MIT Department of Materials Science and Engineering, Cambridge, MA, USA
There has been great interest in electrically manipulating magnetic domain walls in ferromagnetic nanotracks for high-performance memory and logic device applications. In this talk I will describe recently-discovered mechanisms, based on spin-orbit coupling at interfaces, whereby magnetic domain walls can be controlled using very low currents1,2 or by a gate voltage alone3,4. In ferromagnetic tracks thicker than a few nanometers, electrical currents drag domain walls in the direction of electron flow by transferring spin angular momentum to the lattice magnetization. However, since the spin carried by each conduction electron is fixed at ?/2, the spin-transfer efficiency per unit charge current is fundamentally limited. I will show that in ultrathin ferromagnets sandwiched between an oxide and a heavy metal, spin orbit coupling at interfaces leads to unconventional current-induced torques that circumvent this limit1,2. When a charge current flows through the heavy metal, spin-Hall currents pumped into the adjacent ferromagnet drive magnetic domain walls at high speeds with very high efficiency. Surprisingly, we find that this effect derives from chiral symmetry breaking due to a strong Dzyaloshinskii-Moriya interaction (DMI)2 which has previously been identified only at low temperatures in just a few materials whose crystal structures lack inversion symmetry. In such materials, strong DMI can generate new topological spin textures such as spin-spirals and skyrmions, which are of great fundamental and technological interest. Our results identify materials that could for the first time allow such spin textures to be realized at room temperature, in robust thin-film heterostructures that are amenable to device integration. Finally, I will describe how a gate voltage at a ferromagnet/oxide interface can be used to control magnetic domain walls by locally modulating interfacial spin-orbit coupling3,4. We have demonstrated this phenomenon in prototype field-effect devices that highlight new opportunities for ultralow power spin-based memory and logic4.
1. S. Emori, D. Bono, and G. S. D. Beach, Appl. Phys. Lett. 101, 042405 (2012).
2. S. Emori, U. Bauer, S.-M. Ahn, E. Martinez, and G. S. D. Beach, Nature Materials 12, 611 (2013).
3. U. Bauer, M. Przybylski, J. Kirschner, and G. S. D. Beach, Nano Lett. 12, 1437 (2012).
4. U. Bauer, S. Emori, and G. S. D. Beach, Nature Nanotechnology 8, 411 (2013).
Geoffrey Beach is an Associate Professor of Materials Science and Engineering at MIT. He received a B.S. in Physics from Caltech in 1997, and a Ph.D. in Physics from the University of California, San Diego in 2003 at the UCSD Center for Magnetic Recording Research. After his postdoctoral work at the University of Texas at Austin, Prof. Beach joined the MIT faculty and established the Laboratory for Nanomagnetism and Spin Dynamics, which pursues advanced spin-based concepts in data storage, logic, and biomedical applications. His work has been recognized with numerous awards including most recently a Deshpande Center Award for Technological Innovation, the MIT Junior Bose Award for Excellence in Teaching, and the MIT Class of 1958 Chaired Professorship.