Room-Temperature Skyrmions in an Antiferromagnet-Based

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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 Elaine Li, Ania Claire Bleszynski Jayich, Pedram Khalili Amiri, and Kang L. Wang Nano Lett., Just Accepted Manuscript • DOI: 10.1021/acs.nanolett.7b04400 • Publication Date (Web): 22 Dec 2017 Downloaded from http://pubs.acs.org on December 22, 2017

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Room-temperature skyrmions in an antiferromagnet-based heterostructure Guoqiang Yu1,2†*, Alec Jenkins3†, Xin Ma4†, Seyed Armin Razavi1, Congli He1, Gen Yin1, Qiming Shao1, Qing lin He1, Hao Wu2, Wenjing Li2, Wanjun Jiang5,6, Xiufeng Han2, Xiaoqin Elaine Li4, Ania Claire Bleszynski Jayich3, Pedram Khalili Amiri1§ and Kang L. Wang1*

1

Department of Electrical Engineering, University of California, Los Angeles, California 90095, United States

2

Beijing National Laboratory for Condensed Matter Physics, Institute of Physics, Chinese Academy of Sciences, Beijing 100190, China

3

Department of Physics, University of California, Santa Barbara, Santa Barbara, California 93106, USA

4

Department of Physics, The University of Texas at Austin, Austin, Texas 78712, United States

5

State Key Laboratory of Low-Dimensional Quantum Physics and Department of Physics, Tsinghua University, Beijing 100084, China 6

Collaborative Innovation Center of Quantum Matter, Beijing 100084, China †These authors contributed equally to this work. Email address: [email protected], [email protected]

§

Current affiliation: Department of Electrical Engineering and Computer Science, Northwestern

University, Evanston, Illinois 60208, United States 1

ACS Paragon Plus Environment

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Abstract Magnetic skyrmions as swirling spin textures with a non-trivial topology have potential applications as magnetic memory and storage devices. Since the initial discovery of skyrmions in non-centrosymmetric 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 roomtemperature 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, qualitative 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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Skyrmions have been used to describe topologically stable and particle-like objects in nuclear physics. Its application in magnetism is rather recent1-3, with the first experimental observation of magnetic skyrmions in bulk non-centrosymmetric B20 materials at low temperature4-7，which are stabilized by the bulk DzyaloshinskiiMoriya 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 heterostructures11,12, 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 scanning tunneling microscope12. Remarkably lowdensity current is able to drive skyrmion motion effectively, owing to their topological nature13,14. The small size, topological protection, and the ease of manipulation by current all make skyrmions attractive for use in novel memory devices15-20. Most notably, skyrmions were recently found to be stable at room temperature in sputtered magnetic multilayers (e.g., Ta/CoFeB/MgO, Pt/Co) commonly used in spintronic devices21-29, an important step toward its practical applications15,16.

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 3


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adjacent FM layers30-34. 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 DMI35, as well as the optimization of these co-existing interactions. Thus, searching for skyrmions in multilayer 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 Fig. 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. Based on the quantitatively evaluated experimental parameters obtained from nitrogenvacancy (NV)-center based microscopy and Brillouin light scattering technique, our theoretical calculation supports the existence of stable skyrmions at zero external field in the exchange-biased IrMn/CoFeB heterostructure.

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The stack structures studied in this work consist of Ta(2 nm)/Ir22Mn78(IrMn)(4 or 5 nm)/Co20Fe60B20(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 (Fig. 1a) or a Néel-type skyrmion stabilized by exchange bias (Bexchange) at zero field (Fig. 1b) can be realized. The CoFeB layers were grown as a wedge with varying thickness to systematically tune the PMA (see Supplementary 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 Supplementary Information, Figs. S1 and S3).

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 Fig. 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 temperature36. The MOKE images in figures 2b-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 Fig. 2b. The stripe domains typically have Néel-type 5


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domain walls with a particular chirality due to the interfacial DMI37-41, as illustrated in Fig. 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 Fig. 2c and Fig. S4e. Owing to the chiral property of the domain walls, the topological charge of each skyrmion can be calculated and shown to be

2

= 1/4 ∫ ⋅ × = ±1 , 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 = + + ). In the case of labyrinthine domains, the remnant magnetization state at zero field always tends to be magnetically compensated12,26, 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 terms26. The domain wall surface energy density is written as = 4 !"## − |&| , 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 over all existent domain walls. Both !"## 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, 6


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the skyrmion phase becomes stable or metastable, as shown in Fig. 2c and Fig. 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 Fig. 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 Fig. 2d. At zero applied field, small, isolated, circular skyrmions are observed in the MOKE image (Fig. 2e) taken during the field sweep along the red curve shown in Fig. 2d. The skyrmion phase can also be stabilized at zero field for the field sweep along the blue curve as well (see Figs. S5a). However, the skyrmoin densities are very different as we will discuss in more details later.

To image the zero-field skyrmions with higher spatial resolution than that of MOKE, a scanning NV microscope was used42. 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 Fig. 2f is consistent with that from a skyrmion. Similar images are further used to estimate the diameters of the skyrmions (see Supplementary 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 Fig. 2e and Fig. S6), which is likely due to the

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inhomogeneity in film thickness and perpendicular magnetic anisotropy, defects in the film, and non-uniform 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 Figs. 3a-d. 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 based on MOKE images. Samples without and with the perpendicular exchange bias (i.e., 4 nm and 5 nm IrMn layers) are shown in Figs. 3a,b and Figs. 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 (Figs. 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 Figs. 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) has been quantitatively evaluated as a function of CoFeB layer thickness (see Fig. S3b).

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The zero-field skyrmion density varies significantly for two different field sweeping directions as shown in Figs. 3c and d. A higher density of zero-field skyrmions tends to form when they directly evolve from labyrinth domains (dashed line in Fig. 3c). In contrast, a low-density of skyrmions is formed where skyrmions evolve from single domains (dashed line in Fig. 3d). This difference in the skyrmion density can be clearly seen by comparing the magnetic patterns in Fig. 2e and Fig. S5a. This density difference suggests that the field-scanning route can affect the formation of skyrmions.

Inspired by the above results, we now experimentally demonstrate that the fieldscanning 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 Figs. 4b-d (Hset = 0.0 Oe, 6.9 Oe, 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 Fig. 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 Fig. 4e and images in Fig. S7). The dependence of

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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 wall. The pinning field prevents domain wall motion and therefore requires compensation by 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 5nm-thick IrMn sample as shown by the hysteresis loop in Fig. 2d.

To utilize skyrmions in memory devices, current-driven motion is a critical enabling element. We demonstrate that skyrmions 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 sizeable spin Hall angle (∼ 0.057)45, allowing a fairly large damping-like SOT31,45,46. 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 Figs. 5a-d. The direction of skyrmion motion is along the direction of electric current, which indicates a left-handed chirality of the skyrmions (see Supplementary 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 Movie I. 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 10


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skyrmions in the sample with 5 nm thick IrMn layer slightly expand to the left side. The expansion direction follows the electric current 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 effects33, as shown in Fig. 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 devices27.

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 )J/m, obtained from a Brillouin light scattering (BLS) experiment35,47 (see Supplementary 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 the energy terms26, as shown in Figure S11. The calculation demonstrates that skyrmions are indeed 11


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energetically favored for a range of magnetic fields (see Supplementary Information, Section 5), and further, 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 roomtemperature skyrmions in a new AFM/FM material system. We determine proper conditions for skyrmion stabilization at zero-field in a phase diagram. While zerofield skyrmions have been demonstrated in HM/FM multilayers previously, a laterally confined geometry was required22,24. This constraint is removed in the AFM/FM multilayers. The elimination of an external magnetic field for stable skyrmion formation represents an important step towards the application of skyrmions in highdensity 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, 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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Acknowledgements 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 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. Wanjun Jiang 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.Q.Y. would like to thank Dr. Wei Zhang for fruitful discussions. Supporting Information Available：：Additional information is available free of 13


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charge via the Internet at http://pubs.acs.org. 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; Isolated skyrmion energy and diameter as a function of external magnetic field (PDF) Movie showing skyrmions in the sample with exchange bias expand to the left side (AVI) Note：：The authors declare that they have no competing interests.

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Figure Legend 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. The sample of 4 nm-thick IrMn layer (a) does not exhibits exchange bias, because the blocking temperature (TB) is lower than room temperature. The sample with a 5 nm-thick IrMn layer (b) 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 (a)), after an initial negative field saturation. These measurements correspond to the hysteresis path given by the red squares in 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 (d)), after an initial negative field saturation. The measurement corresponds to the hysteresis path given by the red squares in 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 ,- = ±1 spin states of the NV is measured at each point in the scan.

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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-ofplane 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 based on the images of magnetic patterns. Interpolation between experimentally measured data points was applied. The open dots show experimental points, from which the skyrmion density maps are extrapolated. In regions inside the black contours, only skyrmions (no stripes) are observed.

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 a 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 −z direction. For negative Hset values (blue squares), the initial magnetization direction is in +z direction. The field sweeping protocol for the negative Hset values and the corresponding snapshots of zero-field skyrmions are shown in the Supplementary Information, Fig. S7.

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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 a and c indicate the initial positions of the two tracked skyrmions. The red and yellow circles in 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.

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Room-Temperature Skyrmions in an Antiferromagnet-Based

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