 40 K/nm across a carbon tunnel barrier. In the field of spincaloritronics, much weaker thermal gradients of ~1 K/nm were shown to cause a thermal spin-transfer torque (TST) capable of inducing magnetization switching in magnetic tunnel junctions (MTJs). Considering on the one hand, two neighboring grains separated by an insulating grain boundary in an HAMR media can be viewed as an MTJ, and on the other hand, the thermal gradients in HAMR are 1-2 orders of magnitude larger than those used in the conventional spincaloritronic experiments; one may expect a strong impact from these TSTs on magnetization switching dynamics in HAMR recording. This issue has been totally overlooked in the previous investigations on the development of the HAMR technology. This paper combines theory, experiments aiming at determining the polarization of tunneling electrons across the media grain boundaries, and micromagnetic simulations of the recording process taking into account these thermal gradients. It is shown that the thermal in-plane torque can have a detrimental impact on the recording performances by favoring antiparallel magnetic alignment between neighboring grains during the media cooling. Implications on media design are discussed in order to overcome the influence of these thermal torques. Suggestions of spincaloritronic experiments taking advantage of these huge thermal gradients produced by plasmonic antenna are also given.