Generation and Hall effect of skyrmions enabled using nonmagnetic point contacts

Zidong Wang, Xichao Zhang, Jing Xia, Le Zhao, Keyu Wu, Guoqiang Yu, Kang L. Wang, Xiaoxi Liu, Suzanne G. E. te Velthuis, Axel Hoffmann, Yan Zhou, and Wanjun Jiang

Phys. Rev. B 100, 184426 – Published 27 November 2019

Abstract

The generation and manipulation of magnetic skyrmions are perquisites for any skyrmion-based information processing devices, where skyrmions are used as nonvolatile information carriers. In this work, we report experimentally the skyrmion generation through the usage of a nonmagnetic conducting Ti/Au point contact in a device made of a $Ta / CoFeB / Ta O_{x}$ trilayer film. Moreover, the accompanied topological charge-dependent skyrmion dynamics, namely the skyrmion Hall effect, is also observed in the same device. The creation process of a skyrmion has been numerically reproduced through micromagnetic simulations, in which the important role of the skyrmion-antiskyrmion pair formation is identified. The motion and Hall effect of a skyrmion, immediately after its creation, is described using a modified Thiele equation after taking into account the contribution from spatially inhomogeneous spin-orbit torques and the Magnus force. Our results on the simultaneous generation and manipulation of skyrmions using a nonmagnetic point contact are useful for understanding the ultrafast dynamics of skyrmion creation, which could also provide an effective pathway for designing skyrmion-based devices.

Received 21 April 2019
Revised 31 August 2019

DOI:https://doi.org/10.1103/PhysRevB.100.184426

Physics Subject Headings (PhySH)

Dzyaloshinskii-Moriya interaction Hall effect Micromagnetism Skyrmions Spin texture Spin torque

Magnetic multilayers Point contacts

Magneto-optical Kerr effect Micromagnetic modeling

Condensed Matter, Materials & Applied Physics

Authors & Affiliations

Zidong Wang^1,2,*, Xichao Zhang^3,*, Jing Xia^3,*, Le Zhao^1,2, Keyu Wu^1,2, Guoqiang Yu⁴, Kang L. Wang⁵, Xiaoxi Liu⁶, Suzanne G. E. te Velthuis⁷, Axel Hoffmann^7,†, Yan Zhou³, and Wanjun Jiang ^1,2,‡

¹State Key Laboratory of Low-Dimensional Quantum Physics and Department of Physics, Tsinghua University, Beijing 100084, China
²Frontier Science Center for Quantum Information, Tsinghua University, Beijing 100084, China
³School of Science and Engineering, The Chinese University of Hong Kong, Shenzhen, Guangdong 518172, China
⁴Beijing National Laboratory for Condensed Matter Physics, Institute of Physics, Chinese Academy of Sciences, Beijing 100190, China
⁵Department of Electrical Engineering, University of California, Los Angeles, California 90095, USA
⁶Department of Electrical and Computer Engineering, Shinshu University, 4-17-1 Wakasato, Nagano 380–8553, Japan
⁷Materials Science Division, Argonne National Laboratory, Lemont, Illinois 60439, USA

^*These authors contributed equally to this work.
^†Present address: Department of Materials Science and Engineering, University of Illinois at Urbana-Champaign, Urbana, IL 61801, USA
^‡jiang_lab@tsinghua.edu.cn

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Issue

Vol. 100, Iss. 18 — 1 November 2019

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Images

Figure 1
(a) Schematic illustration of the geometrically constricted HM/FM heterostructure device which is connected through a nonmagnetic electrode. (b) The normalized electron-current density distribution in the HM layer. The arrow indicates the direction, and the color as well as the arrow size denote the density. (c) The corresponding spin-current density distribution in the FM layer. The arrows represent the polarization direction of spin currents. The color and arrow size represent the magnitude of spin currents.
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Figure 2
Numerical demonstration of skyrmion generation enabled by inhomogeneous current-induced SOTs around the nonmagnetic contact region. (a) Evolution of magnetization profile (upper panel) and the accompanied topological charge-density profile (lower panel) as a function of time ( $t$ ). In order to show the creation of skyrmions in detail, only the magnetization profiles in the region where the nucleation of skyrmion begins are shown. (b) The calculated time-dependent topological charge (i.e., the skyrmion number $Q$ ) based on the varying magnetization configurations. The dashed line at $t = 1.5$ ns indicates the switching of current from 6 to $0.03 MA c m^{- 2}$ . The transition of topological charge from $Q = 0$ to $Q = - 1$ is evident around $t = 1.7$ ns.
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Figure 3
Time evolution of (a) skyrmion number $Q$ , (b) total micromagnetic energy $E_{total}$ , (c) exchange energy $E_{ex}$ , (d) DMI energy $E_{DMI}$ , (e) anisotropy energy $E_{an}$ , and (f) demagnetization energy $E_{demag}$ in the skyrmion creation process.
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Figure 4
Sequential MOKE images driven by a train of voltage pulses showing continuous creation and the subsequent accumulation of skyrmions at lower side of the device, namely, the occurrence of skyrmion Hall effect. The experiment was performed under a perpendicular magnetic field $B_{⊥} = + 0.5$ mT. The applied voltage pulses are of +15 V in amplitude and 1 ms in duration.
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Figure 5
A diagram of the skyrmion Hall effect for opposite sign of topological charge ( $Q = \pm 1$ ) and opposite direction of voltage/current polarity ( $\pm j_{e}$ ). (a) $- j_{e}$ flows toward the $- x$ direction with $Q = + 1$ ( $B_{⊥} = - 0.5 mT$ ). (b) $- j_{e}$ flows toward the $- x$ direction with $Q = - 1$ ( $B_{⊥} = + 0.5 mT$ ). (c) $+ j_{e}$ flows toward the $+ x$ direction with $Q = - 1$ ( $B_{⊥} = + 0.5 mT$ ). (d) $j_{e}$ flows toward the $+ x$ direction with $Q = + 1$ ( $B_{⊥} = - 0.5 mT$ ). These images were taken after applying 20 pulses of $+ 15$ V in amplitude and 1 ms in duration. Scale bar corresponds to 20 μm.
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