Field-free switching enabled by spin-orbit torques from multiple spin polarization directions
Yu Zhang, Mahendra DC, William Hwang and Shan X. Wang (Stanford)
High spin-orbit torques (SOTs) generated by topological materials and heavy metals interfaced with a ferromagnetic layer show promise for next generation magnetic memory and logic devices. SOT generated from y-spin polarization enables a field-free deterministic in-plane switching for the magnetization along y axis. However, external magnetic fields are required to switch the magnetization along x and z axes via SOTs from y-spin polarization. Here, we report that the unconventional SOT generated from z-spin polarization in sputtered MnPd3/Co/MgO/Ta multilayers can contribute to the switching of perpendicular magnetization. In our experiments, besides to the conventional SOT by y-spin polarization, the out-of-plane SOT from z-spin polarization and the in-plane SOT from x-spin polarization are also observed in SOT characterizations. Although the spin torque efficiency of z-spin and x-spin polarizations in our sample are relatively small, the spin torque efficiency of y-spin (𝜃𝑦) ~ 0.34-0.44 is comparable or better than the widely used SOT heavy metal line as W; and the spin conductivity of y-spin (𝜎𝑦) ~ 5.70-7.30 × 105 ℏ2𝑒⁄ Ω-1m-1 is high enough to avoid current shunting through a conducting ferromagnetic layer. Remarkably, the field-free magnetization switching of perpendicular Co layer has been demonstrated in Hall bar structure. The first-principles calculations indicate that low crystal symmetry present in the (114) oriented MnPd3 thin films can serve as the origin of the observed unconventional SOTs from z-spin and x-spin polarizations. We note that higher density field-free switching SOT-MRAM (4X to 6X over SRAM) is possible while having multiple bits, e.g., one byte, to share a single SOT write line, assisted by STT for selecting individual bits.
ASCENT Theme 2, Task: 2776.056 Exploring new Topological Materials and Interfaces for Advanced SOT-MRAM
Shan X. Wang’s Biography: Dr. Wang is the Leland T. Edwards Professor in the School of Engineering, Stanford University. He is a Professor of Materials Science & Engineering and jointly a Professor of Electrical Engineering, and by courtesy, a Professor of Radiology (Stanford School of Medicine). He directs the Center for Magnetic Nanotechnology and is a leading expert in biosensors, information storage and spintronics. His research and inventions span across a variety of areas including magnetic biochips, in vitro diagnostics, cancer biomarkers, magnetic nanoparticles, magnetic sensors, magnetoresistive random access memory, and magnetic integrated inductors. He has over 300 publications, and holds 70+ issued or pending patents in these and interdisciplinary areas. He was named an inaugural Fred Terman Fellow, and was elected a Fellow of the Institute of Electrical and Electronics Engineers (IEEE) and a Fellow of American Physical Society (APS) for his seminal contributions to magnetic materials and nanosensors. His team won the Grand Challenge Exploration Award from Gates Foundation (2010), the XCHALLENGE Distinguished Award (2014), and the Bold Epic Innovator Award from the XPRIZE Foundation (2017). Through his participation in the Center for Cancer Nanotechnology Excellence (as co-PI of the CCNE) and the Joint University Microelectronics Program (JUMP), he is actively engaged in the transformative research of healthcare and is developing emerging memories for energy efficient computing. Dr. Wang obtained his PhD in Electrical and Computer Engineering from Carnegie Mellon University in 1993, MS in Physics from Iowa State University in 1988, and BS in Physics from the University of Science and Technology of China in 1986.
Yu Zhang’s Biography: Dr. Zhang is a postdoctoral scholar in Prof. Shan Xiang Wang’s group in the Department of Materials Science & Engineering, Stanford University. His research interests lie in the area of Spintronics and his current project is focused on field-free perpendicular SOT switching, aiming to provide a solution for the next generation of SOT-MRAMs with ultrafast access speed, ultralow power consumption and good CMOS compatibility. He is now serving as a manuscript reviewer for several peer reviewed journals, including Physical Review Letter, npj Quantum Materials, Physical Review Applied, APL Materials, Applied Physics Letters, Journal of Applied Physics, Scientific Reports and Transactions on Magnetics. Dr. Zhang obtained his PhDs in Physics from Université Paris-Saclay and in Microelectronics & Solid-State Electronics from Beihang University in 2018, BS in Electrical Engineering from Beihang University in 2011.
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