Bulletin of the American Physical Society
APS March Meeting 2022
Volume 67, Number 3
Monday–Friday, March 14–18, 2022; Chicago
Session B52: Magnetization and Spin Dynamics I: TheoryRecordings Available

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Sponsoring Units: GMAG Chair: Jianxin Zhu, Los Alamos National Laboratory Room: McCormick Place W475A 
Monday, March 14, 2022 11:30AM  11:42AM 
B52.00001: Meron Hall effect signatures of BerezinskiiKosterlitzThouless transition Edward Schwartz, Alexey A Kovalev, Bo Li We have analyzed the meron Hall response and spin superfluidity, in a 2D easyplane Heisenberg model, near the BKT transition. Below the BKT temperature, merons are expected to appear only in tightly bound pairs, suppressing vorticity current. Above the BKT temperature, the thermal contribution to the free energy of the system is sufficient to break these bonds, resulting in a nonzero free meron density. These unbound merons behave as free (topological) charge carriers which are driven in the transverse direction relative to an injected spin current, allowing for a significant vorticity Hall current. Using a combination of numerical methods including Monte Carlo, and spin dynamics simulations, we have calculated this Hall response for different values of anisotropy as a function of temperature. We give proposals how this Hall current may be used to detect the BKT transition in a 2D easy plane ferromagnetic insulators. 
Monday, March 14, 2022 11:42AM  11:54AM 
B52.00002: Spike Pattern Recognition Using Antiferromagnetic Artificial Neurons Hannah Bradley, Vasyl S Tyberkevych It has been shown that antiferromagnetic (AFM) spin Hall oscillators driven by a subthreshold spin current produce ultrashort (∼5 ps) spike in response to an external stimulus and can be used as ultrafast artificial neurons [1]. These AFM neurons can be connected by passive synapses to create neuromorphic circuits [2]. We show that a neural network based on AFM neurons can use a supervised machine learning algorithm, Spike patten association neuron (SPAN) [3]. This algorithm is based on the WidrowHoff learning rule and makes use of the temporal encoding of the neuron’s spikes. SPAN is an example of reservoir computing, as only the weights connected to the output layer are altered. An AFM neural network is trained to recognize letters made from a 9 celled grid of neurons. We use a modified SPAN approach with an inhibitory circuit, made possible by the effective inertia of the AFM neuron [2], as an output layer with constant weights. The inhibitor suppresses any excess unwanted data from the input. This modified SPAN allows for all weights to remain positive, which makes it easier to be implemented in hardware. 
Monday, March 14, 2022 11:54AM  12:06PM 
B52.00003: Magnetostatic surface wave scattering in geometrically modified ferromagnetic films Rodrigo E Arias A problem of interest in the area of Magnonics is the propagation of spin waves in thin ferromagnetic films or stripes. At present, this interest is related with the possibility of using spin waves as a practical mechanism of transfering information within nanodevices, either coded in their amplitudes or phases. In particular, to elucidate the effect of obstacles in their propagation is of relevance for practical applications. The present study focuses on a particular geometry and spin wave modes of interest: we consider the propagation of magnetostatic DamonEshbach surface waves in ferromagnetic films with surfaces that have geometric modulations perpendicular to the direction of propagation of the waves. In particular, we study the scattering of these surface wave of high group velocity by bumps and depressions, with a focus on transmitted energy and the presence of eventual localized modes. We study the propagations of these modes through the Greenextinction theorem, that renders sets of integral equations for the modes evaluated on the geometrically modified surfaces. 
Monday, March 14, 2022 12:06PM  12:18PM 
B52.00004: Spin Wave Propagation through Antiferromagnet/Ferromagnet Interface Oksana Busel, Oksana Gorobets, Oleg A Tretiakov We study the problem of controlling spin waves propagation through an antiferromagnet/ferromagnet interface via tuning material parameters. It is done by introducing the degree of sublattice noncompensation of antiferromagnet (DSNA), which is a physical characteristic of finitethickness interfaces. The DSNA value can be varied by designing interfaces with a particular disorder or curvilinear geometry. We describe spinwave propagation through any designed antiferromagnet/ferromagnet interface considering a variable DSNA and appropriate boundary conditions. As a result, we calculate the physical transmittance and reflectance of the spin waves as a function of frequency and show how to control them via the exchange parameters tuning. 
Monday, March 14, 2022 12:18PM  12:30PM 
B52.00005: Influence of a hot magnon gas on a bullet mode lifetime via incoherent interaction Petro Artemchuk, Vasyl S Tyberkevych, Andrei N Slavin The process of the rapid cooling of a nanometer thick magnetic film has been shown to lead to the transition of the hot magnon gas to BoseEinstein condensate of magnons [1]. In the same film, a nonlinear bullet mode, having frequency below the minimum of the magnon spectrum, can be formed under continuous influence of external spin transfer torque. Although the bullet mode is quite unstable due to its nonlinear nature [3], the bullet mode lifetime (BML) might be much longer at the presence of a hot magnon gas [3] when compared to the BML in the state of thermodynamical equilibrium [2]. We developed a simple model of interaction of a nonlinear bullet mode with a hot magnon gas in the process of a rapid cooling. We demonstrated that the increase of the hot magnon gas temperature leads to the increase of the BML. A significant increase of the BML, which agrees with the experimental results [2], has been observed at the hot magnon gas temperature exceeding a certain critical value. The model developed demonstrates stabilization of the nonlinear bullet mode by incoherent interactions with a nonequilibrium magnon gas. 
Monday, March 14, 2022 12:30PM  12:42PM 
B52.00006: Coherent Spin Pumping Using Easyplane Antiferromagnets Mingda Guo Antiferromagnetic (AFM) spin pumping has been shaping a surging frontier of ultrafast spin generating and detection in the terahertz frequency range. Recently, subTerahertz spin pumping has been confirmed experimentally in uniaxial AFM materials such as Cr_{2}O_{3} and MnF_{2}, which agrees with the theoretical model. However, biaxial (easyplane) AFM materials, which are not only more abundant in nature but also easier to be accessed in frequency than their uniaxial counterparts, are considered to be bad candidates for spin pumping because the Néel vector is linearly polarized due to the easyplane anisotropy, placing a major restriction on the choice of materials for practical applications. We theoretically challenge this seemingly established conclusion by showing that easyplane AFM materials can pump DC spin currents with the help of either the Dzyaloshinskii–Moriya interaction (DMI) or an applied magnetic field without inducing the spinflop transition. The former case, which has been realized in the experiment, is exemplified by αFe_{2}O_{3} in which the canted magnetization exhibits an elliptical precession around its equilibrium direction thanks to the DMI, resulting in a finite DC spin pumping driven by a linearly polarized microwave. The latter case is exemplified by collinear NiO, where a previously unknown chiralityflip can be induced by an applied magnetic field along the inplane easyaxis direction well below the spinflop transition, because of which the spin pumping contributions from different sublattices constructively added up and the overall DC signal turns out to be as strong as that in uniaxial AFM materials. Our finding significantly broadens the materials pool for antiferromagnetic spintronics. 
Monday, March 14, 2022 12:42PM  12:54PM 
B52.00007: Zeeman term for the Neel vector in a two sublattice antiferromagnet Sayak Dasgupta We theoretically investigate the dynamics of solitons in two sublattice antiferromagnets under external perturbations, focusing on the effect of DzyaloshinskyMoriya (DM) interactions. To this end, we construct a micromagnetic field theory for the antiferromagnet in the presence of the external magnetic field, DM interaction, and spintransfer torque. In particular, we show external magnetic field and spin current couple to Néel vector in a Zeemanlike manner when DM interactions present, which can be used to efficiently drive antiferromagnetic solitons of different dimensions. Besides, we study the effect of straining the local lattice. It can serve as an external handle on the Néel field inertia and thus dynamical properties. Our findings may find applications in antiferromagnetic spintronics. 
Monday, March 14, 2022 12:54PM  1:06PM 
B52.00008: In situ quantum state manipulations in μSR experiments on LiY_{0.95}Ho_{0.05}F_{4} via radiofrequency electromagnetic excitation David Billington, Edward Riordan, Stephen P Cottrell, Iain McKenzie, Tom Lancaster, Michael J Graf, Sean R Giblin In a variety of fluorine containing systems, the muon spin relaxation (μSR) signal from entangled fluorinemuonfluorine (F–μ–F) spins has been observed and successfully modelled in terms of magnetic dipoledipole interactions between the muon (spin) and two fluorine (nuclear) magnetic moments. In principle, it should be possible to manipulate the F–μ–F eigenstates and their populations through electromagnetic excitation. Here, we report μSR measurements of the F–μ–F states in single crystal LiY_{0.95}Ho_{0.05}F_{4} excited in situ with a continuous radiofrequency electromagnetic field, tuned to the highest energy transition of the F–μ–F eigenstates. Clear differences in the μSR signal are observed on application of the excitation field. To model the experimental data, a magnetic dipoledipole interaction Hamiltonian was constructed for a μF_{2}Li_{2}Ho cluster whose geometry was determined by calculations of the muon stopping site, to which we add a term representing the interaction of the cluster with the applied excitation field. This work opens new avenues for exploring the manipulation of quantum states within the unique μSR experimental environment. 
Monday, March 14, 2022 1:06PM  1:18PM 
B52.00009: DFTbased description of spin dynamics, lattice dynamics, and spinlattice dynamics of antiferromagnetic and paramagnetic phases at finite temperature Davide Gambino, Oleksandr I Malyi, Zhi Wang, Linding Yuan, Björn Alling, Alex Zunger An outstanding issue in electronic structure of antiferromagnets (AFM) and paramagnets (PM) is obtaining a theoretical description of the finite temperature configurations of spins and ionic positions, to be used in (supercell) DFT calculations of the electronic and magnetic properties vs temperature. We demonstrate this for the respective phases in NiO thus introducing temperature into DFT through statistical mechanics for magnetic insulators. We define three Levels of dynamics for the investigation: (I) dynamics of the spin degrees of freedom via noncollinear Heisenberg Monte Carlo solved with exchange energies obtained from DFT Cluster Expansion (II) Using the foregoing spin configurations while allowing for a vibrating lattice using ab initio molecular dynamics (III) Coupling spin and lattice dynamics via LandauLifshitzGilbert spin dynamics plus ab initio molecular dynamics. Such spinlattice configurations at each of the 3 levels vs temperature are used as input to DFT supercell calculations, from which we illustrate the distribution of magnetic moment, density of states, electronic band structure, band gaps, and shortrange order parameters vs T. This opens the door for ab initio description of the coupling between different local degrees of freedom at all temperatures. 
Monday, March 14, 2022 1:18PM  1:30PM 
B52.00010: Measurement of the Isolated Magnetic Susceptibility. Sean R Giblin, David Billington, Elsa Lhotel, Carley Paulsen, Dharmalingam Prabhakaran, Edward Riordan, Steve T Bramwell The isolated susceptibility may be defined as a (nonthermodynamic) average over the canonical ensemble, but while it has often been discussed in the literature, it has not been clearly measured. In this presentation we demonstrate an unambiguous measurement of the isolated magnetic susceptibility at avoided nuclearelectronic level crossings in a dilute spin ice system, containing wellseparated holmium ions. The measurement is performed with ac suscpetibility up to frequencies of 2 MHz. We show that the isolated suscpetibility quantifies the superposition of quasiclassical spin states at these points, and is a direct measure of state concurrence and state population. We also demonstrate other materials showing direct evidence of isolated magnetic suscpetibiltiy and discuss implications of these observations. 
Monday, March 14, 2022 1:30PM  1:42PM 
B52.00011: Generation of Nonreciprocity of Gapless Spin Waves by Chirality Injection Gyungchoon Go, Seunghun Lee, Se Kwon Kim In chiral magnets with intrinsic inversion symmetry breaking, it has been known that two spin waves moving in opposite directions can propagate at different velocities, exhibiting a phenomenon called magnetochiral nonreciprocity which allows for realizations of certain spin logic devices such as a spinwave diode. Here, we theoretically demonstrate that the spinwave nonreciprocity can occur without intrinsic bulk chirality in certain magnets including easycone ferromagnets and easycone antiferromagnets. Specifically, we show that nonlocal injection of a spin current from proximate normal metals to easycone magnets engenders a nonequilibrium chiral spin texture, on top of which spin waves exhibit nonreciprocity proportional to the injected spin current. One notable feature of the discovered nonreciprocal spin waves is its gapless nature, which can lead to a large thermal rectification effect at sufficiently low temperatures. We envision that nonlocal electric injection of chirality into otherwise nonchiral magnets may serve as a versatile route to realize electrically controllable magnetochiral phenomena in a wide class of materials. 
Monday, March 14, 2022 1:42PM  1:54PM 
B52.00012: Spinorbit torque nucleation and annihilation of magnetic droplet solitons Robin Klause, Axel Hoffmann Magnetic droplet solitons can form in thin films with perpendicular magnetic anisotropy (PMA) and zero damping. They have mostly been realized using nanocontact geometries to locally counteract the intrinsic damping using spintransfer torques. A more efficient pathway for current controlled magnetization dynamics is given by spinorbit torques (SOTs). Due to symmetry restrictions these torques can conventionally only manipulate inplane magnetizations efficiently. However, mirror symmetry breaking in noncolinear antiferromagnets due to magnetic order can generate unconventional outofplane SOTs that can be used to manipulate magnetizations in PMA films. Here, we investigate the nucleation and annihilation behavior of droplets from unconventional spinorbit torques in rectangular films using micromagnetic simulations. We find that the current pulse amplitude and duration affect the number of droplets nucleated and the nucleation time. By precisely controlling the current pulse we can add and subtract droplets from our system on demand. Additionally, an external magnetic field affects the droplet size and position. 
Monday, March 14, 2022 1:54PM  2:06PM 
B52.00013: Slow interminima relaxation and its consequence for BEC of magnons Gang Li, Haichen Jia, Valery L Pokrovsky Two recent reports of the Münster University experimental team led by S.O. Demokritov displayed several important facts contradicting to the existing theories of the BoseEinstein condensate of magnons (BECM). Though a complete theoretical description of these experiments still is in development, we will present simplified theoretical arguments to determine what are properties of the uniform stationary BECM established under the action of permanent parametric pumping that follows from the experimental facts. 
Monday, March 14, 2022 2:06PM  2:18PM 
B52.00014: Suhl instabilities in a magnetic nanoparticle Jinho Lim, Anupam Garg, John B Ketterson We report simulations of largeamplitude responses of a permalloy ellipsoid with z, x, and y diameters of 100×50×50nm^{3} driven by an algorithm that constrains the H_{1} field to lie the xy plane and perpendicular to the sampleaveraged magnetization. For smaller H_{1} uniform precession is observed, but as H_{1} increases instabilities are encountered. The first, signaled by an abrupt increase in the exchange energy, involves a crossover of the uniform mode and a standing spin wave with a 2Pi phase variation along z, characteristic of the second Suhl^{1} processes. The degeneracy arises from a strong dependence of the 2Pi mode frequency on the uniform precession amplitude. Above the crossover these two modes appear to merge into a bound state, in which the spins precess nonuniformly but in phase and the two modes periodically exchange energy back and forth. At the second instability two low frequency modes with different mode numbers appear with frequencies summing to the boundstate mode, characteristic of an asymmetric parametric instability, a generalized form of the first Suhl processes that satisfies angular momentum conservation. The system subsequently evolves in a highly irregular manner. 
Monday, March 14, 2022 2:18PM  2:30PM 
B52.00015: Theory for the nuclear spin Seebeck effect Derek Reitz, Yaroslav Tserkovnyak The spin Seebeck effect (SSE) involves transfer of spin angular momentum between a magnet and a metal from internal thermal fluctuations. SSE is usually dominated by electronic, rather than nuclear, spins, since interfacial exchange is much stronger than interfacial hyperfine coupling. At low temperatures, however, electronic magnons freeze out while nuclear spins remain thermally active. Along these lines, we introduce a theory for a new type of SSE: the nuclear SSE, and compare our results to novel ultralowtemperature experiments where this physics is believed to have been observed for the first time. The dominant mechanism for nuclear spin relaxation in a metal is into the Fermi sea, called Korringa relaxation, which drives the nuclear spin current in our theory. The nuclear SSE is then determined by competing rates: thermalization with phonons via hyperfine coupling to electrons in the magnet, and thermalization with metallic electrons via an interfacial Korringalike interaction. 
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