United States

Skyrmions are typically manipulated by driving them with an electrical current in a racetrack and have been proven to be advantageous, due to their high mobility, energy efficient drivability and potential in overcoming edge roughness-related pinning occurring in domain wall (DW)- based racetrack devices [1]-[3]. However, to have high density skyrmion based memory devices, it is essential to create and manipulate skyrmions in magnetic tunnel junctions (MTJs). Previously, it has been shown with micromagnetic simulations that voltage-controlled magnetic anisotropy (VCMA) can be used in perpendicular MTJs to create an intermediate skyrmion starting from a ferromagnetic up/down state and subsequently annihilating it to form a ferromagnetic down/up state [4] and thus energy efficient electrical field switching robust to defects and thermal noise can be achieved. Towards the experimental realization of this switching strategy, skyrmions have to be created and manipulated with VCMA in a confined geometry where the skyrmion lateral dimension equals the lateral dimension of a nanodot. However, so far creation and annihilation of fixed magnetic skyrmions using VCMA has only been experimentally observed through magnetic force microscopy (MFM) imaging in thin ferromagnetic films where the skyrmions are much smaller in lateral dimension compared to the thin films and the imaging was performed after withdrawal of the applied electric field (Fig.1) [5]. Here, we will study experimental creation and annihilation of skyrmions using VCMA in a confined geometry of &amp;lt;1μm lateral dimension with in-situ application of voltage while imaging. This will enable visualization of the skyrmion/ferromagnet state as a function of the applied electric field and the mechanism through which skyrmions can be manipulated with VCMA induced change in PMA in the presence of interactions with the boundary of the patterned nanodot. Fig. 2 shows an example of a nanostructure in which such experiments will be carried out.&lt;br/&gt;&lt;br/&gt;Acknowledgement: NSF grant CCF #1909030

![](https://assets.underline.io/uploads/markdown_image/1/image/8ddae612ae2d0e4ae2fc5e04ca640dfb.jpg)

Fig. 1. Magnetization state before application of any electric field (a) and after application of negative (b), positive (c) voltage pulse [5]

![](https://assets.underline.io/uploads/markdown_image/1/image/a01a9085cbf4c95176982da23375450e.jpg)

Fig. 2. Skyrmions to be studied in confined geometries



2022 Joint MMM-INTERMAG

Creation and Annihilation of Magnetic Skyrmions in Confined Geometry using Voltage Control of Magnetic Anisotropy

The 15th Joint MMM-INTERMAG Conference (2022 Joint) is sponsored jointly by AIP Publishing and the IEEE Magnetics Society. Members of the international scientific and engineering communities interested in recent developments in fundamental and applied magnetism are invited to attend and contribute to the technical sessions. The technical program will include invited and contributed papers in oral and poster sessions, invited symposia, a plenary session, and an evening session, with about 1500 presentations overall. This Conference provides an outstanding opportunity for worldwide participants to meet their colleagues and collaborators and discuss developments in all areas of magnetism research.

The 2022 Joint Conference will offer both in-person and prerecorded on-demand content.

For tickets to the event, please register at the following link: https://magnetism.org/register

Please register for the Joint MMM-Intermag conference to join the event.

This Conference provides an outstanding opportunity for worldwide participants to meet their colleagues and collaborators and discuss developments in all areas of magnetism research.

Skyrmions are typically manipulated by driving them with an electrical current in a racetrack and have been proven to be advantageous, due to their high mobility, energy efficient drivability and potential in overcoming edge roughness-related pinning occurring in domain wall (DW)- based racetrack devices [1]-[3]. However, to have high density skyrmion based memory devices, it is essential to create and manipulate skyrmions in magnetic tunnel junctions (MTJs). Previously, it has been shown with micromagnetic simulations that voltage-controlled magnetic anisotropy (VCMA) can be used in perpendicular MTJs to create an intermediate skyrmion starting from a ferromagnetic up/down state and subsequently annihilating it to form a ferromagnetic down/up state [4] and thus energy efficient electrical field switching robust to defects and thermal noise can be achieved. Towards the experimental realization of this switching strategy, skyrmions have to be created and manipulated with VCMA in a confined geometry where the skyrmion lateral dimension equals the lateral dimension of a nanodot. However, so far creation and annihilation of fixed magnetic skyrmions using VCMA has only been experimentally observed through magnetic force microscopy (MFM) imaging in thin ferromagnetic films where the skyrmions are much smaller in lateral dimension compared to the thin films and the imaging was performed after withdrawal of the applied electric field (Fig.1) [5]. Here, we will study experimental creation and annihilation of skyrmions using VCMA in a confined geometry of &lt;1μm lateral dimension with in-situ application of voltage while imaging. This will enable visualization of the skyrmion/ferromagnet state as a function of the applied electric field and the mechanism through which skyrmions can be manipulated with VCMA induced change in PMA in the presence of interactions with the boundary of the patterned nanodot. Fig. 2 shows an example of a nanostructure in which such experiments will be carried out. Acknowledgement: NSF grant CCF #1909030

![](https://assets.underline.io/uploads/markdown_image/1/image/8ddae612ae2d0e4ae2fc5e04ca640dfb.jpg)

Fig. 1. Magnetization state before application of any electric field (a) and after application of negative (b), positive (c) voltage pulse [5]

![](https://assets.underline.io/uploads/markdown_image/1/image/a01a9085cbf4c95176982da23375450e.jpg)

Fig. 2. Skyrmions to be studied in confined geometries

technical paper

Materials for magnetocaloric refrigeration near room temperature attract main attention, but compounds exhibiting large magnetocaloric effects (MCE) in the cryogenic regime are also of importance.1,2 In particular, MCE materials that can support liquefaction of certain gases, from hydrogen (boils at 20 K) to natural gas (~110 K), are of interest for diversification of energy supplies.2 Here we report a large MCE in GdNi1-xCox series in the temperature range between 70 and 115 K that can be controlled by varying x(Co) between 0 and 0.15. The compounds undergo second-order magneto-elastic transformations where the transitions from paramagnetic to ferromagnetic states are coupled with anisotropic changes in lattice parameters without significant changes in phase volume.3,4 Lattice symmetry also remains unperturbed across the transitions.3,4 While the transition temperatures increase nearly linearly with x(Co), doping with Co slightly reduces the maximum magnetic entropy changes: from -14 J/ Kg K at 71 K when x(Co) = 0 to -10 J/Kg K at 115 K when x(Co) = 0.15 (all values are quoted for a 50 kOe magnetic field change). The calculated temperature-averaged-entropy changes (TECs), one of the important figures of merit for assessing the potential of magnetic refrigerants,5 are comparable with the best known potential magnetocaloric materials exhibiting second-order phase transition. Our detailed analysis of the magnetic field dependence of the magnetic entropy changes along with critical exponent analysis explore the role of magneto-elastic coupling on the magnetocaloric properties of GdNi1-xCox alloys. Acknowledgement: This work was performed at Ames Laboratory and was supported by the Division of Materials Science and Engineering of the Office of Basic Energy Sciences of the U.S. Department of Energy (DOE). Ames Laboratory is operated for the U.S. DOE by Iowa State University under Contract No. DE-AC02-07CH11358.

Large cryogenic magnetocaloric effect in GdNi1-xCox (0 ≤ x ≤ 0.15)

Long-range ferromagnetic order in the single-layer limit appears to be possible in several chromium-based chalcogenides and halides with layered structure. Ab initio calculations based on the density functional theory of different Cr-based chalcogenides are analyzed approaching the twodimensional limit. The focus will be on magneto-crystalline anisotropy and how to obtain an out-of-plane magnetic polarization that would help circumvent the restrictions of the Mermin-Wagner theorem. This will be studied through various means like strain, pressure, the stability of charge density waves (CDW)[1], stoichiometry and other different structural details. The electronic structure of these compounds will be discussed as an approach to the understanding of the magnetic properties of the transition metal chalcogenide family in general.

Two-dimensional Magnetic Order in Cr-based Transition Metal Chalcogenides

Spin-orbit torque enables electrical control of the magnetic state of ferromagnets or antiferromagnets. The quintessential system exhibiting spin-orbit torque is a bilayer composed of a heavy metal and a ferromagnet. The heterostructure bilayer geometry breaks the inversion symmetry and allows the spin-orbit torque arising from the spin current flow from the heavy metal layer to the ferromagnetic layer. In this work, we consider the intrinsic spin-orbit torque generated in inversion symmetry breaking magnetic materials directly without stacking with other materials. In the first example of ferromagnetic monolayer Fe3GeTe2, we calculate the intrinsic spin-orbit torque which arises due to the global inversion symmetry breaking. In addition, spin-orbit torque in Fe3GeTe2 has the unique property that the lowest order contribution is fieldlike and time-reversal even because of the C3z rotation symmetry. This is in contrast to the conventional case in which the lowest order is time-reversal odd and fieldlike and the next lowest-order is time-reversal even and dampinglike. We next show that the next lowest-order spin-orbit torque in Fe3GeTe2 is time-reversal odd and dampinglike which can play a more important role in switching the magnetization in the ensuring dynamics, as described by the Landau-Lifshitz-Gilbert equation. In the second example of antiferromagnetic bilayer CrI3, we find staggered dampinglike (time-reversal even) torque and uniform fieldlike (time-reversal odd) torque due to local inversion symmetry breaking and global inversion plus time-reversal symmetry. This is similar to the previous studies of CuMnAs and Mn2Au, where the Neel order switching is demonstrated using uniform fieldlike torque. However, CrI3 has a unique property in that the anisotropy energies and exchange energies are similar in magnitude. This leads to qualitatively different results in the switching dynamics: the staggered dampinglike torque is the dominant switching mechanism. The primary role of staggered dampinglike torque applies to other antiferromagnets in which the product of Gilbert damping and exchange field is smaller than the anisotropy field.

Intrinsic spin-orbit torque in van der Waals magnetic materials

Due to the long spin diffusion length and the high mobility, monolayer graphene is an intriguing material element for spintronic applications [1]. For example, if a monolayer graphene can be magnetized, Dirac electrons and holes will carry opposite spins, leading to a good spin filter or a convenient generator of pure spin currents. However, a prestine monolayer graphene does not have net magnetization. Previous experiments have demonstrated that when a monolayer graphene is transferred to the surface of a magnet, the p orbitals on the carbon atoms can overlap with the magnetic atoms in the adjacent layer, leading to an induced magnetization in the graphene layer [2]. This is known as the proximity effect, and has been reported in different types of ferromagnetic heterostructures. Compared to ferromagnets, antiferromagents are better candidates to magnetize graphene. Due to the zero net magnetization, an antiferromagnet does not respond to external magnetic noise, and therefore devices using antiferromagnetic heterostructures have good scalbility and stability. Also, antiferromagents can switch almost three orders of magnitude faster than conventional ferromagnets, which may lead to high-speed information processing. Here, we show that a monolayer graphene can indeed be magnetized through the proximity effect provided by a thin film of hexagonal chromium selenide (Fig. 1), a semiconducting antiferromagnet [3]. The induced spin splitting energy is experimentally measured to be 134meV at 2K. Such exchange splitting is experimentally confirmed by the shift of quantum oscillations in the graphene layer. The magnitude of the splitting energy is obtained by its modulation through field cooling, with the exchange splitting energy increasing driven by positive cooling fields and decreasing by negative cooling fields (Fig. 2). The magnitude of the exchange coupling and the Laudau-level indices are confirmed by a machine-learning search for the best fit in a large parameter space. Such spin splitting is also supported by polarized neutron reflectometry and magneto-optic Kerr measurements. Acknowledgements: The transport measurement and theoretical modelling in this work were supported by Spins and Heat in Nanoscale Electronic Systems (SHINES), an Energy Frontier Research Center funded by the US Department of Energy (DOE), Office of Science, Basic Energy Sciences (BES) under award #SC0012670. We are also grateful for support from the National Science Foundation (NSF) (DMR-1411085 and DMR-1810163) and the ARO programme (contract no. W911NF-15-1-10561). Research was performed in part at the NIST Center for Nanoscale Science and Technology. This work used the Extreme Science and Engineering Discovery Environment (XSEDE), which is supported by NSF grant no. OCI-1053575. Specifically, it used the Bridges system (supported by NSF award no.ACI-1445606) at the Pittsburgh Supercomputing Center (PSC). Certain commercial equipment, instruments or materials are identified in this paper to foster understanding. Such identification does not imply recommendation or endorsement by the National Institute of Standards and Technology, nor does it imply that the materials or equipment identified are necessarily the best available for the purpose.

![](https://assets.underline.io/uploads/markdown_image/1/image/17681d6778d9ccff54b5964943012fc1.jpg)

Fig. 1. The schematic of the graphene/CrSe heterostructure. Hexagonal CrSe has a layered AFM structure. Each layer contains a non-colinear spin texture with a non-zero magnetization along z. Adjacent layers cancel the moments of each other, resulting in a net antiferromagnet.

![](https://assets.underline.io/uploads/markdown_image/1/image/d699ad42c2ba992a04094c15c97d5021.jpg)

Fig. 2. Quantum oscillations are shifted by field cooling in the heterostructure. The dips corresponding to each Landau level are denoted by dashed lines.

Large Exchange Splitting in Monolayer Graphene Magnetized by an Antiferromagnet

Topological insulators (TI) have gained much interest in the field of spintronics for the generation of pure spin currents. Indeed, three-dimensional TIs are predicted to host exotic properties like topologically protected surface states (TSS), which show Dirac-like band dispersion and spin-momentum locking [1]. One of the main strategies is to take advantages of the spin polarization of the TSSs to manipulate the magnetization of an adjacent ferromagnetic thin film (FM) using the spin-orbit torque (SOT) mechanism [2]. In the past few years, the community attempted to replace the traditional heavy metals by a TI in order to enhance the SOT efficiency with limited success. It now appears that the interface sharpness and the high chemical affinity between Bi-based TIs and classical 3d FMs is a major hurdle to reach the predicted breakthrough in magnetization switching power-efficiency [3]. The emergence of ferromagnetism in two dimensions in 2017, which started a new field in condensed matter physics, could bring a solution to this issue. The van der Waals (vdW) nature of the interaction between the TI and the 2D-FM should limit chemical reactions, interface intermixing and hybridization of state between the two layers. Our approach focuses on stacking and growing 2D-FM (Fe3GeTe2) onto high-quality compensated TIs ((Bi0.4Sb0.6 )2Te3) grown by molecular beam epitaxy. We use both low and high field second harmonic magnetotransport measurement to characterize the SOT generation, finding a strong temperature dependence of the harmonic signals. We also reports the SOT current-induced magnetization switching operation at low critical current density (Jc &lt; 1010 A/m2) in the vdW heterostructure.

![](https://assets.underline.io/uploads/markdown_image/1/image/276785c92c28212f866028b086fcb79c.jpg)

Fig.1: Second harmonic spin-orbit torque measurement in BST/FGT bilayers. a) Measurement geometry. b) Anomalous Hall effect (AHE) recorded by applying the magnetic field along z. c) AHE recorded by sweeping the field along y. d) Damping-like torque second harmonic measurements. e) Field-like torque second harmonic measurements. The sample temperature is 70 K and the applied current density is 1.3 × 1010 A.m-2.

Spin-orbit torques in topological insulator - two-dimensional ferromagnet heterostructures

Iron selenide telluride, FeTeSe is a family of Fe-chalcogenide and Fe-based superconductor. This material is a compound material and possesses an in-plane symmetry breaking. From spintronic viewpoints, FeTeSe allows Ising-type superconductivity, which enables the locking of spin in out-of-plane directions. In addition, FeTeSe can be a great candidate for topological superconductor which can arise the S=1 spin-triplet Cooper pairs at the surface. To detect possible perpendicular spin-triplet states in FeTeSe electrically, ferromagnetic electrodes with perpendicular magnetic anisotropy (PMA) is necessary. However, an interplay of ferromagnetism of the electrodes and superconductivity of FeTeSe has not been sufficiently explored. We prepared exfoliated thin FeTe0.6Se0.4 (FTS) flakes and deposited PMA electrodes on the FTS, which is made of [Pt/Co]10 multilayer (“10” denotes a stacking number). The conventional four-terminal measurement was implemented as the in-plane and perpendicular external magnetic field (Hext) was applied up to 9 T. The superconducting transition is observed at T = 15 K. More importantly, the upper critical field of the FTS is anisotropic and the in-plane upper critical field is measured to be 145 T [1]. Y. Saito et al. reported the enhancement of the in-plane upper critical field of a member of Ising-type superconductivity, MoS2 [2]. Therefore, these results show that the superconductivity of exfoliated thin film FTS is robust and FTS is a potential material for Ising-type superconductor, which can open a new pathway to super-spintronics. More detailed discussion will be given in the presentation.

![](https://assets.underline.io/uploads/markdown_image/1/image/67764886b292d9077025b348fa1b86b9.jpg)

Fig. 1: An optical microscopy image of the mechanically exfoliated FTS single crystal.

![](https://assets.underline.io/uploads/markdown_image/1/image/de178bfdb99000c880b2a6f5e358f073.jpg)

Fig. 2: Temperature dependence of the normalized resistance of the FTS with PMA electrodes, when in-plane magnetic field is applied. The normalization was carried out using the normal-state resistance R0 at 16 K.

Observation of a superconducting state of possible Ising-type topological superconductor FeTe0.6Se0.4, equipping ferromagnetic electrodes with perpendicular magnetic anisotropy

The interfacial Dzyaloshinskii–Moriya interaction (DMI) emerges in ferromagnet/heavy metal thin films with broken z-mirror symmetry, stabilizing Néel domain walls and skyrmions of one chirality as dictated by its Rashba-type field. On the other hand, both Bloch chiralities remain energetically degenerate at static equilibrium for mixed domain walls in films with a moderate DMI. Multiple research groups have nevertheless independently reported significant Bloch chirality asymmetries in the Bz-stabilized domain microstructures of different interfacial DMI systems [1, 2]. In this work, we demonstrate how the asymmetric wall energy landscape in such systems can produce a deterministic Bloch chirality when domain walls are driven away from static equilibrium. For D &lt; Dc, the two ground state domain walls—having Bloch components of opposite chiralities—exhibit different restoring torques and steady state configurations. As dictated by the driving field’s direction and sign of DMI, one wall, which we term the lax wall, becomes labile at a lower Bz prior to the Walker field. At this lability point, the wall lies at an inflection point in its energy landscape and the domain wall susceptibility (χdw ∝ (σΦΦ)-1) diverges. The lax wall therefore reorients at higher fields to the steady state configuration of its counterpart, which we call the locked wall (Fig. 1). Using the Slonczewski equations and the 1D wall energy model, we analytically derived a DMI–Bz phase diagram of steady state domain wall Bloch chirality (Fig. 2), which includes an astroid analogous to the Stoner–Wohlfarth domain switching model. While these equations predict that locked walls precess beyond the Walker field, it was observed that they can instead persist due to DMI-induced asymmetries in vertical Bloch line (VBL) evolution—extending the Bloch monochirality regimes [3]. Here, we generalize these Bloch chirality preferences to arbitrary VBL behavior in these systems, before discussing how the theory of horizontal Bloch line evolution could similarly mediate Bloch chirality preferences [4].

![](https://assets.underline.io/uploads/markdown_image/1/image/c78283a8de7f53f55949028b259b7207.jpg)

Steady state Bloch monochirality via DMI-induced asymmetric evolution of lax & locked walls.

![](https://assets.underline.io/uploads/markdown_image/1/image/e73fbc654808325ad8a75b63241d4af0.jpg)

DMI–Bz phase diagram of steady state domain wall Bloch chirality

Deterministic Bloch Chirality in Interfacial DMI Systems: An Astroid for DW Switching

Research into magnetic skyrmions and their spintronics applications has revealed complex chiral spin textures. However, direct characterizations of the structure of skyrmion and related spin textures pose an experimental challenge. To better understand their structure, we use Lorentz transmission electron microscopy (LTEM) to track their evolution from well-studied domain walls (DWs) into skyrmion and biskyrmion phases. Magnetic skyrmions were first predicted as a result of the Dzyaloshinskii-Moriya interaction (DMI) found in materials with broken inversion symmetry1. However, topologically equivalent chiral bubbles, or dipole skyrmions, can be stabilized in achiral thin films by competing long-range dipole and short-range DW energies2. Recently, a novel skyrmion phase consisting of bound skyrmion pairs with opposite helicities was reported in achiral Fe/Gd multilayers3 (Fig. 1). However, the primary magnetic imaging techniques for thin films are LTEM, which yields projected in-plane magnetic field, and resonant X-ray techniques, which yield projected out-of-plane magnetization. This complicates identification of complex magnetic spin textures, as multiple spin textures can yield qualitatively similar projected magnetic fields, and there has been subsequent debate over the structure of the reported skyrmion phases4,5. By tracking the field-dependent evolution of magnetic spin textures from the well-studied magnetic stripes to the novel skyrmion phases, we can identify key components of these spin textures. First, by applying an out-of-plane magnetic field with a small in-plane component, stripe DWs form, alternatingly aligned and anti-aligned with the in-plane component, matching previous results2. As the field is further increased, the stripe DWs split, and the aligned DWs form the center of the novel texture, while the anti-aligned form closure loops (Fig. 2), supporting that the magnetization is a pair of contour-rotating vortices. Fully understanding these textures will require knowledge of the 3D structure, so surface sensitive techniques like SEMPA, SPLEEM, and MFM will be vital moving forward.

![](https://assets.underline.io/uploads/markdown_image/1/image/26875a594ef774ade1f7585132522bfd.jpg)

Fig. 1. The projected in-plane field of the novel texture.

![](https://assets.underline.io/uploads/markdown_image/1/image/c339839e8a22aef9d6e717998b111ef8.jpg)

Fig. 2. Stripe domains breaking into the novel phase.

Evolution of Novel Chiral Spin Textures in Fe/Gd Based Multilayers

Anti-symmetric exchange, or the Dzyaloshinskii–Moriya interaction (DMI), is responsible for non-collinear spin textures such as canting, helices and skyrmions in bulk systems or films with asymmetric structure [1]. Analogously, thin films without DMI can have competing anisotropies which stabilise non-collinear magnetic textures. Using resonant soft x-ray scattering, we study the rigid stripe rotation in an Fe/Gd multilayer which accompanies the transition from the stripe phase to the six-fold dipole skyrmion lattice. The sample was measured in transmission and the small-angle diffraction gave the average period and in-plane orientation, Φ, of the perpendicular stripes. By changing the sample tilt, ω, we control the resultant field vector which changes the threshold, Honset, and abruptness of the observed rotation (Fig. 1a). We used micromagnetic simulation software MuMax3 [2] to model the system using similar parameters to previous work [3]. By carefully setting the initial state and polarity of the Bloch center of the helical domain wall we reproduce the rotational behaviour observed experimentally (Fig. 1a(inset), 1b). We show on inspection of the simulated 3D spin structure that as the magnetic field is increased the helical domain wall region tries to unwind before rotation and then reverts back to a more wound structure in the rotated state before transitioning into the skyrmion/bubble lattice and finally saturating.

![](https://assets.underline.io/uploads/markdown_image/1/image/1fa3e6fa70435914350d7a2a9c56d6db.jpg)

(a) The Φ angle of the diffracted peaks is plotted for different values of ω as a function of the modulus of applied field where solid symbols are Hz and open symbols are -Hz. Increasing Φ shows clockwise (CW) motion for ω<1 and counter clockwise (CCW) motion for ω>1 . (Inset) The experimental in-plane (IP) component of Honset along with the corresponding values from the simulations. (b) Micromagnetically simulated rotation of the stripe domains for combined IP and OOP magnetic field. H is indicated at the bottom and is in the z direction for all panels except the bottom row where it denotes an in-plane field along the x direction. Simulated area was 2x2 μm2, color wheel indicates direction of in-plane magnetisation and white and black are up and down perpendicular magnetisation, respectively.

Frustration of coupled helices in weakly perpendicular magnetic thin films

Magnetic racetrack memories offer the possibility of increasing the density and speed of data storage. Yet, achieving fast current-induced motion of domain walls (DWs) and skyrmions at room temperature remains a challenge. We study nearly compensated ferrimagnetic CoGd alloys inserted between two heavy metals for which we have already demonstrated the optimization of spin-orbit torques (SOTs)1 and interfacial Dzyaloshinskii-Moriya interactions2. Ferrimagnets are attractive materials for their low stray field and fast spin dynamics which has been theoretically predicted to enable formation of ultrasmall3 and fast skyrmions. However, so far, the reported skyrmion velocities4 are much lower than of DWs3. Here, we investigate the nucleation and dynamics of chiral magnetic textures induced by current pulses in Pt/CoGd(5 nm)/(W or Ta) films. First, we demonstrate fast motion of DWs driven by SOTs at room temperature. We observe a maximum DW velocity of about 460 m s-1 (Fig. 1). Hence, we confirm that our ferrimagnetic materials have a very low DW pinning and density of defects, which is ideal for fast current-induced motion of DWs and skyrmions. Most literature studies focus on materials with a skyrmionic ground state. Here, our CoGd films have a sharp magnetic hysteresis loop and skyrmions are metastable. In Fig. 2a, we show that we can achieve field free nucleation of skyrmion bubbles with a single 5-ns current pulse and with a density of 2.7 x1012 A m-2. In addition, sequential magneto-optical Kerr (MOKE) imaging (Fig. 2b-f) after applying a single 5-ns current pulse with a density of 1.5 x1012 A m-2 reveals fast skyrmion motion with a velocity of about 400 m s-1. A small out-of-plane magnetic field of -5.7 mT is applied to stabilize the skyrmion bubble. In this study, we demonstrate that our low pinning ferrimagnetic films enable one of the fastest skyrmion motion thus far reported5. Our results could pave the way for ultrafast racetrack memory operations. This research was supported by DARPA award No. D18AP00009.

![](https://assets.underline.io/uploads/markdown_image/1/image/675f52758a6a530350f50604ed06cb0b.jpg)

Average DW velocity driven by SOTs in Pt/CoGd/(W or Ta).

![](https://assets.underline.io/uploads/markdown_image/1/image/c0bb1144e41d0a2de9c390f652bdf62b.jpg)

Field free current-induced nucleation (a) and motion (b-f) of skyrmion bubbles by a single 5-ns current pulse.

Creation and Annihilation of Magnetic Skyrmions in Confined Geometry using Voltage Control of Magnetic Anisotropy

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