Bulletin of the American Physical Society
APS March Meeting 2021
Volume 66, Number 1
Monday–Friday, March 15–19, 2021; Virtual; Time Zone: Central Daylight Time, USA
Session M40: Magnetization Dynamics in Novel MaterialsFocus Live
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Sponsoring Units: GMAG Chair: Gen Yin, University of California, Los Angeles |
Wednesday, March 17, 2021 11:30AM - 12:06PM Live |
M40.00001: Magnetization dynamics and spin transport in compensated ferrimagnets Invited Speaker: Kyung-Jin Lee For magnetization dynamics, compensated ferrimagnets combine the best features of antiferromagnets and ferromagnets. Antiferromagnets are of considerable interest because the exchange torques between the two sublattices give a time scale that is much faster than that in ferromagnets and the lack of magnetization and net angular momentum lead to minimal perturbation by stray fields and eased constraints due to angular momentum conservation. A compensated ferrimagnet has all these virtues. At the same time, the lack of symmetry between the two sublattices in a compensated ferrimagnet means that quantities like average spin currents are not zero making the systems potentially easier to manipulate and detect the consequences. We will describe calculations and measurements of domain wall and skyrmion motion at the angular momentum compensation point. At this point with no net spin density, the rotational motion of the magnetic textures (domain walls and skyrmions) is absent. As a result, domain walls move fast (Refs. [1, 2]) since there is no tilting of domain wall angle. For the same reason, the skyrmion Hall effect vanishes (Ref. [3]) and the magnon-photon coupling enhances (Ref. [4]). We will also discuss the increased efficiency of spin torques due to the weakened dephasing in compensated ferrimagnets. Combining experiments with theoretical studies, Refs. [5] and [6] show large torques for ferrimagnetic multilayers and for ferrimagnetic domain walls, respectively. |
Wednesday, March 17, 2021 12:06PM - 12:18PM Live |
M40.00002: Effect of Notch Defects on Superfluid-Like Spin Transport David Smith, Fernando Ramos-Diaz, Matthew Simmers, So Takei, Satoru Emori We examine the effects of pinning potentials (notch defects) on superfluid-like spin transport [1] in magnetic nanostrips using a one-dimensional model and micromagnetic simulations. Our results reveal two distinct dynamic regimes. At driving current densities slightly above the threshold required to induce easy-plane precessional dynamics, a train of individual domain walls with the same chirality propagates along the nanostrip toward the notches. The notches act as a local potential minimum, causing pinning of the domain walls. At higher current densities, we find that a dense train of domain walls constitutes a superfluid-like spin flow that is largely unaffected by the presence of the notches. Such immunity to pinning is more evident in synthetic antiferromagnets, where instabilities via vortex formation are suppressed up to high current densities. Our results suggest that superfluid-like spin transport is robust to edge defects, which might arise due to lithographic patterning, and is therefore promising for future device applications. |
Wednesday, March 17, 2021 12:18PM - 12:30PM Live |
M40.00003: Ultra-low critical velocity switching of magnetic vortex core polarity Mahdi Mehrnia, Jeremy Trimble, Olle Heinonen, Jesse A. Berezovsky Topological structures in magnetism are subject of great interest both from physics and technological perspectives. For instance, the topology of a magnetic vortex results in an extremely stable magnetic vortex core polarity, which makes vortices attractive for applications such as information storage. Switching of the magnetic vortex core polarity, as a result of this high stability, is typically achieved either with large magnetic fields or strong dynamic driving. Here, we show that the interaction of the magnetic vortex core with localized defects results in the reduction of the strong stability by more than an order of magnitude. We excite vortex dynamics in thin Permalloy disks, and track the magnetic vortex core orbits using 3D time-resolved Kerr microscopy. In pristine samples with weak defects, we observe normal gyrations of the vortex core. After laser-induced generation of strong defects, however, we observe repeated vortex core reversal at much-reduced driving strength. Micromagnetic simulations reveal how local reduction of exchange coupling can create vortex core reversal sites for deterministic vortex core switching at ultra-low critical velocities. |
Wednesday, March 17, 2021 12:30PM - 12:42PM Live |
M40.00004: Challenges and opportunities for skymionic racetrack devices Hamed Vakilitaleghani, Golam Morshed, Avik Ghosh Skyrmions are potential low energy information carrying bits for high density memory and logic applications. We propose to use the current-induced displacement of voltage gated skyrmions as a native memory for temporal computing1 as well as conventional Boolean logic operations2. One of the main challenges however is the positional stability of skyrmions. In a magnetic racetrack, large skyrmions tend to thermally diffuse while small skyrmions pin. Our models show that by engineering notches we can create controlled pinning sites in a racetrack, leading to a reliable skyrmionic device with small diffusion and moderate unpinning current. |
Wednesday, March 17, 2021 12:42PM - 12:54PM Live |
M40.00005: Electrically reconfigurable skyrmion lattice based self-adaptive oscillating neurons Priyamvada Jadaun, Can Cui, Jean Anne C. Incorvia Neuromorphic computing promises to realize the transformative potential of Artificial Intelligence (AI) by enabling ultra-low power, advanced AI. Spintronic materials are particularly attractive for neuromorphic computing as they have a small footprint, use low power and can mimic the complex properties of the brain. Here, we utilize an electrically reconfigurable skyrmion lattice to design and simulate a novel artificial neuron that incorporates two advanced neural behaviors: oscillatory dynamics and neuromodulation. Neuromodulation is the self-adaptive ability of a neuron to regulate its dynamics in response to its environment. Here, neuromodulation arises from the reconfigurability of the skyrmion lattice, i.e, skyrmions in a lattice are rearranged via electrical currents, shifting the resonant frequencies and altering the amplitudes of oscillation of the neuron. The neuron is implemented with a lattice of five magnetic skyrmions in a thulium iron garnet and platinum bilayer. We utilize the neuron to demonstrate 2 high-level cognitive processes: context-aware decision making and feature binding. These results can be used for advanced AI applications including biomedicine, neuro-prosthesis and human-machine interaction. |
Wednesday, March 17, 2021 12:54PM - 1:06PM Live |
M40.00006: Guided Motion for Skyrmions Along Square Obstacle Arrays Interface Nicolas Vizarim, Cynthia Reichhardt, Pablo A. Venegas, Charles Reichhardt In this work we simulate the skyrmion motion when subjected to an ac drive in the presence of a square obstacle array. The obstacle array has two regions with different obstacle array structures. We consider drive amplitudes at which skyrmions in the bulk lattice undergo only localized orbit motion. However, when the interface is introduced, direct skyrmion transport occurs along the interface. We show that the skyrmion can be guided by the interface, even turning 90° corners. This results may be promising for developing interface structures for the skyrmion to follow, hence controlling its motion. |
Wednesday, March 17, 2021 1:06PM - 1:42PM Live |
M40.00007: Chemisorption-Induced Dzyaloshinskii-Moriya Interactions Invited Speaker: Gong Chen Chiral spin textures lead to a host of fascinating phenomena due to their topologically protected spin configurations and emergent electromagnetic field, offering great potential for novel concepts in low dissipation magnetic information storage [1-3]. The most widely used mechanism to stabilize magnetic chirality is the Dzyaloshinskii–Moriya interaction (DMI), originating from broken inversion symmetry. To date, sufficiently large DMI has been found in a limited set of bulk materials with chiral lattice, and in magnetic thin films adjacent to heavy metals or oxides. |
Wednesday, March 17, 2021 1:42PM - 1:54PM Live |
M40.00008: Skyrmion generation engineered by ionic liquid gating in ultra-thin films Yao Zhang, Guy Dubuis, Simon Granville Magnetic skyrmions show topological spin textures with nanoscale size, which have great potential for spintronics applications. In this work, we investigated the skyrmion generation in MgO/Mn2CoAl/Pd ultra-thin films with perpendicular magnetic anisotropy (PMA) by ionic liquid gating (ILG). For samples with a weak PMA, skyrmions can be generated directly by just applying a negative gate voltage, -2.5 V, which can be ascribed to the reduction of effective magnetic anisotropy into a spin-reorientation transition. In the case of samples with an as-grown strong PMA, skyrmions can also appear at -2.5 V after cycling positive and negative voltages which induces an accumulation of PdOx layer after each cycling. These gating behaviours are non-volatile due to the oxygen ions diffuse into the Pd layer. On the other hand, by just applying a large gate voltage, -3 V, to etch away the nature oxide PdOx layer, volatile skyrmions can be generated at a small positive gate voltage due to the electrostatic charge accumulation. Our results suggest that ILG is a useful method to engineer the magnetic properties of samples and generate skyrmions without designing samples in specific parameters, like thickness and weak PMA. |
Wednesday, March 17, 2021 1:54PM - 2:06PM Live |
M40.00009: Late stages in the ordering of magnetic skyrmion lattices James Stidham, Michel Pleimling The late-stage ordering of interacting magnetic skyrmions is studied numerically through extensive Langevin molecular dynamics simulations. Defining skyrmion displacements that change the connectivity of cells obtained in a Voronoi tesselation as events, we investigate event histograms as a function of the time elapsed since preparing the system as well as the histograms of consecutive events as a function of the time separating these two events. These histograms, which provide unique insights into the transient properties during the ordering process of skyrmion matter, show a characteristic behavior that allows the Magnus-force-dominated regime, where the Magnus force accelerates the relaxation process, to be distinguished from the noise-dominated regime, where the Magnus force enhances the effects of thermal noise. In the Magnus-force-dominated regime, the different histograms display power-law tails with exponents that depend on the strength of the Magnus force. |
Wednesday, March 17, 2021 2:06PM - 2:18PM Live |
M40.00010: Operando Control of Skyrmion Density in a Lorentz Transmission Electron Microscope with Current Pulses Albert Park, Zhen Chen, Xiyue S Zhang, Lijun Zhu, David Anthony Muller, Gregory Fuchs The demonstration of sub-100 nm magnetic skyrmions at room temperature in ferromagnetic multilayers raises the realistic possibility of implementing skyrmion device technology. The key to this effort is understanding the stability and behavior of skyrmions on this scale. We study the response of skyrmions to external stimuli such as magnetic field and electric current pulses by fabricating a device compatible with operando current pulsing in a Lorentz transmission electron microscope. We investigate the behavior of skyrmions that are strongly bound to magnetic pinning sites using this method. While the pinning sites obstruct the skyrmions' motion, we observe that skyrmions are created and deleted by current pulses. Our investigation of the skyrmion density over a range of current pulse energies and magnetic fields reveal that we can control skyrmion density with current pulses. We use micromagnetic simulation to clarify the mechanism of thermally assisted skyrmion nucleation and annihilation. Finally, we show that high density skyrmion states are more stable over the magnetic field perturbation than isolated skyrmion states through the suppression of stretching of skyrmions into stripe domains at low magnetic fields. |
Wednesday, March 17, 2021 2:18PM - 2:30PM On Demand |
M40.00011: Skyrmion lattice formation and destruction mechanisms probed with SANS Namila Liyanage, Nan Tang, Lizabeth Quigley, Guo-Jiun Shu, Fangchang Chou, Nicholas Butch, Markus Bleuel, Julie Borchers, Lisa DeBeer-Schmitt, Dustin Gilbert A magnetic Skyrmions is a local whirl of the spin configuration in a magnetic material. These topologically stabilized quasi-particles have interesting applications in spintronic devices [1] also these structures cannot be continuously created or destroyed. |
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