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講 師：Daniel C. Worledge
所 属：IBM基礎研究所（アルマデン）および磁気ランダムアクセスメモリ（MRAM）部門のシニアマネージャー
題 目：Spin-Transfer Torque Physics and Applications
日 時：2025 年 10 月 14 日(火)15:00～
場 所：理学部４号館1220号室
URL：https://www.phys.s.u-tokyo.ac.jp/colloquium/46903/
Spin-transfer torque was invented at IBM by John Slonczewski in the early 1990s. By using a spin-polarized current, instead of a magnetic field, to switch a magnetic layer in a magnetic tunnel junction, the switching current naturally decreases with area, providing attractive scaling for memory technology. The discovery of high magnetoresistance in MgO tunnel barriers at IBM by Stuart Parkin, and later independently by Shinji Yuasa, enabled sufficient signal to efficiently read magnetic tunnel junctions. The discovery of perpendicular magnetic anisotropy in thin CoFeB/MgO layers at IBM and independently by Tohoku University enabled a dramatic reduction in the spin-transfer torque switching current, and opened the way to practical perpendicular magnetic tunnel junctions for dense Spin-Transfer-Torque MRAM.
This talk will provide an overview of Spin-Transfer-Torque MRAM, including the physics of the three basic building blocks described above: spin-transfer torque, tunnel magnetoresistance, and perpendicular magnetic anisotropy. I will then discuss the research at IBM in 2009 that led to our discovery of perpendicular anisotropy in thin CoFeB/MgO layers, and our use of these layers to make the first practical perpendicular magnetic tunnel junctions and the first demonstration of reliable writing in Spin-Transfer-Torque MRAM. The second half of the talk will review our recent results on methods to lower the spin-transfer torque switching current, including optimized magnetic materials, double magnetic tunnel junctions, and ordered alloys. In particular, the theory of double magnetic tunnel junctions will be reviewed, and experimental data on the double spin-torque magnetic tunnel junction will be shown, including our demonstration of reliable 250 ps switching.