Room-temperature skyrmions in an antiferromagnet-based

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Letter Cite This: Nano Lett. 2018, 18, 980−986

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Room-Temperature Skyrmions in an Antiferromagnet-Based Heterostructure Guoqiang Yu,*,†,‡ Alec Jenkins,§ Xin Ma,∥ Seyed Armin Razavi,† Congli He,† Gen Yin,† Qiming Shao,† Qing lin He,† Hao Wu,‡ Wenjing Li,‡ Wanjun Jiang,⊥,# Xiufeng Han,‡ Xiaoqin Li,∥ Ania Claire Bleszynski Jayich,§ Pedram Khalili Amiri,† and Kang L. Wang*,† †

Department of Electrical Engineering, University of California, Los Angeles, California 90095, United States Beijing National Laboratory for Condensed Matter Physics, Institute of Physics, Chinese Academy of Sciences, Beijing 100190, China § Department of Physics, University of California, Santa Barbara, Santa Barbara, California 93106, United States ∥ Department of Physics, The University of Texas at Austin, Austin, Texas 78712, United States ⊥ State Key Laboratory of Low-Dimensional Quantum Physics and Department of Physics, Tsinghua University, Beijing 100084, China # Collaborative Innovation Center of Quantum Matter, Beijing 100084, China

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‡

S Supporting Information *

ABSTRACT: Magnetic skyrmions as swirling spin textures with a nontrivial topology have potential applications as magnetic memory and storage devices. Since the initial discovery of skyrmions in noncentrosymmetric B20 materials, the recent effort has focused on exploring room-temperature skyrmions in heavy metal and ferromagnetic heterostructures, a material platform compatible with existing spintronic manufacturing technology. Here, we report the surprising observation that a room-temperature skyrmion phase can be stabilized in an entirely different class of systems based on antiferromagnetic (AFM) metal and ferromagnetic (FM) metal IrMn/CoFeB heterostructures. There are a number of distinct advantages of exploring skyrmions in such heterostructures including zero-field stabilization, tunable antiferromagnetic order, and sizable spin−orbit torque (SOT) for energy-efficient current manipulation. Through direct spatial imaging of individual skyrmions, quantitative evaluation of the interfacial Dzyaloshinskii−Moriya interaction, and demonstration of current-driven skyrmion motion, our findings firmly establish the AFM/FM heterostructures as a promising material platform for exploring skyrmion physics and device applications. KEYWORDS: Skyrmion, antiferromagnet, exchange bias, zero field, room temperature, thin films

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monly used in spintronic devices,21−29 an important step toward its practical applications.15,16 To date, skyrmions have only been studied in magnetic multilayers involving ferromagnetic (FM) metal and heavy metal (HM) layers. Recently, incorporating antiferromagnets (AFMs) in magnetic multilayers has proven to be a successful strategy, leading to a number of important technical advances in spintronics such as current-driven zero-field magnetization switching and ultrafast control of spins in adjacent FM layers.30−34 The revival of AFMs in spintronics takes advantage of tunable interfacial interactions at the AFM/FM interface including perpendicular magnetic anisotropy (PMA), exchange bias, and DMI,35 as well as the optimization of these coexisting interactions. Thus, searching for skyrmions in multilayer

kyrmions have been used to describe topologically stable and particle-like objects in nuclear physics. Its application in magnetism is rather recent,1−3 with the first experimental observation of magnetic skyrmions in bulk non-centrosymmetric B20 materials at low temperature,4−7 which are stabilized by the bulk Dzyaloshinskii−Moriya interaction (DMI).8−10 Since then, the field of magnetic skyrmions has evolved rapidly. Skyrmions with a few nanometer dimensions have been reported in ultrathin film heterostructures,11,12 where they are stabilized by the interfacial DMI. Individual nanometer-scale skyrmions have been created and deleted at low temperature using local spin-polarized current from a scanning tunneling microscope.12 Remarkably low-density current is able to drive skyrmion motion effectively, owing to their topological nature.13,14 The small size, topological protection, and ease of manipulation by current all make skyrmions attractive for use in novel memory devices.15−20 Most notably, skyrmions were recently found to be stable at room temperature in sputtered magnetic multilayers (e.g., Ta/CoFeB/MgO, Pt/Co) com© 2017 American Chemical Society

Received: October 15, 2017 Revised: December 21, 2017 Published: December 22, 2017 980

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Figure 1. Schematic illustration of a labyrinthine domain pattern with Néel-type domain walls and a Néel-type skyrmion stabilized by exchange bias (Bexchange) at zero field in IrMn/CoFeB/MgO structures. (a) The sample with a 4 nm thick IrMn layer does not exhibit exchange bias, because the blocking temperature (TB) is lower than room temperature. (b) The sample with a 5 nm thick IrMn layer exhibits clear exchange bias due to the increased blocking temperature with thicker IrMn layer.

Figure 2. Skyrmions in IrMn/CoFeB heterostructures with and without exchange bias. (a) Hysteresis loop for the CoFeB layer thickness of tCoFeB = 0.96 nm and the IrMn layer thickness of tIrMn = 4 nm. The inset shows the structure of the studied samples. (b, c) Polar-MOKE images at different out-of-plane external magnetic field values (labeled in part a), after an initial negative field saturation. These measurements correspond to the hysteresis path given by the red squares in part a. The bright (dark) areas represent Mz > 0 (Mz < 0). (d) Hysteresis loop for a CoFeB layer thickness of tCoFeB = 0.91 nm and an IrMn layer thickness of tIrMn = 5 nm. (e) Polar-MOKE image for zero out-of-plane external magnetic field values (labeled in part d), after an initial negative field saturation. The measurement corresponds to the hysteresis path given by the red squares in part d. (f) Scanning NV image of a single zero-field skyrmion showing the measured magnitude of the magnetic field along the NV axis. The splitting between the ms = ±1 spin states of the NV is measured at each point in the scan.

supports the existence of stable skyrmions at zero external field in the exchange-biased IrMn/CoFeB heterostructure. The stack structures studied in this work consist of Ta(2 nm)/Ir22Mn78 (IrMn)(4 or 5 nm)/Co20Fe60 B20(CoFeB)(wedge)/MgO(2 nm)/Ta(2 nm) sputtered films. The films were then annealed under a magnetic field to enhance their PMA and introduce the exchange bias. In the studied heterostructures, the PMA and AFM order critically depend on the thickness of both IrMn and CoFeB layers. By tuning the thickness of both layers, either a labyrinthine domain pattern with Néel-type domain walls (Figure 1a) or a Néel-type skyrmion stabilized by exchange bias (Bexchange) at zero field (Figure 1b) can be realized. The CoFeB layers were grown as a wedge with varying thickness to systematically tune the PMA (see the Supporting Information, section 1). The antiferromagnetic order was also tuned through changing the IrMn layer thickness. Here, we study and compare two structures with different IrMn thicknesses at room temperature: one exhibiting no exchange bias (4 nm) and the other with exchange bias (5 nm) (see the Supporting Information, Figures S1 and S3).

heterostructures incorporating an AFM may provide new opportunities to further improve device performance. Here, we report the discovery of a stable skyrmion phase at room temperature in an AFM/FM heterostructure, IrMn/ CoFeB. We further demonstrate a number of distinct advantages of skyrmions hosted in such IrMn/CoFeB heterostructures. First, the exchange bias at the IrMn/CoFeB interface can eliminate the need for an external magnetic field, leading to zero-field skyrmion formation, as illustrated in Figure 1b. Second, the antiferromagnetic order and PMA can be tuned by the IrMn and CoFeB film thicknesses, lending additional flexibility in interfacial control. Third, a sizable spin−orbit torque (SOT) in the IrMn permits energy-efficient current control of skyrmions. We determine a phase diagram revealing the conditions for skyrmion formation, and we also provide a protocol for manipulating skyrmion density. On the basis of the quantitatively evaluated experimental parameters obtained from nitrogen-vacancy (NV)-center-based microscopy and the Brillouin light scattering technique, our theoretical calculation 981

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Figure 3. Skyrmion phase diagrams. The skyrmion phase diagrams as a function of the nominal CoFeB thickness and out-of-plane magnetic field for the samples without (a, b) and with (c, d) exchange bias. (a, c) Phase diagrams are obtained at various out-of-plane magnetic fields after a negative field saturation (Minitial < 0). (b, d) Phase diagrams are obtained at various out-of-plane magnetic fields after a positive field saturation (Minitial > 0). The color scale represents the skyrmion density, which is obtained on the basis of the images of magnetic patterns. The open dots show experimental points, from which the skyrmion density maps are extrapolated. Interpolation between experimentally measured data points was applied. In regions inside the black contours, only skyrmions (no stripes) are observed.

First, with the assistance of an external magnetic field, we demonstrate a stable skyrmion phase in the 4 nm thick IrMn sample. As shown in Figure 2a, the hysteresis loop measured from the magneto-optic Kerr effect (MOKE) for this sample is centered at zero field. The absence of an exchange field in this sample with a thin AFM layer is owing to a low blocking temperature.36 The MOKE images in Figure 2b and c depict the evolution of magnetic domain patterns as an out-of-plane magnetic field is changed from 0 to 10.4 Oe. At zero external field, the film exhibits a labyrinthine pattern, as shown in Figure 2b. The stripe domains typically have Néel-type domain walls with a particular chirality due to the interfacial DMI,37−41 as illustrated in Figure 1a. When the magnetic field reaches a critical value, the labyrinthine phase is transformed into a stable skyrmion phase through shrinking stripe domains into skyrmions, as shown in Figure 2c and Figure S4e. Owing to the chiral property of the domain walls, the topological charge of each skyrmion can be calculated and shown to be N = (1/ 4π)∫ d2r(m·∂xm × ∂ym) = ±1,2 where m is the unit magnetization vector. The observed evolution of the magnetic pattern results from the competition of three energy terms: magnetic dipole energy (ED), domain wall energy (EDW), and Zeeman energy (EZee) (the total energy is written as Etot = ED + EDW + EZee). In the case of labyrinthine domains, the remnant magnetization state at zero field always tends to be magnetically compensated,12,26 with equal areas of upward- and downward-pointing spins/ magnetizations. This is because compensated magnetic order helps to lower ED and stabilizes the labyrinthine domain

pattern. The evolution to the skyrmion necessarily requires tuning the other two energy terms.26 The domain wall surface energy density is written as δ W = 4 AKeff − π |D|, where A is the exchange stiffness, Keff is the effective perpendicular anisotropy energy, and D is the DMI coefficient. EDW is calculated as the integral of δw over all existent domain walls. Both Keff and D are tunable via coupling at the IrMn/CoFeB interface, enabling the modification of EDW. The Zeeman energy term is readily tunable via a magnetic field perpendicular to the sample plane. When the magnetic field reaches a critical value, the skyrmion phase becomes stable or metastable, as shown in Figure 2c and Figure S4e. Compared to HM/FM multilayers, one key advantage of using the AFM/FM structure for skyrmion formation is that the external magnetic field can be replaced by the perpendicular exchange bias introduced by the AFM layer, as illustrated in Figure 1b. This situation is realized in a sample with a thicker IrMn layer (5 nm). The magnitude of this exchange bias (∼28 Oe) is measured from the shifted hysteresis loop, as shown in Figure 2d. At zero applied field, small, isolated, circular skyrmions are observed in the MOKE image (Figure 2e) taken during the field sweep along the red curve shown in Figure 2d. The skyrmion phase can also be stabilized at zero field for the field sweep along the blue curve as well (see Figure S5a). However, the skyrmoin densities are very different, as we will discuss in more detail later. To image the zero-field skyrmions with higher spatial resolution than that of MOKE, a scanning NV microscope 982

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Nano Letters was used.42 The NV microscope measures the stray magnetic fields emanating from the sample surface with nanometer-scale spatial resolution. The stray field image as shown in Figure 2f is consistent with that from a skyrmion. Similar images are further used to estimate the diameters of the skyrmions (see the Supporting Information, section 2). At zero external field, the average skyrmion diameter is measured to be 600 ± 190 nm. It is noted that the sizes of skyrmions are not identical (see Figure 2e and Figure S6), which is likely due to the inhomogeneity in film thickness and perpendicular magnetic anisotropy, defects in the film, and nonuniform exchange bias. The experimentally determined skyrmion phase diagrams as a function of tCoFeB and the applied magnetic field for the two multilayer structures are summarized in Figure 3. The measurements were performed on samples with a wedged CoFeB layer, in which the thickness of the CoFeB layer varied systematically across the sample. The density of the skyrmions was counted visually on the basis of MOKE images. Samples without and with the perpendicular exchange bias (i.e., 4 and 5 nm IrMn layers) are shown in Figure 3a,b and Figure 3c,d, respectively. In regions inside the black contours, only skyrmions are present, rather than mixtures of stripe domains and skyrmions. Without an exchange bias (Figure 3a and b), it is clear that the pure skyrmion phase can only be observed under a finite external magnetic field. However, when exchange bias is present in the 5 nm thick IrMn sample, the phase diagram shifts along the magnetic field axis such that the stable skyrmion region intersects the zero-field axis (white dashed lines in Figure 3c and d). It is noted that the observed zero-field skyrmion phase only exists within a limited range of tCoFeB (see the phase diagram). The sensitive dependence of the skyrmion density on the CoFeB thickness originates from the changes in perpendicular magnetic anisotropy field (Hk). Both the Hk and the exchange bias field (Hex) have been quantitatively evaluated as a function of CoFeB layer thickness (see Figure S3b). The zero-field skyrmion density varies significantly for two different field sweeping directions, as shown in Figure 3c and d. A higher density of zero-field skyrmions tends to form when they directly evolve from labyrinth domains (dashed line in Figure 3c). In contrast, a low density of skyrmions is formed where skyrmions evolve from single domains (dashed line in Figure 3d). This difference in the skyrmion density can be clearly seen by comparing the magnetic patterns in Figure 2e and Figure S5a. This density difference suggests that the fieldscanning route can affect the formation of skyrmions. Inspired by the above results, we now experimentally demonstrate that the field-scanning route controls the density of zero-field skyrmions in both branches of the hysteresis loop. Figure 4a depicts the field sweeping protocol. Starting from a single domain saturated along −z (Minitial < 0), the applied field H was swept from a negative starting value to positive Hset (step I). The applied field was then reduced to zero (step II). Different values of Hset give different densities of zero-field skyrmions, as shown in Figure 4b−d (Hset = 0.0, 6.9, and 17.3 Oe, respectively). As Hset increases and the sample magnetization approaches saturation in the +z direction, the skyrmion density decreases, as quantitatively shown in Figure 4e (red curve). Tuning the skyrmion density can also be achieved by sweeping the field in the opposite direction when Minitial > 0 (blue squares in Figure 4e and images in Figure S7). The dependence of zero-field skyrmion density on the sweeping route can be attributed to the presence of a pinning field, which is the threshold external field required to move the domain

Figure 4. Dependence of zero-field skyrmion densities on the magnetic field sweeping protocol. (a) Schematic diagram for setting the magnetic field to zero from an initial state of Mz < 0. The magnetization is initially saturated in the −z direction. The magnetic field is then increased to Hset (step I) and subsequently set back to 0.0 Oe (step II). (b−d) Snapshots of zero-field skyrmions for Hset = 0.0 Oe (b), 6.9 Oe (c), and 17.3 Oe (d). (e) Zero-field skyrmion density for different Hset values. For positive Hset values (red dots), the initial magnetization direction is in the −z direction. For negative Hset values (blue squares), the initial magnetization direction is in the +z direction. The field sweeping protocol for the negative Hset values and the corresponding snapshots of zero-field skyrmions are shown in the Supporting Information, Figure S7.

wall. The pinning field prevents domain wall motion and therefore requires compensation by an external magnetic field, making the creation and elimination of skyrmions depend on the field-scanning route. This pinning effect is likely introduced along with the exchange bias36 and can be seen from the enhanced coercivity of the 5 nm thick IrMn sample, as shown by the hysteresis loop in Figure 2d. To utilize skyrmions in memory devices, current-driven motion is a critical enabling element. We demonstrate that skyrmion motion can be driven by an electric current via a sizable SOT43,44 in the IrMn layer. The direction of the motion further reveals the chirality of the skyrmions. It has been recently reported that IrMn has a sizable spin Hall angle (∼0.057),45 allowing a fairly large damping-like SOT.31,45,46 Utilizing this electric-current-induced SOT, the skyrmions in the 4 nm thick IrMn sample (sustained by out-of-plane magnetic fields) can be displaced by current pulses with a magnitude of 3.57 × 1010 A/m2, as shown in Figure 5. The direction of skyrmion motion is along the direction of electric current, which indicates a left-handed chirality of the skyrmions (see the Supporting Information, section 3). However, the zero-field skyrmions in the 5 nm thick IrMn sample (sustained by exchange bias) cannot be effectively moved by current pulses of up to 1.09 × 1011 A/m2, as shown in the Supporting Information movie. This is due to the pinning effect that is likely originated from the nonuniformity of the exchange bias. Nonetheless, the movie shows that many of the skyrmions in the sample with a 5 nm thick IrMn layer slightly expand to the left side. The expansion direction follows the electric current 983

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FM material system. We determine proper conditions for skyrmion stabilization at zero field in a phase diagram. While zero-field skyrmions have been demonstrated in HM/FM multilayers previously, a laterally confined geometry was required.22,24 This constraint is removed in the AFM/FM multilayers. The elimination of an external magnetic field for stable skyrmion formation represents an important step toward the application of skyrmions in high-density magnetic memories, data storage, and computing devices. Our work paves the way to explore a new class of magnetic multilayers to explore skyrmions based on specific antiferromagnetic materials. The rich properties of AFMs, such as zero net magnetization, tunable antiferromagnetic order, tunable SOT strength, and large exchange bias, combined with the fact that the AFM order can be electrically switched48 provide many future opportunities to explore and optimize skyrmion-based applications.

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ASSOCIATED CONTENT

S Supporting Information *

Figure 5. Current-induced skyrmion motion. (a−d) The motion of skyrmions with N = 1 driven by current pulses. The current directions are in the positive direction (a and b) and negative direction (c and d), respectively. The red and yellow circles in parts a and c indicate the initial positions of the two tracked skyrmions. The red and yellow circles in parts b and d indicate the final positions of the two tracked skyrmions after applying current pulses. The skyrmions are stabilized by an out-of-plane external field of 10.4 Oe.

The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acs.nanolett.7b04400. The CoFeB layer thickness dependence of perpendicular magnetic anisotropy and exchange bias; scanning nitrogen-vacancy measurements of skyrmion size; determination of chirality based on current-driven skyrmion motion; DMI measurement using Brillouin light scattering experiments; and isolated skyrmion energy and diameter as a function of external magnetic field (PDF) Movie showing skyrmions in the sample with exchange bias expanded to the left side (AVI)

direction (current flowing from right to left), again confirming the left-handed chirality of the skyrmions. Further increasing the current density (above 1.25 × 1011 A/m2) weakens the exchange bias due to thermal effects,33 as shown in Figure S9. While there is an apparent compromise in skyrmion mobility via the pinning effect and stability induced by exchange bias in the experiments reported here, we believe that this is not a fundamental limitation. The pinning effect can be suppressed by enhancing the uniformity of the exchange bias through further material development, and it is an important issue to address for the practical application of the AFM/FM heterostructures such as in skyrmion shift devices.27 Microscopically, a sufficiently strong DMI is essential to produce skyrmions, whose chirality is determined by the sign of the DMI. The left-handed chirality we observe in the measurements above corresponds to a negative DMI, which is consistent with the measured DMI, D = −102 ± 4 μJ/m2, obtained from a Brillouin light scattering (BLS) experiment35,47 (see the Supporting Information, section 4). It is noted that the DMI value is inversely proportional to the CoFeB layer thickness. The value obtained above is for the position where zero-field skyrmions are observed (tCoFeB = 0.91 nm). Using the measured DMI value, the static magnetic energy of a single skyrmion can be calculated as a function of its radius by considering all of the energy terms,26 as shown in Figure S11. The calculation demonstrates that skyrmions are indeed energetically favored for a range of magnetic fields (see the Supporting Information, section 5). The required magnetic field agrees reasonably well with the measured exchange bias field of 28 Oe for the 5 nm thick IrMn sample. Thus, simulations support the existence of zero-field skyrmions. Finally, the simulated skyrmion diameters are hundreds of nanometers, consistent with the experimental values measured using the scanning NV microscope. In conclusion, we report the demonstration and current manipulation of room-temperature skyrmions in a new AFM/

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AUTHOR INFORMATION

Corresponding Authors

*E-mail: [email protected]. *E-mail: [email protected]. ORCID

Guoqiang Yu: 0000-0002-7439-6920 Qiming Shao: 0000-0003-2613-3031 Xiufeng Han: 0000-0001-8053-793X Kang L. Wang: 0000-0001-9129-2202 Present Address

P.K.A.: Department of Electrical Engineering and Computer Science, Northwestern University, Evanston, Illinois 60208, United States. Author Contributions

G.Y., A.J., X.M.: These authors contributed equally to this work. Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS The authors at UCLA and UT-Austin were supported by the Spins and Heat in Nanoscale Electronic Systems (SHINES), an Energy Frontier Research Center funded by the U.S. Department of Energy (DOE), Office of Science, Basic Energy Sciences (BES) under Award No. SC0012670. The authors at UCLA were also partially supported by the National Science Foundation (ECCS 1611570). The authors at UCLA were also partially supported by CSPIN and FAME, two of six centers of 984

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(16) Iwasaki, J.; Mochizuki, M.; Nagaosa, N. Current-induced skyrmion dynamics in constricted geometries. Nat. Nanotechnol. 2013, 8, 742. (17) Zhang, X.; Zhou, Y.; Ezawa, M. Magnetic bilayer-skyrmions without skyrmion Hall effect. Nat. Commun. 2016, 7, 10293. (18) Zhang, X.; Ezawa, M.; Zhou, Y. Magnetic skyrmion logic gates: conversion, duplication and merging of skyrmions. Sci. Rep. 2015, 5, 9400. (19) Barker, J.; Tretiakov, O. Static and Dynamical Properties of Antiferromagnetic Skyrmions in the Presence of Applied Current and Temperature. Phys. Rev. Lett. 2016, 116, 147203. (20) Zhang, X.; Zhou, Y.; Ezawa, M. Antiferromagnetic Skyrmion: Stability, Creation and Manipulation. Sci. Rep. 2016, 6, 24795. (21) Jiang, W.; Upadhyaya, P.; Zhang, W.; Yu, G.; Jungfleisch, M.; Fradin, F.; Pearson, J.; Tserkovnyak, Y.; Wang, K.; Heinonen, O.; et al. Blowing magnetic skyrmion bubbles. Science 2015, 349 (6245), 283. (22) Woo, S.; Litzius, K.; Kruger, B.; Im, M.; Caretta, L.; Richter, K.; Mann, M.; Krone, A.; Reeve, R.; Weigand, M.; et al. Observation of room-temperature magnetic skyrmions and their current-driven dynamics in ultrathin metallic ferromagnets. Nat. Mater. 2016, 15, 501. (23) Moreau-Luchaire, C.; Moutafis, C.; Reyren, N.; Sampaio, J.; Vaz, C.; Van Horne, N.; Bouzehouane, K.; Garcia, K.; Deranlot, C.; Warnicke, P.; et al. Additive interfacial chiral interaction in multilayers for stabilization of small individual skyrmions at room temperature. Nat. Nanotechnol. 2016, 11, 444. (24) Boulle, O.; Vogel, J.; Yang, H.; Pizzini, S.; Chaves, D.; Locatelli, A.; Mentes, T.; Sala, A.; Buda-Prejbeanu, L.; Klein, O.; et al. Roomtemperature chiral magnetic skyrmions in ultrathin magnetic nanostructures. Nat. Nanotechnol. 2016, 11, 449. (25) Chen, G.; Mascaraque, A.; N’Diaye, A.; Schmid, A. Room temperature skyrmion ground state stabilized through interlayer exchange coupling. Appl. Phys. Lett. 2015, 106, 242404. (26) Yu, G.; Upadhyaya, P.; Li, X.; Li, W. Y.; Kim, S. K.; Fan, Y. B.; Wong, K. L.; Tserkovnyak, Y.; Amiri, P. K.; Wang, K. L. RoomTemperature Creation and Spin Orbit Torque Manipulation of Skyrmions in Thin Films with Engineered Asymmetry. Nano Lett. 2016, 16, 1981. (27) Yu, G.; Upadhyaya, P.; Shao, Q.; Wu, H.; Yin, G.; Li, X.; He, C.; Jiang, W.; Han, X.; Amiri, P.; et al. Room-Temperature Skyrmion Shift Device for Memory Application. Nano Lett. 2017, 17, 261. (28) Jiang, W.; Zhang, X.; Yu, G.; Zhang, W.; Jungfleisch, M. B.; Pearson, J. E.; Heinonen, O.; Wang, K. L.; Zhou, Y.; Hoffmann, A.; et al. Direct Observation of the Skyrmion Hall Effect. Nat. Phys. 2016, 13, 162. (29) Litzius, K.; Lemesh, I.; Kruger, B.; Bassirian, P.; Caretta, L.; Richter, K.; Buttner, F.; Sato, K.; Tretiakov, O. A.; Forster, J.; et al. Skyrmion Hall effect revealed by direct time-resolved X-ray microscopy. Nat. Phys. 2017, 13, 170. (30) Fukami, S.; Zhang, C.; DuttaGupta, S.; Kurenkov, A.; Ohno, H. Magnetization switching by spin-orbit torque in an antiferromagnetferromagnet bilayer system. Nat. Mater. 2016, 15, 535. (31) Oh, Y.; Baek, S.; Kim, Y.; Lee, H.; Lee, K.; Yang, C.; Park, E.; Lee, K.; Kim, K.; Go, G.; et al. Field-free switching of perpendicular magnetization through spin-orbit torque in antiferromagnet/ferromagnet/oxide structures. Nat. Nanotechnol. 2016, 11, 878. (32) van den Brink, A.; Vermijs, G.; Solignac, A.; Koo, J.; Kohlhepp, J.; Swagten, H.; Koopmans, B. Field-free magnetization reversal by spin-Hall effect and exchange bias. Nat. Commun. 2016, 7, 10854. (33) Razavi, S.; Wu, D.; Yu, G.; Lau, Y.; Wong, K.; Zhu, W.; He, C.; Zhang, Z.; Coey, J.; Stamenov, P.; et al. Joule Heating Effect on FieldFree Magnetization Switching by Spin-Orbit Torque in ExchangeBiased Systems. Phys. Rev. Appl. 2017, 7, 024023. (34) Ma, X.; Fang, F.; Li, Q.; Zhu, J.; Yang, Y.; Wu, Y.; Zhao, H.; Lupke, G. Ultrafast spin exchange-coupling torque via photo-excited charge-transfer processes. Nat. Commun. 2015, 6, 8800. (35) Ma, X.; Yu, G.; Razavi, S. A.; Sasaki, S. S.; Li, X.; Hao, K.; Tolbert, S. H.; Wang, K. L.; Li, X. Dzyaloshinskii-Moriya interaction across an antiferromagnet-ferromagnet interface. arXiv:1706.00535, 2017.

STARnet, a Semiconductor Research Corporation program, sponsored by MARCO and DARPA. The authors at UCLA were also partially sponsored by the Army Research Office and was accomplished under Grant Number W911NF-16-1-0472. The authors at UCLA would like to acknowledge the collaboration of this research with the King Abdul-Aziz City for Science and Technology (KACST) via The Center of Excellence for Green Nanotechnologies (CEGN). The authors at UCSB were supported by an Air Force Office of Scientific Research PECASE award. Probe fabrication was done in the UC Santa Barbara Nanofabrication Facility, part of the NSF funded NNIN network. W.J. was supported by National Key R&D Program of China under contract number 2017YFA0206200, 2016YFA0302300, National Science Foundation of China under contract number 11774194, the 1000Youth talent program of China, the State Key Laboratory of Low-Dimensional Quantum Physics, the Beijing Advanced Innovation Center for Future Chip (ICFC). G.Y. would like to thank Dr. Wei Zhang for fruitful discussions.

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REFERENCES

(1) Fert, A.; Cros, V.; Sampaio, J. Skyrmions on the track. Nat. Nanotechnol. 2013, 8, 152. (2) Nagaosa, N.; Tokura, Y. Topological properties and dynamics of magnetic skyrmions. Nat. Nanotechnol. 2013, 8, 899. (3) Jiang, W.; Chen, G.; Liu, K.; Zang, J.; te Velthuis, S. G. E.; Hoffmann, A. Skyrmions in magnetic multilayers. Phys. Rep. 2017, 704, 1. (4) Muhlbauer, S.; Binz, B.; Jonietz, F.; Pfleiderer, C.; Rosch, A.; Neubauer, A.; Georgii, R.; Boni, P. Skyrmion Lattice in a Chiral Magnet. Science 2009, 323, 915. (5) Neubauer, A.; Pfleiderer, C.; Binz, B.; Rosch, A.; Ritz, R.; Niklowitz, P. G.; Boni, P. Topological Hall Effect in the A Phase of MnSi. Phys. Rev. Lett. 2009, 102, 186602. (6) Yu, X. Z.; Onose, Y.; Kanazawa, N.; Park, J. H.; Han, J. H.; Matsui, Y.; Nagaosa, N.; Tokura, Y. Real-space observation of a twodimensional skyrmion crystal. Nature 2010, 465, 901. (7) Yu, X. Z.; Kanazawa, N.; Onose, Y.; Kimoto, K.; Zhang, W. Z.; Ishiwata, S.; Matsui, Y.; Tokura, Y. Near room-temperature formation of a skyrmion crystal in thin-films of the helimagnet FeGe. Nat. Mater. 2011, 10, 106. (8) Dzyaloshinsky, I. Thermodynamic theory of weak ferromagnetism in antiferromagnetic substances. Sov. Phys. JETP 1957, 5, 1259. (9) Moriya, T. Anisotropic Superexchange Interaction and Weak Ferromagnetism. Phys. Rev. 1960, 120, 91. (10) FERT, A.; LEVY, P. ROLE OF ANISOTROPIC EXCHANGE INTERACTIONS IN DETERMINING THE PROPERTIES OF SPIN-GLASSES. Phys. Rev. Lett. 1980, 44, 1538. (11) Heinze, S.; von Bergmann, K.; Menzel, M.; Brede, J.; Kubetzka, A.; Wiesendanger, R.; Bihlmayer, G.; Blugel, S. Spontaneous atomicscale magnetic skyrmion lattice in two dimensions. Nat. Phys. 2011, 7, 713. (12) Romming, N.; Hanneken, C.; Menzel, M.; Bickel, J. E.; Wolter, B.; von Bergmann, K.; Kubetzka, A.; Wiesendanger, R. Writing and Deleting Single Magnetic Skyrmions. Science 2013, 341, 636. (13) Jonietz, F.; Muhlbauer, S.; Pfleiderer, C.; Neubauer, A.; Munzer, W.; Bauer, A.; Adams, T.; Georgii, R.; Boni, P.; Duine, R. A.; et al. Spin Transfer Torques in MnSi at Ultralow Current Densities. Science 2010, 330, 1648. (14) Yu, X. Z.; Kanazawa, N.; Zhang, W. Z.; Nagai, T.; Hara, T.; Kimoto, K.; Matsui, Y.; Onose, Y.; Tokura, Y. Skyrmion flow near room temperature in an ultralow current density. Nat. Commun. 2012, 3, 988. (15) Sampaio, J.; Cros, V.; Rohart, S.; Thiaville, A.; Fert, A. Nucleation, stability and current-induced motion of isolated magnetic skyrmions in nanostructures. Nat. Nanotechnol. 2013, 8, 839. 985

DOI: 10.1021/acs.nanolett.7b04400 Nano Lett. 2018, 18, 980−986

Letter

Nano Letters (36) Nogues, J.; Schuller, I. Exchange bias. J. Magn. Magn. Mater. 1999, 192, 203. (37) Emori, S.; Bauer, U.; Ahn, S. M.; Martinez, E.; Beach, G. S. D. Current-driven dynamics of chiral ferromagnetic domain walls. Nat. Mater. 2013, 12, 611. (38) Ryu, K. S.; Thomas, L.; Yang, S. H.; Parkin, S. Chiral spin torque at magnetic domain walls. Nat. Nanotechnol. 2013, 8, 527. (39) Chen, G.; Zhu, J.; Quesada, A.; Li, J.; N’Diaye, A.; Huo, Y.; Ma, T.; Chen, Y.; Kwon, H.; Won, C.; et al. Novel Chiral Magnetic Domain Wall Structure in Fe/Ni/Cu(001) Films. Phys. Rev. Lett. 2013, 110, 77204. (40) Yu, G.; Upadhyaya, P.; Wong, K.; Jiang, W.; Alzate, J.; Tang, J.; Amiri, P.; Wang, K. Magnetization switching through spin-Hall-effectinduced chiral domain wall propagation. Phys. Rev. B: Condens. Matter Mater. Phys. 2014, 89, 104421. (41) Hrabec, A.; Porter, N.; Wells, A.; Benitez, M.; Burnell, G.; McVitie, S.; McGrouther, D.; Moore, T. Marrows, C. Measuring and tailoring the Dzyaloshinskii-Moriya interaction in perpendicularly magnetized thin films. Phys. Rev. B: Condens. Matter Mater. Phys. 2014, 90, 020402. (42) Pelliccione, M.; Jenkins, A.; Ovartchaiyapong, P.; Reetz, C.; Emmanouilidou, E.; Ni, N.; Jayich, A. Scanned probe imaging of nanoscale magnetism at cryogenic temperatures with a single-spin quantum sensor. Nat. Nanotechnol. 2016, 11, 700. (43) Miron, I. M.; Garello, K.; Gaudin, G.; Zermatten, P. J.; Costache, M. V.; Auffret, S.; Bandiera, S.; Rodmacq, B.; Schuhl, A.; Gambardella, P. Perpendicular switching of a single ferromagnetic layer induced by in-plane current injection. Nature 2011, 476, 189. (44) Liu, L.; Pai, C.; Li, Y.; Tseng, H.; Ralph, D.; Buhrman, R. SpinTorque Switching with the Giant Spin Hall Effect of Tantalum. Science 2012, 336, 555. (45) Wu, D.; Yu, G.; Chen, C.-T.; Razavi, S. A.; Shao, Q.; Li, X.; Zhao, B.; Wong, K. L.; He, C.; Zhang, Z.; et al. Spin-orbit torques in perpendicularly magnetized Ir22Mn78/Co20Fe60B20/{MgO} multilayer. Appl. Phys. Lett. 2016, 109, 222401. (46) Zhang, W.; Jungfleisch, M.; Jiang, W.; Pearson, J.; Hoffmann, A.; Freimuth, F.; Mokrousov, Y. Spin Hall Effects in Metallic Antiferromagnets. Phys. Rev. Lett. 2014, 113, 196602. (47) Ma, X.; Yu, G.; Li, X.; Wang, T.; Wu, D.; Olsson, K.; Chu, Z.; An, K.; Xiao, J.; Wang, K.; et al. Interfacial control of DzyaloshinskiiMoriya interaction in heavy metal/ferromagnetic metal thin film heterostructures. Phys. Rev. B: Condens. Matter Mater. Phys. 2016, 94, 180408. (48) Wadley, P.; Howells, B.; Zelezny, J.; Andrews, C.; Hills, V.; Campion, R.; Novak, V.; Olejnik, K.; Maccherozzi, F.; Dhesi, S.; et al. Electrical switching of an antiferromagnet. Science 2016, 351, 587.

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DOI: 10.1021/acs.nanolett.7b04400 Nano Lett. 2018, 18, 980−986