Seminar organized by Graduate Program in Spintronics“Spin-Orbitronics: Interfacial
Design of Spintronic
Materials and Devices”
Speaker: Professor Geoffrey Beach(Massachusetts Institute of Technology)

HEREFebruary 18 (Thu.) 2016
10:30-12:00
Venue: Room A401
Laboratory for
Nanoelectronics andSpintronics, RIEC
(Katahira campus)

Contact to Shunsuke Fukami
E-mail: s-fukami@csis.tohoku.ac.jp, Phone: 022-217-5555

Spin-Orbitronics: Interfacial Design of Spintronic Materials and Devices
Geoffrey BeachMIT Department of Materials Science and Engineering, Cambridge, MA, USA

There is great interest in electrically manipulating the magnetization in nanoscale materials
for high-performance memory and logic device applications. In this talk I will describe
recently-discovered mechanisms, based on symmetry breaking and spin-orbit coupling atinterfaces, whereby the magnetization can be controlled using very low currents1-6 or by a gate
voltage alone.5-8 I will focus on ultrathin transition metal ferromagnets sandwiched between
an oxide and a nonmagnetic heavy metal, in which magnetic, electronic and ionic effects at theinterface can be exploited in new and unexpected ways.
I first focus on the heavy-metal/ferromagnetic interface, where spin-orbit coupling
influences not only spin transport, but the nature of magnetism itself in the ferromagnet. In
nonmagnetic heavy metals, spin-orbit coupling leads to a left-right scattering asymmetry suchthat spin “up” and spin “down” electrons pile up on either side of a material when a charge
current flows through it. I will show how this spin Hall effect can be used to drive
magnetization switching and domain wall motion in an adjacent ferromagnetic film,1-4 anddiscuss its enhancement through interface engineering5. In these same materials, broken
inversion symmetry can lift the chiral degeneracy and generate new topological spin textures
such as spin-spirals and skyrmions through the interfacial Dzyaloshinskii-Moriya interaction(DMI). I will show that chiral magnetism can persist at room temperature in common
transition metal ferromagnets3,4, and discuss the role of DMI in domain wall dynamics3,4 andspin-orbit torque switching5. I then show that DMI in engineered heterostructures can be used
to stabilize room-temperature magnetic skyrmions6, which have recently been proposed asscalable, thermally-stable bits for advanced spintronics devices. We have demonstrated theability to generate stable skyrmion lattices and drive trains of individual skyrmions by shortcurrent pulses along a magnetic racetrack at speeds exceeding 100 m/s, opening the door toroom-temperature skyrmion spintronics in robust thin-film heterostructures6.
Finally, I will turn to the ferromagnet/oxide interface7-11 and describe our discovery of a
new class of “magneto-ionic” materials,9,10 in which a gate voltage can be used toelectrochemically switch the interfacial oxidation state to realize unprecedented control overmagnetic properties. Here we use Pt/Co/Gd2O3- ultrathin film stacks, where Gd2O3- serves as
an efficient oxygen ion conductor. I show that the magnetization in the thin Co layer can beswitched between perpendicular and in-plane orientations, or quenched entirely, by driving O2towards or away from the Co/GdOx interface with a small gate voltage10. I then show thatmagneto-ionic gates can be used to locally tune magnetic properties to create a magneticanalog of field-effect transistors,9 and to electrically control spin-orbit torques11.
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. S. Emori, E. Martinez, Kyung-Jin Lee, Hyun-Woo Lee, U. Bauer, S.-M. Ahn, P. Agrawal, D. C. Bono, and G.
S. D. Beach, Physical Review B 90, 184427 (2014).
4. N. Perez, E. Martinez, L. Torres, S.-H. Woo, S. Emori, and G. S. D. Beach, Appl. Phys. Lett. 104, 092403(2014)
5. S. Woo, M. Mann, A. J. Tan, L. Caretta, and G. S. D. Beach, Appl. Phys. Lett. 105, 212404 (2014).
6. S. Woo, K. Litzius, B. Krüger, M.-Y. Im, L. Caretta, K. Richter, M. Mann, A. Krone, R. Reeve, M. Weigand,P. Agrawal, P. Fischer, M. Kläui, and G. S. D. Beach, Nature Materials in press (2016); arXiv:1502.07376
7. U. Bauer, M. Przybylski, J. Kirschner, and G. S. D. Beach, Nano Lett. 12, 1437 (2012).
8. U. Bauer, S. Emori, and G. S. D. Beach, Appl. Phys. Lett. 100, 192408; ibid 101, 172403 (2012).
9. U. Bauer, S. Emori, and G. S. D. Beach, Nature Nanotechnology 8, 411 (2013).
10. U. Bauer, L. Yao, S. Emori, H. L. Tuller, S. van Dijken, and G. S. D. Beach, Nature Materials 14, 174(2015)11. S. Emori, U. Bauer, S. –H. Woo, and G. S. D. Beach, Appl. Phys. Lett.105, 222401 (2014).