Bulletin of the American Physical Society
APS March Meeting 2022
Volume 67, Number 3
Monday–Friday, March 14–18, 2022; Chicago
Session N54: Stability, Lifetime, and Nucleation of SkyrmionsFocus Recordings Available

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Sponsoring Units: GMAG Chair: Claudia Mewes, University of Alabama Room: McCormick Place W476 
Wednesday, March 16, 2022 11:30AM  12:06PM 
N54.00001: Direct Demonstration of Topological Stability of Magnetic SkyrmionsviaTopology Manipulation Invited Speaker: SoongGeun Je The topological concept, initially introduced in particle physics, brought new excitements to many other fields of physics, such as topological insulators, ultracold atoms, and topological lasers to unveil various robust states therein. In nanomagnetism, the most prominent example governed by topology is the magnetic skyrmion. Since magnetic skyrmions exhibit a nontrivial topological property, it is expected that skyrmions are robust against external perturbations. Therefore, it has sparked considerable interest due to the academic interest and the prospect of employing the skyrmions as the nanometerscale nonvolatile information carriers. Although profound the concept is, the realistic situation that magnetic moments are localized in a discrete atomic lattice raises an immediate question of whether the topological concept is still viable in the real system. Despite the fundamental importance, the thorough assessment has remained elusive due to the challenge of controlling topology and thereby directly comparing the topologically nontrivial skyrmions and trivial bubbles in a single specimen. 
Wednesday, March 16, 2022 12:06PM  12:18PM 
N54.00002: An abinitio paradigm for exploring the stability of skyrmions Swarnava Ghosh, Markus Eisenbach Magnetic skyrmions are topological defects in the magnetization texture. The typical sizes of skyrmions are small with diameters in the nanometer range. They behave as particles that can be created, moved, and annihilated. Topological properties strongly influence the stability and dynamics of skyrmions. Long living stable skyrmions are potential candidates for spintronic materials. Skyrmions are stabilized by Dzyaloshinskii–Moriya or asymmetric exchange interactions. Certain class of deformations and disorder influence the Dzyaloshinskii–Moriya interactions and hence the stability and dynamics of skyrmions. In this talk, we will present a multiscale paradigm to simulate skyrmions from firstprinciples calculations. We will talk about the role of disorder and lattice strains on the stability and dynamics of skyrmions. 
Wednesday, March 16, 2022 12:18PM  12:30PM 
N54.00003: Dynamics of skyrmions driven by spin transfer and spin orbit torques Jacob B Freyermuth, PoKuan Wu, Denis Pelekhov, Roland K Kawakami, Mohit Randeria Skyrmions are topological magnetic textures with great potential for technological applications in magnetic devices. For efficient manipulation of skyrmions, it is important to understand how the motion of skyrmions can be driven by a current. We report on the motion of skyrmions in narrow channels in two different classes of materials: metallic materials with a spin transfer torque drive and ferromagnetic insulatorheavy metal bilayers with spin orbit torque from the spin Hall effect. Using numerical simulations and analytical approaches, we compare the two cases to elucidate the fastest possible speed that the skyrmions can achieve before becoming unstable. 
Wednesday, March 16, 2022 12:30PM  12:42PM 
N54.00004: Dynamical Generation of Interactions and Floquet Skyrmions in Periodically Driven Spin Systems Abhishek Kumar, Babak Seradjeh, Philip Richerme, Gerardo Ortiz, William Holdhusen Motivated by applications in synthetic quantum simulators, we study manybody Floquet spin systems with periodically driven twobody interactions and external fields in the highfrequency limit. We show that a hierarchy of multispin interactions can be generated in powers of inversefrequency, such as Dzyaloshinskii–Moriya interaction (DMI), three and fourspin interactions, and additional Zeeman terms. We consider different types of drives and lattice geometries in one and two dimensions to explore possible types of dynamically generated interactions. In particular, we focus on the DMI term in twodimensional square and triangular lattices to study the dynamical generation of quantum skyrmions, controlled by drive parameters. Using exact diagonalization and hierarchical meanfield theory calculations, we characterize the energetic and topological signatures of possible skyrmion phases. This study is relevant to the experimental realization of Floquet skyrmion, for example, in recently demonstrated twodimensional lattices of trapped ions. 
Wednesday, March 16, 2022 12:42PM  12:54PM 
N54.00005: Skyrmion lattice formation and destruction mechanisms probed with SANS Namila C Liyanage, Nan Tang, Lizabeth J Quigley, GuoJiun Shu, FangCheng Chou, Nicholas Butch, Markus Bleuel, Julie A Borchers, Lisa M DeBeerSchmitt, Dustin A Gilbert A magnetic Skyrmions is a local whirl of the spin configuration in a magnetic material. These structures have promising applications in spintronic devices [1] due to their topological protection, which prevents them from being continuously created or destroyed. The formation and destruction time scales of skyrmions are important when it comes to applications. Typically, magnetic dynamics occur on the nanosecond (109 s) timeframe, however, the time scale of skyrmion formation may be much longer. In this work, we have measured the dynamics of skyrmion lattice formation in B20 materials MnSi, FeCoSi, Cu2OSeO3 using smallangle neutron scattering. The formation rate of skyrmions was a surprisingly slow 10's of ms. Using a stepped magnetic field, the material is moved into/out of the skyrmion stability window. Using timedomain slicing of the SANS data, the dynamics formation and destruction of the skyrmion lattice is captured in realtime. These results reveal that the formation/destruction dynamics occur between 2740 ms. 
Wednesday, March 16, 2022 12:54PM  1:06PM 
N54.00006: Computational Design of LongLifetime RoomTemperature Skyrmions Chung T Ma, Markus Hoffmann, Stefan Blügel, S. Joseph Poon Magnetic skyrmions are spin textures that can serve as the key components in future logic devices. To retain information, longterm stability of the skyrmions at room temperature is necessary. Atomistic simulations have been developed to help identify the desirable skyrmionic materials [13]. We calculate the lifetimes of skyrmions at room temperature. To evaluate the activation energy for a collapsing skyrmion, the geodesic nudged elastic band (GNEB) method is employed. Additionally, the lifetime prefactor is obtained by using the harmonic approximation to the transitionstate theory (HTST). Using these methods, skyrmions, with at least a yearlong lifetime, are predicted in a 10 nm thick amorphous ferrimagnetic CoGd film. Additionally, we investigate the mechanism of skyrmion collapse along different reaction paths in the energy landscape. Furthermore, using machine learning, we explore several tunable parameters, including thickness, saturation magnetization, and DzyaloshinskiiMoriya interactions (DMI), to optimize a heterostructure to host longlife roomtemperature skyrmions. 
Wednesday, March 16, 2022 1:06PM  1:18PM 
N54.00007: Lifetime of metastable skyrmions in thin films of chiral magnets PoKuan Wu, Mohit Randeria, James R Rowland Studying the lifetime of a metastable skyrmion in chiral magnetic materials is of fundamental interest and also of great importance for applications. We use a continuum field theory with exchange J, interfacial DzyaloshinskiiMoriya interaction D, anisotropy K, and external field H, and compute the free energy barrier for the destruction for a skyrmion, using harmonic transition state theory (HTST) to take into account entropic effects. We show that the nature of the saddle point configuration changes from a Bloch point singularity in the 2D limit to a chiral bobber beyond a certain thickness in thin films. We discuss how the size and lifetime of metastable skyrmions vary across the (KJ/D^{2}, HJ/D^{2}) phase diagram for the case of films with perpendicular magnetic anisotropy. 
Wednesday, March 16, 2022 1:18PM  1:30PM 
N54.00008: Mechanism for ultrafast electricfield driven skyrmion nucleation Louise Desplat We show how, at ultrashort timescales, a DzyaloshinskiiMoriya interaction can be generated in metallic thin films by an electromagnetic pulse. This interaction originates from the spinorbit coupling between the pulseinduced electric field and the spins of the delocalized electrons of the material [1]. We perform density functional theory calculations to estimate the strength of this fieldinduced interaction in centrosymmetric Fe, Co, Ni, and Mn monolayers, as well as FePt and MnPt alloys. Last, using atomistic simulations, we demonstrate how an isolated antiferromagnetic skyrmion can be coherently nucleated from the collinear background by an electric field pulse at the 100fs timescale [2]. These results provide a new handle for an ultrafast, coherent control of noncollinear magnetic states. 
Wednesday, March 16, 2022 1:30PM  1:42PM 
N54.00009: Visually informed guesses of ChainOfStates methods to find saddle points of the energy landscape in chiral magnetic nanodisks. Gabriel D ChavesO'Flynn, Daniel L Stein, Piotr Kuswik, Katarzyna Kotus Magnetic systems with an interfaciallyinduced DzyaloshinskiiMoriya Interaction (DMI) are currently the focus of active research as candidates for devices in information technologies [16]. A crucial figure of merit is the height of the minimum energy barrier that separates adjacent minima, since it determines stability against thermal fluctuations. It is difficult to determine analytically the transition states for thermal activated switching. This difficulty is exacerbated in the presence of DMI which roughens the energy landscape and introduces many minima. Numerically, a family of numerical algorithms relies on the evolution of chainsofstates to identify transition states [79]. These methods use initial guesses for transition paths that evolve downwards in energy until convergence is achieved. We highlight the importance of considering visual features of energy minima in the initial guesses for chainofstates. Two advantages of our improved guesses are a dramatic reduction of relaxation steps, and avoidance of topological defects that correspond to singularities of the energy landscape. 
Wednesday, March 16, 2022 1:42PM  1:54PM 
N54.00010: Skyrmion nucleation on a surface of topological insulators Daichi Kurebayashi, Oleg A Tretiakov Skyrmion nucleation induced by spintransfer torques at an interface of a topological insulator and a ferromagnetic insulator is investigated. Due to strong spinorbit coupling on a surface of topological insulators, which enhances the effect of spin torques, efficient manipulation of skyrmions is expected and therefore, topological insulators could provide the ideal platform to achieve highperformance skyrmionic devices. Using micromagnetic simulations and energetics, we evaluate properties of the skyrmion nucleation on a surface of topological insulators, such as nucleation time, critical electric field, and skyrmion numbers. We show that the nucleation time is inversely proportional to the applied electric field, while the skyrmion number linearly increases with the field in the vicinity of the critical field. We also identify the Gilbert damping and temperature dependences of the critical field. Furthermore, we analytically evaluate the effect of the DzyaloshinskiiMoriya interaction and demonstrate that the temperature dependence can be explained by the reduction of a magnon excitation gap due to the selfenergy corrections. 
Wednesday, March 16, 2022 1:54PM  2:30PM 
N54.00011: Optimizing skyrmion properties for technological applications: an atomistic perspective Invited Speaker: Markus Hoffmann Chiral magnetic skyrmions are not only of great scientific interest but also of potential relevance in information technology, data storage, and neuromorphic computing. Their desirable properties – such as their lifetime, mobility, and robustness with respect to external influences – depend hereby on the specific technological application. Therefore, the skyrmion properties need to be tuned accordingly to be able to compete with existing technologies or to even outperform those. 
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