Wednesday, April 21, 2021 -
Spin-Transfer-Torque MRAM was invented at IBM by John Slonczewski in the early 1990s. By using a spin-polarized current, instead of a magnetic field, to write a magnetic free layer in a magnetic tunnel junction, the required write current naturally decreases with area, providing attractive technology scaling. The discovery of high magnetoresistance in MgO tunnel barriers at IBM by Stuart Parkin, and later independently by Shinji Yuasa, enabled sufficient read 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 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 three basic building blocks described above. I’ll give an introduction to the basic physics of spin-transfer torque and applications of Spin-Transfer-Torque MRAM. Then I will review why perpendicular magnetic anisotropy is advantageous for MRAM compared to in-plane anisotropy, and the materials challenges of perpendicular anisotropy. I will 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. Finally I will review our recent results on methods to lower the switching current of Spin-Transfer-Torque MRAM by using optimized magnetic materials and double magnetic tunnel junctions, and our recent demonstration of the world’s first 14 nm node MRAM.
Dr. Worledge received a BA with a double major in Physics and Applied Mathematics from UC Berkeley in 1995, receiving the Department Scholar Award in physics and the Dorothea Klumpke Roberts Prize in mathematics. He then received a PhD in Applied Physics from Stanford University in 2000, with a thesis on spin-polarized tunneling in oxide ferromagnets, measuring the largest tunneling spin-polarization in (LaSr)MnO3 and the first negative tunneling spin-polarization in SrRuO3. After joining the Physical Sciences Department at the IBM T. J. Watson Research Center as a Post-doc in 2000, he became a Research Staff Member in 2001, inventing and developing Current-in-Plane Tunneling as a fast turn-around measurement method for magnetic tunnel junctions. In 2003, Dr. Worledge became the manager of the MRAM Materials and Devices group, and in 2013 he became Senior Manager of MRAM. In 2014 he was promoted to Principle Research Staff Member and in 2015 to Distinguished Research Staff Member. He has worked on developing Toggle and then Spin Torque MRAM, including discovering perpendicular magnetic anisotropy in Ta|CoFeB|MgO, and leading the IBM team that developed perpendicular magnetic tunnel junctions and demonstrated the first integrated perpendicular spin torque MRAM, with ultra-low bit error rate. His current research interests include magnetic devices and their behavior at small dimensions, magnetic devices for logic applications, and new hardware approaches to machine learning. Dr. Worledge has received four IBM Outstanding Technical Achievement Awards and the IBM Research Client Award.