Bulletin of the American Physical Society
APS March Meeting 2022
Volume 67, Number 3
Monday–Friday, March 14–18, 2022; Chicago
Session W54: Transport and Dynamics of SkyrmionsFocus Recordings Available

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Sponsoring Units: GMAG Chair: Gang Qiu, UCLA Room: McCormick Place W476 
Thursday, March 17, 2022 3:00PM  3:12PM 
W54.00001: Fewnm tracking of magnetic vortex core trajectories near and inside defects Mahdi Mehrnia, Jesse A Berezovsky Nanoscale magnetic textures such as vortices and skyrmions have attracted attention for applications in memory, logic, or microwave sources. For these applications, the effect of defects on the magnetization dynamics is critical, which can cause pinning or deviations of the trajectory as predicted by the Thiele equation. We report on the experimental observation of magnetic vortex core motion inside and near defects. We use a 3D timeresolved Kerr microscope to track the trajectory of a magnetic vortex core with spatial resolution of several nanometers and nanosecond time resolution [1]. We position the vortex core within a thin permalloy disk using a static magnetic field and excite the vortex core dynamics using a fast magnetic pulse. In a region relatively free of defects, the trajectory of the vortex core is a circular spiral path, decaying to the equilibrium position. When the moving vortex core approaches a defect that causes significant vortex pinning, however, we observe that the vortex core continues moving but deviates from its circular path in response to the defect. Sufficiently close to a large defect, we also observe enhanced damping to a new pinned equilibrium within the defect. This technique provides a method for probing the motion of nanoscale spin textures as they interact with defects in the material either leading to pinning or altered dynamics. 
Thursday, March 17, 2022 3:12PM  3:24PM 
W54.00002: Relaxation of magnetic skyrmions driven in a channel Michel Pleimling, James Stidham Understanding the behavior of magnetic skyrmions on tracks is an important step in the pursuit of skyrmionbased devices. Many proposed devices confine magnetic skyrmions to narrow magnetic strips that are thin enough to be considered twodimensional. We investigated how driven magnetic skyrmions would relax when subjected to a drive after starting in an initially disordered state due to the strength of the Magnus force. We used a coarse grain particle model for magnetic skyrmions, to determine the relaxation behavior of skyrmions in a 2d channel with a constant and periodic drive. To do this, we looked at a covering factor to qualitatively understand the positioning of skyrmions in respect to their position along the height of the channel. We also looked at how the average distance between skyrmions is affected as a function of time. We show that even in this constrained geometry, the skyrmions order faster with increasing strength of the Magnus force. Conversely, when the skyrmions are subjected to a periodic drive after reaching the steady state, they relax faster with a decrease in the strength of the Magnus force. 
Thursday, March 17, 2022 3:24PM  3:36PM 
W54.00003: Local Ferromagnetic Resonance of Mesoscopic Structures Using Deterministically Placed Nanodiamonds Jeffrey G Rable Nitrogenvacancy (NV) centers in diamond offer a powerful method of measuring local magnetization dynamics in ferromagnetic materials. To date, measurements of ferromagnetic resonance (FMR) using NV centers have relied on a stochastic distribution of NV centers or their rough placement on a micron scale. Here, we demonstrate sub100 nm placement accuracy of NVcontaining nanodiamonds using an atomic force microscope and use this capability to perform optically detected FMR measurements on mesoscopicallypatterned permalloy structures. By probing FMR using nanodiamonds deterministically placed at different sites on individual nanoscale ferromagnetic structures, we detect localized differences in the FMR signal and compare these with results from micromagnetic simulations. The technique has potential application in the study of localized spin dynamics of individual magnetic nanostructures as well for probing the dynamics of individual domain walls. 
Thursday, March 17, 2022 3:36PM  4:12PM 
W54.00004: Topological Spin Textures in Three Dimensions Invited Speaker: Jiadong Zang Chiral magnets are a series of magnets with broken inversion symmetry. A new type of spin interaction therein, the DzyaloshinskiiMoriya interaction, stimulates the formation of many novel topological spin textures. One important example is the emergence of magnetic skyrmion, whose nontrivial topology enables unique dynamical property and thermal stability, and gives rise to promising applications in future spintronic devices. However, skyrmion is just a twodimensional texture. Its threedimensional (3D) counterparts are widely unexplored. In this talk, I will discuss realization of 3D skyrmionic textures, including target skyrmion^{1}, skyrmion bundle^{2}, and skyrmionic vortices^{3}. I will also discuss the magnetic hopfion, a rigorously defined 3D topological texture. We propose the presence of zerofield hopfion in synthetic chiral magnetic multilayers^{4}. All these 3D textures exhibit novel dynamical properties that can be formulated in terms of collective coordinates^{5}. These works could stimulate the development of 3D spintronics. 
Thursday, March 17, 2022 4:12PM  4:24PM 
W54.00005: Magnetization switching and spin oscillation dynamics in noncollinear antiferromagnets MyoungWoo Yoo, Virginia O Lorenz, David G Cahill, Axel Hoffmann Antiferromagnets (AFs) are promising candidates for nextgeneration spintronic devices, because of distinct benefits, such as fast magnetization dynamics and reduced crosstalk from magnetostatic interactions [1]. Towards this end, noncollinear AFs are particularly interesting for AF spintronics, since they exhibit a large anomalous Hall effect which can be used for detecting the magnetic order [2], which furthermore can be modulated electrically by the spinorbit torques [3]. However, the detailed spin dynamics in the noncollinear AFs have not been satisfactorily explored. Here, we theoretically investigate spin dynamics in the noncollinear AFs, Mn3Sn. We prepare a minimal atomisticspin model on a twodimensional kagome lattice. From the model, we numerically calculate the switching dynamics among six degenerate stable configurations and compute AF resonance oscillations and the frequency. From the result, we find that different responses can be expected from inplane and outofplane measurements, because of phase differences among the spin precessions. This work provides further insights required for noncollinear AF spintronics. 
Thursday, March 17, 2022 4:24PM  4:36PM 
W54.00006: Imaging Neel Vector Switching Using Nitrogen Vacancy Centers in Diamond Alex Melendez, Shekhar Das, Justin Michel, Inhee Lee, Fengyuan Yang, P Chris Hammel Control of the orientation of the antiferromagnetic order parameter in systems with multiple easyaxes is provides a promising route to high frequency information processing. NV relaxometry arising from the resulting terahertz magnetic dynamics could provide diagnostic imaging. Nitrogen vacancy (NV) centers are sensitive, local and noninvasive probes of magnetic dynamics but have until recently been limited to detection of dynamics at or below the NV resonant frequency (~3 GHz). It has recently been shown that two magnons in ferromagnetic and antiferromagnetic systems may scatter emitting electromagnetic noise at their difference frequency. Thus THz frequency dynamics associated antiferromagnetic magnons, which would otherwise be undetectable using an NV, can be detected when this difference frequency is resonant with the NV precession. Switching the Neel vector from one easyaxis to another can be achieved by means of currentinduced spinorbit torque. 
Thursday, March 17, 2022 4:36PM  4:48PM 
W54.00007: Fielddriven resonances of magnetic Hopfions David Raftrey, Peter J Fischer Here, we report on micromagnetic simulations of fast dynamics in three dimensional spin textures. We focus on the resonant spin wave modes of magnetic Hopfions up to 15 GHz driven by external magnetic fields. A sharp transition is found around 66 mT coinciding with a transition from Hopfions to torons. The modes exhibit characteristic amplitudes in frequency space accompanied by unique localization patterns in real space, and are found to be robust to damping around topological features, particularly vortex lines in Hopfions and Bloch points in torons. Most remarkably, we have identified pronounced differences in spin wave spectra between Hopfions, torons and target skyrmions that can serve as fingerprints in future experimental validation studies of these novel 3d topological spin textures. 
Thursday, March 17, 2022 4:48PM  5:00PM 
W54.00008: Electrical generation and deletion of magnetic skyrmions via vertical current injection Chanyong Hwang, Seungmo Yang, KyoungWoong Moon, TaeSeong Ju Due to its distinct topological features, magnetic skyrmions have recently been considered to be a novel information carrier in spintronics[1, 2]. The electrical creation/deletion of a magnetic skyrmion is ecessential in realizing skyrmionbased spintronics devices. Therefore, various experimental demonstrations of skyrmion creation/deletion have been reported using diverse methods [3, 4]. However, most of them have been created near a randomly created defect site, disturbing the skyrmion motion after the creation. Here, we introduce a new method for electrical creating, deleting, and shifting magnetic skyrmions by adopting a vertical current injection. We will show proofofconcept demonstration of skyrmion reacetrack memory. 
Thursday, March 17, 2022 5:00PM  5:12PM 
W54.00009: Orbital angular momentum of a domain wall and geometrically twisted magnons Seungho Lee, Se Kwon Kim We theoretically study the dynamics of a domain wall in a ferromagnetic nanotube driven by electrons and magnons by investigating the electrondomainwall and magnondomain wall interactions. Due to the geometry of the sample, domain walls are classified by the Skyrmion charge which counts the winding number of magnetic textures on the domain wall [1]. The domain wall with a nonzero Skyrmion charge generates an emergent magnetic field for interacting particles, which exerts the Lorentz force on moving electrons and magnons and thereby deflects their trajectories. This deflection is manifested as the generation of the finite orbital angular momenta of the electrons and magnons that traverse the domain wall [2,3]. For the case of electrons, we can interpret the exchange of orbital angular momenta between electrons and the domain wall as a currentinduced torque. From this peculiar torque, the domain wall with the nonzero Skyrmion charge can be driven by an arbitrary small current without the Walker breakdown. For the case of magnons, we obtain the exact solution for the magnon on the Skyrmiontextured domain wall and also their scattering properties with the domain wall with the aid of supersymmetric quantum mechanics (SUSY QM) [4]. We also show that there is a critical wavenumber for the total reflection and it is discretized by the Skyrmion charge of the domain wall. Our results show that the orbital angular momenta of magnetic systems and quasiparticles can be intertwined in a curved geometry. 
Thursday, March 17, 2022 5:12PM  5:24PM 
W54.00010: Triplon Spin Current Masahiro Sato, Yao Chen, Yifei Tang, Yuki Shiomi, Koichi Oyanagi, Takatsugu Masuda, Yusuke Nambu, Masaki Fujita, Eiji Saitoh Spin current is a central concept of modern spintronics. Spin Seebeck effect (SSE), i.e., the spin current generation driven by a thermal gradient is one of the representative methods of creating DC spin current. SSE has been investigated from the viewpoint of its potential applications such as temperature sensor. Therefore, so far famous functional magnets such as ferro or ferrimagnets have been the main targets in SSE experiments. However, in recent years, it is gradually revealed that SSE can be an excellent tool to observe fundamental magnetic natures of attractive magnets. Namely, SSE is very powerful to detect dynamical features of various magnets including spin liquids and frustrated magnets. In fact, using SSE, we have uncovered several new features of different magnetic states such as a 1D spin liquid and a spinnematic liquid. In this meeting, I will report our recent work about the SSE in the dimerized spinPeierls phase of a quantum spin chain magnet CuGeO_{3}, whose elementary excitations are given by spin1 triplons. We observed a new spin current carried by triplon excitations for the first time and developed the microscopic theory of the triplon spin current. I will explain the essential aspects of the triplon spin current. 
Thursday, March 17, 2022 5:24PM  6:00PM 
W54.00011: Emergence of spinorbit coupled ferromagnetic surface state derived from Zak phase in a nonmagnetic insulator FeSi Invited Speaker: Naoya Kanazawa A chiral compound FeSi is a prototypical example of the stronglycorrelated delectron insulators. Its peculiar charge and spin dynamics have provoked many important physical concepts such as delectron Kondo insulator and insulatortometal transition due to onsite strong Coulomb interaction. Triggered by recent new insights into topological aspects of correlated insulators, FeSi is attracting renewed attention. However, it remains highly nontrivial that compounds made of light (and usually common) elements, like FeSi, can bear topological characteristics. 
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