报告题目：Magnetic Skyrmions: From Topology to Technology

报 告 人：Geoffrey S. D. Beach，Department of Materials Science and Engineering, Massachusetts Institute of Technology

报告时间：2018-10-08 14:00

报告地点：物理系理科楼C302

报告摘要：Magnetic skyrmions [1] are particle-like chiral spin textures that are topologically protected from being continuously ‘unwound’. Their topological nature gives rise to rich behaviors including ordered lattice formation, emergent electrodynamics and robust current-driven displacement at remarkably low current densities. However, magnetic skyrmions have until recently been restricted to just a few materials and observed only at low temperatures, limiting the experimental accessibility and technological application of these unique topological objects. This talk focuses on magnetic skyrmions in ultrathin ferromagnetic transition metal multilayers in which interfaces with heavy metals generate a strong Dzyaloshinskii-Moriya interaction (DMI) [2] that can stabilize chiral magnetic skyrmions at room temperature [3,4]. Using high-resolution transmission x-ray microscopy, we reveal room-temperature magnetic skyrmions and skyrmions lattices in engineered heterostructures, and provide key insights into their stability and dynamics. We show pure spin currents can be used to drive skyrmions at speeds >100 m/s [4], and to write and delete them on sub-ns timescales [5], providing experimental realizations of the key functionalities of the recently-proposed skyrmion racetrack memories [6]. We find that current-induced shifting is repeatable over billions of cycles, and we discover an analogue to the conventional Hall effect [7,8], in which the skyrmion trajectory depends on its topological charge much as a particle in a magnetic field is deflected due to its electric charge. Finally, we present an analytical framework [9] for computing the energy and structure of any skyrmion in any material, providing a solution the inverse materials design problem of achieving skyrmions with desired properties through informed materials selection. We show that whereas ferromagnets possess fundamental limits for skyrmion speed and size, multisublattice materials provide a path toward achieving ultrasmall and ultrafast skyrmions at room temperature, as we recently verified experimentally in a compensated ferrimagnet [10]. These results demonstrate the promise of using skyrmions as topological bit carriers in spin-based devices for low-power memory and logic.

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[2] S. Emori, et al., Nat. Mater. 12, 611 (2013).

[3]. W. Jiang, et al., Science 349, 283 (2015).

[4] S. Woo, et al., Nature Mater. 15, 501 (2016)

[5] F. Büttner, et al., Nature Nano. 12, 1040 (2017).

[6] A. Fert, V. Cros., and J. Sampaio, Nature Nano. 8, 152 (2013).

[7] W. Jiang, et al., Nature Phys. 13, 162 (2017).

[8] K. Litzius, et al., Nature Phys. 13, 170 (2017).

[9] F. Büttner, et al., Sci. Rep. 8, 4464 (2018).

[10] L. Caretta, et al., Nature Nano. Adv. Online Publication (2018).