Bulletin of the American Physical Society
APS March Meeting 2023
Volume 68, Number 3
Las Vegas, Nevada (March 5-10)
Virtual (March 20-22); Time Zone: Pacific Time
Session M44: Topological Magnetic TexturesFocus
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Sponsoring Units: DMP Chair: Hong Li, Boston College Room: Room 316 |
Wednesday, March 8, 2023 8:00AM - 8:36AM |
M44.00001: Ab initio magnetic structure prediction for topological magnets Invited Speaker: Ryotaro Arita To theoretically design functional magnets having a non-trivial spin configuration, we need a reliable method for magnetic structure prediction. While the numerical cost for structure prediction is generally expensive, we recently formulated an efficient scheme based on the cluster multipole theory [1]. |
Wednesday, March 8, 2023 8:36AM - 9:12AM |
M44.00002: Single skyrmion true random number generator using local dynamics and interaction between skyrmions Invited Speaker: Gang Xiao Magnetic skyrmions are topologically protected quasi-particles that can exist in certain magnetic materials. They are of great interest to both fundamental research and technological applications. Of particular interest is their potential for applications in logic-in-memory and unconventional computing devices. Magnetic skyrmions exhibit many desirable properties including stability, small size, and highly efficient controllability, which make them effective carriers in racetrack memory devices. In this talk, we show that the local dynamics of skyrmions, in contrast to the global dynamics of a skyrmion, can be introduced to provide effective functionalities for versatile computing. Our physical system is a magnetic multilayer with perpendicular magnetic anisotropy, substrate/Ta(1.6 nm)/Co40Fe40B20(CoFeB) (0.95 nm)/MgO(1.6 nm)/TaOx(2.0 nm). We can generate a single or multiple skyrmion(s). A single skyrmion interacting with local pinning centers under thermal effects can fluctuate in time and switch between a small-skyrmion (S) and a large-skyrmion (L) state, thereby serving as a robust true random number generator (TRNG) for probabilistic computing. The skyrmion mimics a robust stochastic neuron. We demonstrate that the probability of a skyrmion being in the S state or in the L state can be tuned by an applied magnetic field and/or spin current due to the effects of the Zeeman-energy variation and spin-orbit torque. Furthermore, we also discovered that neighboring skyrmions exhibit an anti-correlated coupling in their fluctuation dynamics. Both the switching probability and the dynamic coupling strength can be tuned by modifying the applied magnetic field and spin current. All these attributes offer opportunities to implement skyrmionic devices based on either a single skyrmion or a skyrmion network with a high degree of tunability and controllability. With a better understanding of the local dynamics of skyrmions, versatile skyrmionic devices can be designed and realized experimentally. |
Wednesday, March 8, 2023 9:12AM - 9:24AM |
M44.00003: Manipulation of the Topological Hall Effect with Electric Field in Top-Gated (Bi,Sb)2Te3 on EuIG Jyun-Fong Wong, Ko-Hsuan Chen, Jui-Min Chia, Zih-Ping Huang, Sheng-Xin Wang, Pei-Tze Chen, Lawrence Boyu Young, Yen-Hsun Glen Lin, Shang-Fan Lee, Chung-Yu Mou, Minghwei Hong, Jueinai Kwo Topological Hall effect (THE), a new member of the “Hall” family, is an electrical transport feature of systems hosting chiral spin textures like skyrmions. Recently, THE has been observed in topological insulator (TI) heterostructures possessing a strong spin-orbit coupling and a broken spatial inversion symmetry when interfaced with a magnetic layer. However, the electrical manipulation of THE has rarely been addressed. In this work, we report a giant THE signal exceeding 3.1 µΩ·cm coexisted with AHE (3.9 µΩ·cm) at 2 K in a 4 nm TI (Bi,Sb)2Te3 (BST) heterostructure with a ferrimagnetic insulator europium iron garnet. Dependences of temperature, magnetic field angle, and gate bias were investigated in a four-terminal Hall bar device implemented with a top electrical gate. The THE diminished with increasing temperatures and disappeared above 75 K. Furthermore, the observation of THE at zero fields suggested a stable skyrmion phase without the support of an external magnetic field. The magnitude of THE remained nearly the same up to 70° as the magnetic field gradually tilted away from the surface normal (0°) to the in-plane direction (90°). Most importantly, the sign of THE was switched from negative to positive as carriers in BST were altered from electrons to holes via the gate bias, consistent with the prediction of a skyrmion-driven THE. Our demonstration of electrical manipulation of THE in TI-based heterostructures has important technological implications for ultralow power skyrmion-based spintronics. |
Wednesday, March 8, 2023 9:24AM - 9:36AM |
M44.00004: Topological spin textures stabilised by Weyl fermions Juba Bouaziz, Gustav Bihlmayer, Julie Staunton, Stefan Blügel Rare-earth intermetallic (REI) constitute a playground for the realization of topological spin textures |
Wednesday, March 8, 2023 9:36AM - 9:48AM |
M44.00005: Dynamical switching of magentic topology in microwave-driven itinerant magnet Rintaro Eto, Masahito Mochizuki Magnetic skyrmion crystals (SkXs) are magnetic textures characterized by a topological invariant called skyrmion number Nsk, which are often stabilized by the Dzyaloshinskii-Moriya interactions (DMI) in chiral magnets. Recently, SkXs have been discovered successively in centrosymmetric itinerant magnets without active DMI as well, and they are considered to be stabilized by a different mechanism, i.e., conduction-electron-mediated spin-spin interactions. Because the DMI is absent, the SkXs in such magnets have several unfrozen degrees of freedom associated with the swirling spin configurations, e.g., vorticity and helicity, which provide us with a unique opportunity to switch the magnetic topology by external stimuli. In this work, we theoretically demonstrate that the magnetic topology of SkX in Kondo-lattice magnets can be switched among Nsk=2, 1, and 0 by irradiating with circularly polarized microwave field via intensely exciting a resonance mode [1]. This topology switching exhibits various behaviors possibly due to different dynamical landscape of energies, that is, deterministic irreversible switching, probabilistic irreversible switching, and temporally random fluctuation. |
Wednesday, March 8, 2023 9:48AM - 10:00AM |
M44.00006: Fabrication and Magnetism of Thin Film Copper Selenate David King, Jinke Tang, John Ackerman The chiral magnet Cu2OSeO3 is an insulator with a magnetic skyrmion lattice phase, making it an interesting material for spintronic applications. We have successfully fabricated polycrystalline thin film Cu2OSeO3. We present our method of thin film fabrication and our progress in enhancing thin film quality and controlling film growth. We also present data displaying the magnetic properties of the thin films. Specifically, we focus on the AC susceptibility of the films which we use to probe the magnetic phases of the material. |
Wednesday, March 8, 2023 10:00AM - 10:12AM |
M44.00007: Magnetic skyrmion resonance with density control Maciej W Olszewski, Audre Lai, Xiyue S Zhang, David A Muller, Gregory D Fuchs, Daniel C Ralph Magnetic skyrmions are a candidate for next-generation spintronic devices. Their topological nature makes them robust, allowing for a non-volatile, high-speed, and low-power-consumption method of storing and processing information. In addition, recent advances in skyrmionics have demonstrated both electrical and thermal techniques for controlling skyrmion densities in thin films. In this talk we present measurements of magnetic resonance in ferromagnetic multilayers with interfacial Dzyaloshinskii-Moriya interaction. We have performed room temperature experiments utilizing both conventional field-driven ferromagnetic resonance and spin-torque ferromagnetic resonance. We find a difference in the resonance readout between the phases with and without skyrmions, indicating a resonance readout of skyrmions themselves. This observation is further supported by micromagnetic simulations and Lorentz transmission electron microscopy with spatial resolution on the order of a few nanometers, where we directly observe skyrmions in our thin films. |
Wednesday, March 8, 2023 10:12AM - 10:24AM |
M44.00008: Evidence of non-coplanar spin textures in Weyl semi-metal candidate β iron germanide Brian W Casas, Moises Bravo, Julia Chan, Luis Balicas The exploitation of skyrmions, merons, or other non-coplanar spin textures as dynamic components in novel computational schemes has been of substantial interest since the experimental discovery of skyrmions in the B20 compounds. Naturally, systems that show ferromagnetic order up to or surpassing room temperature have significant potential for device applications. We report here the examination of the magnetic and electronic behavior of the under studied Fe1.7Ge, otherwise known as β iron germanide. Evidence of non-coplanar spin textures arises from both the conventional and so-called "unconventional" topological Hall effect, both of which have been shown to be sensitive to a wide array of non-coplanar spin textures such as Bloch domain walls, skyrmions, and merons. This material system supplies a test bed for understanding the interplay between strong electron correlations, electronic topology, and non-coplanar spin textures. |
Wednesday, March 8, 2023 10:24AM - 10:36AM |
M44.00009: Large anomalous Nernst effect in canted antiferromagnet YbMnBi2 Yu Pan, Congcong Le, Bin He, Sarah J Watzman, Mengyu Yao, Johannes Gooth, Joseph P C Heremans, Yan Sun, Claudia Felser A large anomalous Nernst effect (ANE) is crucial for thermoelectric energy conversion applications because the associated unique transverse geometry facilitates module fabrication. Topological ferromagnets with large Berry curvatures show large ANEs; however, they face drawbacks such as strong magnetic disturbances and low mobility due to high magnetization. YbMnBi2, as a canted antiferromagnet, surprisingly shows a large ANE conductivity of ~10 A m−1 K−1 that surpasses large values observed in other ferromagnets (3–5 A m−1 K−1). The canted spin structure of Mn guarantees a non-zero Berry curvature, but generates only a weak magnetization three orders of magnitude lower than that of general ferromagnets. The heavy Bi with a large spin–orbit coupling enables a large ANE and low thermal conductivity, whereas its highly dispersive px/y orbitals ensure low resistivity. The high anomalous transverse thermoelectric performance and extremely small magnetization make YbMnBi2 an excellent candidate for transverse thermoelectrics. |
Wednesday, March 8, 2023 10:36AM - 10:48AM |
M44.00010: Theory of surface orbital magnetization Daniel Seleznev, David Vanderbilt The theory of bulk orbital magnetization has been formulated both in reciprocal space based on Berry curvature and related quantities, and in real space in terms of the spatial average of a local marker. Here we consider a three-dimensional antiferromagnetic material having a vanishing bulk but a nonzero surface orbital magnetization. We ask whether the surface-normal component of the surface magnetization is well defined, and if so, how to compute it. As the physical observable corresponding to this quantity, we identify the macroscopic current running along a hinge shared by two facets. However, the hinge current only constrains the difference of the surface magnetizations on the adjoined facets, leaving a potential ambiguity. By performing a symmetry analysis, we find that only crystals exhibiting a pseudoscalar symmetry admit well-defined magnetizations at their surfaces. We then explore the possibility of computing surface magnetization via a coarse-graining procedure applied to a local marker. We show that multiple expressions for the local marker exist, and apply constraints to filter out potentially meaningful candidates. Using several tight-binding models as our theoretical test bed and several potential markers, we compute surface magnetizations for slab geometries and compare their predictions with explicit calculations of the macroscopic hinge currents of rod geometries. We find that only a particular form of the marker consistently predicts the correct hinge currents. |
Wednesday, March 8, 2023 10:48AM - 11:00AM |
M44.00011: Higher-Order Nonlinear Anomalous Hall Effects Induced by Berry Curvature Multipoles Cheng-Ping Zhang, Xuejian Gao, Yingming Xie, Hoi Chun Po, Kam Tuen Law In recent years, it has been shown that Berry curvature monopoles and dipoles play essential roles in the anomalous Hall effect and the nonlinear Hall effect respectively. In this work, we demonstrate that Berry curvature multipoles (the higher moments of Berry curvatures at the Fermi energy) can induce higher-order nonlinear anomalous Hall (NLAH) effect. Specifically, an AC Hall voltage perpendicular to the current direction emerges, where the frequency is an integer multiple of the frequency of the applied current. Importantly, by analyzing the symmetry properties of all the 3D and 2D magnetic point groups, we note that the quadrupole, hexapole and even higher Berry curvature moments can cause the leading-order frequency multiplication in certain materials. To provide concrete examples, we point out that the third-order NLAH voltage can be the leading-order Hall response in certain antiferromagnets due to Berry curvature quadrupoles, and the fourth-order NLAH voltage can be the leading response in the surface states of topological insulators induced by Berry curvature hexapoles. Our results are established by symmetry analysis, effective Hamiltonian and first-principles calculations. Other materials which support the higher-order NLAH effect are further proposed, including 2D antiferromagnets and ferromagnets, Weyl semimetals and twisted bilayer graphene near the quantum anomalous Hall phase. |
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