Bulletin of the American Physical Society
APS March Meeting 2021
Monday–Friday, March 15–19, 2021; Virtual; Time Zone: Central Daylight Time, USA
Session P40: Domain WallsFocus Live

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Sponsoring Units: GMAG DMP Chair: Alexey Kovalev, University of Nebraska  Lincoln 
Wednesday, March 17, 2021 3:00PM  3:12PM Live 
P40.00001: Chiral couplinginduced asymmetric domain wall motion Zhentao Liu, Zhaochu Luo, Trong Phuong Dao, Laura J Heyderman, Pietro Gambardella, Aleš Hrabec The engineering of chiral coupling in thin magnetic films with inhomogeneous anisotropy [1] allows for designing synthetic magnetic systems of fundamental and technological interest [2,3]. This coupling arises from the interfacial DzyaloshinskiiMoriya interaction, which, in combination with spinorbit torques, leads to the local manipulation of chiral magnetic order [4]. So far, the chiral coupling between nanomagnets with patterned magnetic anisotropy has been studied only in Pt/Co/AlOx trilayers. 
Wednesday, March 17, 2021 3:12PM  3:24PM Live 
P40.00002: Dielectric loss due to soft domainwall localized electromagnons in cycloidal magnets Francesco Foggetti, Sergey Artyukhin Competing magnetic exchange interactions often result in noncollinear magnetic states, such as spin spirals, which break the inversion symmetry and induce ferroelectric polarization. The resulting strong interactions between spins and ferroelectric polarization give rise to electromagnons and open the door to a technologically important possibility to control magnetic order by electric fields. Here we study the effects of chiral domain walls on dielectric and magnetoelectric properties of cycloidal spirals. We use a quasi1D model Hamiltonian with competing Heisenberg exchange interactions, leading to a spin spiral, and DzyaloshinskiiMoriya interactions, responsible for magnetoelectric coupling. The results suggest that low frequency dielectric anomalies in spiral magnets, such as TbMnO$_3$ and MnWO$_4$, may originate from hybrid magnonpolar phonon excitations associated with domain walls. 
Wednesday, March 17, 2021 3:24PM  4:00PM Live 
P40.00003: Anomalous Chiral Exchange Drag in Synthetic Antiferromagnets Invited Speaker: SeeHun Yang Longrange interactions between quasiparticles give rise to a ‘drag’ that affects the fundamental properties in condensed matter physics like Coulomb drag [1]. Drag typically involves the exchange of linear momentum between quasiparticles affecting transport properties. 
Wednesday, March 17, 2021 4:00PM  4:12PM Live 
P40.00004: Nonlinear soliton confinement in weakly coupled antiferromagnetic spin chains Harry Lane, Chris Stock, SangWook Cheong, Franz Demmel, Russell Ewings, Frank Kruger It has long been known that the lowenergy dynamics of spin systems are welldescribed by classical linear spinwave theory. This is particularly true in the case of systems with a large spin moment, where one can expand in powers of 1/S. For largespin antiferromagnets, linear spin wave theory predicts dispersive magnon modes which are gapped in the presence of anisotropy. Such spectra are found time and again in the literature, but can the quantum nature of spin give rise to exotic excitations that are not well described by linear spinwave theory? 
Wednesday, March 17, 2021 4:12PM  4:24PM Live 
P40.00005: Nonlinear Optical Studies of Neel Order and Spin Domains in Antiferromagnetic MnN Joongwon Lee, Zexuan Zhang, Debdeep Jena, Farhan Rana Antiferromagnetic (AF) materials has been drawing increasing attention for practical applications due to their ultrafast spin dynamics. θMnN, a metallic AF, exhibits many properties that are interesting for spin switching applications such as a low anisotropy field and a high Neel temperature (650 K) [1]. A challenge in studying spin domains, and their switching, in AF materials is the lack of readily available experimental techniques that can be used to determine the orientation of the Neel vector. In this work, we use optical second harmonic generation to study the Neel order and spin domains in singlecrystal epitaxiallygrown MnN. Our results show that spins in θMnN are ferromagnetically aligned in planes perpendicular to the caxis and have a large outofplane tilt which results in 2/m1’ magnetic point group. Different spin domains correspond to the inplane component of the spins aligned along the two inplane crystal cubic axes. Our measurements reveal that the sizes of the spin domains are smaller than a micron. We will also present results from nonlinear optical imaging of spin switching in MnN/Pt devices in which spin orbit torque is used to switch the Neel vector. [1] Phys. Rev. Research 2, 013347 (2020). 
Wednesday, March 17, 2021 4:24PM  4:36PM Live 
P40.00006: Magnetic Domain Wall Leaky IntegrateandFire Neurons with ShapeBased Configurable Activation Functions Wesley Brigner, Naimul Hassan, Xuan Hu, Christopher H Bennett, Felipe GarciaSanchez, Matthew J. Marinella, Jean Anne C. Incorvia, Joseph S. Friedman Neuromorphic computing attempts to mimic the neurons and synapses in a human brain in order to provide significant improvements in the computation of unstructured, realworld data. In past research, we have proposed three separate leaky integrateandfire (LIF) neurons that provide the leaking, integrating, and firing characteristics without the use of any additional circuitry [1][3]. These neurons are therefore able to significantly reduce the area and energy overhead of neuromorphic circuits. To improve the biomimicry of these neurons and to better match the neurons to different neuromorphic schema and algorithms, it is desirable for neuron leaking to implement specific mathematical functions in addition to exhibiting the three basic LIF neuronal functionalities. By varying the shape of the devices, it is possible to implement a variety of leaking characteristics. In this work, we will discuss the implementation of linear and sigmoidal leaking characteristics. 
Wednesday, March 17, 2021 4:36PM  4:48PM Live 
P40.00007: Dynamics of Muliferroic Domain Wall in an External Field Koji Kawahara, Hirokazu Tsunetsugu GdFeO_{3 }is a typical material showing magnetoelectric (ME) effects. It has three order parameters, antiferromagnetic moments of Fe and Gd ions and electric polarization, and they interact with each other. Tokunaga et al. [1] pointed out the presence of composite domain walls (DW), and discussed ME effects related to DW motion as well as DW splitting induced by impurity pinning. 
Wednesday, March 17, 2021 4:48PM  5:00PM Live 
P40.00008: Strain Controlled Domain Wall Synapse with Quantized Weights in the Presence of Thermal Noise and Edge Roughness Walid Al Misba, Tahmid Kaisar, Mark Lozano, Damien Querlioz, Caroline Anne Ross, Jayasimha Atulasimha We perform micromagnetic simulations to show that energy efficient [1] strain control of domain wall (DW) in a perpendicularly magnetized racetrack can realize a multistate synapse well suited to neuromorphic computingbased classification tasks. In conjunction with a fixed current exerting spin orbit torque (SOT), the racetrack is strained to modulate its anisotropy by applying voltage across the piezoelectric substrate on which the racetrack is patterned to control the translation of the DW to different positions in the racetrack. Simulations that include edge roughness and thermal noise showed that 5state and 3state synapse are possible in a 500 nm long and 50 nm wide racetrack [2]. Such limited state DW synapse is attractive to implement quantized neural network which is proven to achieve near equivalent accuracy to fullprecision network [3] even in the presence of device variability [4]. Preliminary experiments with such racetracks will also be presented. 
Wednesday, March 17, 2021 5:00PM  5:12PM Live 
P40.00009: Annihilation and control of chiral domain walls with magnetic fields Sunil Karna, M. Marshall, Weiwei Xie, Lisa DeBeerSchmitt, David P Young, I Vekhter, W Shelton, A. Kovács, Michalis Charilaou, John DiTusa The control of domain walls (DW) is central to nearly all magnetic technologies, particularly for information storage and spintronics. Creative attempts to increase storage density need to overcome volatility due to thermal and quantum fluctuations of nanoscopic domains and heating limitations. In contrast, topological defects, such as solitons, skyrmions, and merons, maybe much less susceptible to fluctuations, owing to topological constraints, while also being controllable with relatively low current densities. Here we present, the first evidence for soliton/soliton and soliton/antisoliton DW in a thin lamella of the hexagonal chiral magnet Mn_{1/3}NbS_{2} that respond asymmetrically to small magnetic fields and exhibit pairannihilation. This is important because it suggests that it may be possible to control the occurrence of soliton pairs and to make use of small fields or, more interestingly, small currents to control these nanoscopic magnetic domains. Specifically, our data suggest we can stabilize a state of either soliton/soliton or soliton/antisoliton pairs by tuning the balance between intrinsic exchange interactions and longrange magnetostatics in restricted geometries. 
Wednesday, March 17, 2021 5:12PM  5:24PM Live 
P40.00010: Control of domain wall patterning and anomalous response functions in ferrimagnetic spinels Lazar Kish, Alexander N. Thaler, Minseong Lee, Matthew Frith, Brian Wolin, Xu Wang, Alexander Zakrzewski, Matthias D Frontzek, Ken Littrell, Raffi Budakian, Haidong Zhou, Vivien Zapf, Adam Aczel, Lisa DeBeerSchmitt, Greg MacDougall

Wednesday, March 17, 2021 5:24PM  5:36PM Live 
P40.00011: Magnetic Domain Dynamics in an Insulating Ferromagnet Christopher Tang, Daniel Marc Silevitch, Karin Andrea Dahmen, Thomas F Rosenbaum Drag effects in domainwall dynamics in metallic ferromagnets have been observed in a wide range of materials, and traditionally have been attributed to eddy current backaction. Recently, drag effects have been observed in the electricallyinsulating disordered Ising ferromagnet, LiHo_xY_1xF_4, which cannot sustain eddycurrents. It was hypothesized that these drag effects were instead driven by disorderinduced randomfield pinning. We test this hypothesis by measuring the statistics and form of magnetic avalanches due to domainwall motion in the pure compound, LiHoF_4, free from randomfield pinning. Following the approach derived from the ABBM model, we examine the average scaled shape of Barkhausen events and test for drag effects by characterizing the skewness. We find that shorter Barkhausen events approach a symmetric lineshape, indicating a dragfree response, whereas longer avalanches do retain a weak drag signature. Furthermore, the dynamics of the system can be tuned by enhancing the quantum tunneling rate via an applied magnetic field transverse to the Ising axis. Tuning from a classical regime to a quantum regime dominated by tunneling reveals the structure of the freeenergy surface and barriers to domain reorientation, with applications to quantum annealing. 
Wednesday, March 17, 2021 5:36PM  5:48PM Live 
P40.00012: Degradation of domains with sequential field application Nirvana Caballero Recent experiments show striking unexpected features when sequences of alternating magnetic field pulses are applied to ferromagnetic samples: domains show area reduction and domain walls change their geometrical properties. In this work, we use a simple scalarfield model which only considers two preferential values for an order parameter, shortrange exchange, disorder, and temperature. By proposing a numerical protocol that mimics the experimental one, we show that domains described by the model are also subject to area reduction under sequential field application. We study two domain geometries common in ferromagnetic films: a bubble and a stripe domain. In both cases, the area reduction as a function of the number of alternating field cycles follows a linear combination of an exponentially and a linearly decreasing functions, as reported for the experiments. We characterize the domain walls geometry by computing its roughness exponents. The obtained roughness exponents are indistinguishable from the ones observed experimentally under alternating fields and also when the standard protocol to measure velocities is numerically emulated. 
Wednesday, March 17, 2021 5:48PM  6:00PM On Demand 
P40.00013: Purely Spintronic Perceptron with Unsupervised Learning Enabled by FourTerminal Domain WallMagnetic Tunnel Junction Neuron Naimul Hassan, Wesley Brigner, Christopher H. Bennet, Alvaro Velasquez, Xuan Hu, Otitoaleke G. Akinola, Can Cui, Felipe GarciaSanchez, Matthew J. Marinella, Jean Anne C. Incorvia, Joseph S. Friedman We propose a fourterminal domain wallmagnetic tunnel junction (DWMTJ) neuron that enables the firstever CMOSfree purely spintronic perceptron with unsupervised learning. The proposed leaky integrateandfire neuron has a ferromagnetic DW track coupled to a free layer of a binary MTJ by an electrically insulated layer. Current through the ferromagnetic track performs integration by moving the DW. Intrinsic leaking occurs by moving the DW in the opposite direction of integration due to either dipolar magnetic field, anisotropy gradient, or shape variation. When the DW passes underneath the electricallyisolated MTJ, it fires by switching between the antiparallel resistive state and parallel conductive state. Additionally, by exploiting stray magnetic field interactions, these neurons perform lateral inhibition. 
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