Tomohito Mizuno

CPP-GMR Film With ZnO-Based Novel Spacer for Future High-Density Magnetic Recording

A new type of CPP-GMR film, which has ZnO-based novel spacer, was studied. A high MR ratio of 21.4% at RA of about 0.2 ¿¿m2 at room temperature was obtained by optimization of the fabrication condition of ZnO layer. Based on HRTEM observation, the ZnO spacer is crystalline which have c-axis orientation with wurtzite structure, and there are no metallic portions. The measured noise is in good agreement with Johnson noise fitted at T = 400(K). This result suggests that the contact at our ZnO-based spacer interface with magnetic electrode is ohmic. First principle calculations that used a simple model supported the existence of large spin dependent scattering at the interface of ZnO layer. These results indicate that higher signal to noise ratio can be achieved in this type of CPP GMR head even with lower MR ratio than MTJ head and it is very attractive candidate of future magnetic read sensor in HDDs.

Transport and Magnetic Properties of CPP-GMR Sensor With CoMnSi Heusler Alloy

We investigated structural and magnetic properties of CoMnSi (CMS) Heusler alloy films and giant magneto-resistance (GMR) ratio of current-perpendicular-to-plane (CPP) GMR sensor elements. The films were deposited with magnetron sputtering method and annealed at 673 K and 773 K. The CMS films were identified as B2 structure by using X-ray diffraction and the magnetization of the films were evaluated as 0.95 T. We fabricated the CPP-GMR sensors varying the composition of the CMS films, Co48 Mn21 Si31 (at. %) as Si-rich CMS and Co51Mn25Si24 (at. %) as CMS-ref, having a FeCo layer between the CMS and Cu spacer. It was found that the element with Si-rich CMS exhibited higher GMR ratio of 9.0% than that with CMS-ref. Our calculations indicate that larger spin polarization of contact region between the CMS film and the FeCo film relates to larger GMR ratio.

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.