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 F38: Antiferromagnetic SpintronicsFocus Live

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Sponsoring Units: GMAG DMP FIAP Chair: Yi Wang, Dalian University of Technology 
Tuesday, March 16, 2021 11:30AM  11:42AM Live 
F38.00001: Novel Spin Torques in Antiferromagnetic FeRh Jonathan Gibbons, Takaaki Dohi, Vivek P Amin, Fei Xue, Hanu Arava, Hilal Saglam, Yuzi Liu, John Pearson, Amanda K PetfordLong, Paul M Haney, Mark D Stiles, Shunsuke Fukami, Axel F Hoffmann Magnetic order can reduce the symmetry of the underlying crystal lattice, allowing for the generation of spin torques with novel symmetries. By controlling the magnetic ordering of the material, it is therefore possible to control the orientations of the torque and generate torques that are better suited to applications such as switching perpendicularly magnetized nanomagnets and driving dynamics in spin torque oscillators for neurotrophic tasks. Antiferromagnets are better suited to this task than ferromagnets, as they can be prepared into a magnetically ordered state that is robust against external influence. However, spin torques with new symmetries have not yet been observed in collinear antiferromagnetic materials. We report large, highly temperaturedependent unconventional spin torques generated in collinear antiferromagnetic FeRh, and further show the effects of the magnetic ordering direction on spin torque geometries using FeRh. These experimental observations are supported by theoretical calculations of spin torques from FeRh. 
Tuesday, March 16, 2021 11:42AM  11:54AM Live 
F38.00002: Staggered spinorbit torques in antiferromagnets Fei Xue, Paul M Haney Spinorbit torques in a heterostructure composed of heavy metals and ferromagnets have been widely studied, with the aim of efficient electrical switching of the magnetization. Compared to ferromagnets, the order parameter of antiferromagnets (the Neel vector) is more robust to external magnetic fields, however, it is also more difficult to switch. Recent studies have demonstrated efficient control and switching of antiferromagnets with a fieldlike torque, due to a staggered electricfield induced spin density. In this work, we consider the electric fieldinduced staggered spin current and resulting staggered spinorbit torque in a collinear antiferromagnet. We use orthorhombic CuMnAs as an example to compute the timereversaleven and timereversalodd components of the electric fieldinduced torque and study the ensuing dynamics of the antiferromagnetic order parameter. 
Tuesday, March 16, 2021 11:54AM  12:06PM Live 
F38.00003: Nuclear spin Seebeck effect in antiferromagnets 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 magnon thermal occupation numbers are exponentially suppressed, while nuclear spins remain active. The nuclear spins are paramagnetic, weakly polarized in the large hyperfine field of the Neel order. One source for nuclear SSE is interfacial nuclear, metal spin flipflops, known as Korringa relaxation. Nuclear SSE is then determined by competing rates: thermalization with phonons via hyperfine coupling to electrons in the magnet, and Korringa relaxation into the metal. 
Tuesday, March 16, 2021 12:06PM  12:18PM Live 
F38.00004: Temperature dependence of the anisotropic magnetoresistance of the metallic antiferromagnet Fe_{2}As Junyi Wu, Manohar H Karigerasi, Daniel Shoemaker, Virginia O Lorenz, David G. Cahill Electrical readout of metallic antiferromagnet (AF) memories is typically realized by measuring the anisotropic magnetoresistance (AMR), but the mechanisms for enhanced AMR are not yet established. We study AMR of single crystals of AF Fe_{2}As from T=5 K to above the Néel temperature, T_{N} ∼ 353 K. With an applied magnetic field B rotating in the (001) plane, we observe a peaktopeak AMR change of 1.3% for B>1 T at T=5 K, one order of magnitude larger than in CuMnAs, a widely studied candidate for AF spintronics. The AMR varies strongly with temperature, decreasing by a factor of ∼ 10 at T∼ 200 K, and depends on the residual resistance. Our results suggest large AMR in easyplane AFs may require Néel temperatures that greatly exceed room temperature. 
Tuesday, March 16, 2021 12:18PM  12:30PM Live 
F38.00005: Quantum spin torque driven transmutation of antiferromagnetic Mott insulator Marko Petrovic, Priyanka Mondal, Adrian Feiguin, Branislav Nikolic The standard model of spintransfer torque in antiferromagnetic spintronics considers the exchange of angular momentum between quantum spins of flowing electrons and noncollineartothem localized spins treated as classical vectors. These vectors are assumed to realize Néel order in equilibrium, ↑↓…↑↓, and their dynamics is described by the LandauLifshitzGilbert (LLG) equation. However, many experimentally employed materials (such as NiO) are strongly correlated antiferromagnetic Mott insulators (AFMI) where the ground state is quite different from the unentangled Néel state. The ground state is entangled by quantum spin fluctuations, leading to zero expectation value of all localized spins so the LLG dynamics cannot even be initiated. Instead, a fully quantum treatment of both electrons and localized spins is necessary to capture the spin transfer. Here, we use the timedependent density matrix renormalization group approach to predict how the injection of spin current into a normal metal coupled to AFMI induces a nonzero expectation value of AFMI localized spins. 
Tuesday, March 16, 2021 12:30PM  12:42PM Live 
F38.00006: Exceptional Point Location Dependence on Dipolar Interactions in Synthetic Antiferromagnets Teresa Jeffrey, Wei Zhang, Joseph N Sklenar Synthetic antiferromagnets (SAFMs) have recently been suggested as ideal macroscopic systems to search for Exceptional Points (EPs). Both optical and acoustic modes exist within SAFM structures [1]. Within SAFMs, dipolar interactions and symmetrybreaking external fields can hybridize acoustic and optical magnons [2]. Physically, an EP is the point at which the optical and acoustic modes coalesce and are indistinguishable from one another. Beyond this, only a single magnon branch exists. This implies that there is no magnonmagnon interaction. 
Tuesday, March 16, 2021 12:42PM  12:54PM Live 
F38.00007: Neuromorphic Memory Rings based on Artificial Antiferromagnetic Neurons Hannah Bradley, Vasyl Tyberkevych Antiferromagnetic (AFM) spin Hall oscillators driven by an external subthreshold current can create ultrashort spikes (with duration on the order of a few ps) in response to a weak external stimulus and therefore can be used as ultrafast artificial neurons [1, 2]. One of specific features of AFM neurons is an effective inertia that originates from exchange coupling between two magnetic sublattices [3]. Inertial effects allow one to develop AFMbased neural networks with new functionalities not possible with conventual artificial neurons. Here we investigate the dynamical behavior of neuromorphic memory rings – periodic pulse generators formed by several AFM neurons coupled in a ring structure. We show, that spike generation in a neuromorphic ring can be initiated by applying an external spike to any single neuron in the ring. Dynamic suppression of the spike propagation can be achieved by embedding an additional inhibitor circuit in the ring, thus turning a ring structure into a fully controllable neuromorphic memory device. 
Tuesday, March 16, 2021 12:54PM  1:30PM Live 
F38.00008: Spontaneous Hall and Nernst effects in antiferromagnets Invited Speaker: Helena Reichlova The anomalous Hall effect and its thermoelectric counterpart, the anomalous Nernst effect, were for a long time believed to be exclusively present in ferromagnetic materials. The development in the past years, however, revealed that these effects can be finite also in certain noncollinear antiferromagnets owing to the nontrivial topology of their band structure. A typical representative of this class of antiferromagnets is Mn_{3}Sn. In the first part of this talk, we therefore discuss magneto thermal transport measurements in Mn3Sn thin films. We demonstrate that the local magnetothermal response can be employed for a spatially resolved visualization of the magnetic properties of the antiferromagnet [1]. In the second part of the talk, we then address mechanisms for spontaneous Hall effects (i.e. a Hall effect present in absence of magnetic fields), including the crystal Hall effectrecently proposed by theory [2] and corroborated by first experimental indications in RuO_{2} [3]. This so far overlooked mechanism enables a spontaneous Hall effect arising from the crystal and spin symmetry of a particular compound, which tremendously broadens the pool of materials in which spontaneous Hall and Nernst effects can be expected. We finally introduce one particular example of a collinear antiferromagnet Mn_{5}Si_{3} in which a finite spontaneous Hall effect is enabled via yet another combination of spinspace symmetry, and address the various contributions to the Hall signal together with their microscopic origins [4]. 
Tuesday, March 16, 2021 1:30PM  1:42PM Live 
F38.00009: Observation of magnonpolarons in a uniaxial antiferromagnetic insulator Cr_{2}O_{3} Junxue Li, Haakon Thømt Simensen, Derek Reitz, Qiyang Sun, Wei Yuan, Chen Li, Yaroslav Tserkovnyak, Arne Brataas, Jing Shi Magnonpolarons, a type of hybridized excitations between magnons and phonons, were first reported in yttrium iron garnet as anomalies in the spin Seebeck effect responses. In antiferromagnetic insulators, the magnon spectrum is distinctly different due to the existence of two equivalent, antiferromagnetically coupled spin sublattices. Here we report an observation of antiferromagnetic (AFM) magnonpolarons in a uniaxial AFM insulator Cr_{2}O_{3}. Despite the relatively higher energy of magnon than that of the acoustic phonons, near the spinflop transition of ~ 6 T, the lefthanded magnon spectrum shifts downward to hybridize with the acoustic phonons to form AFM magnonpolarons, which can also be probed by the spin Seebeck effect. The spin Seebeck signal is founded to be enhanced due to the magnonpolarons at low temperatures. 
Tuesday, March 16, 2021 1:42PM  1:54PM Live 
F38.00010: Spintorque driven selfoscillations in noncollinear coplanar antiferromagnets Ankit Shukla, Shaloo Rakheja We study the spin torque driven selfoscillations in thinfilm noncollinear coplanar antiferromagnets (AFMs) with spin polarization perpendicular to the plane of the film. Owing to their intrinsic THz spin dynamics, negligible stray fields, and the ability to host textures, AFMs can be used to realize various ultrafast and lowpower devices including spintorque oscillators, spinbased memories, and spin interconnects. While there are several prior reports on spintorquedriven oscillations and switching in collinear AFMs like NiO, similar analyses in the case of planar noncollinear AFMs is currently lacking. In this work, we use a combination of micromagnetics and an effective pendulum equation to model the oscillation dynamics in chiral systems like Mn3Ir and Mn3Sn. We examine the effects of the inhomogeneous exchange interaction and the DzyaloshinskiiMoriya interaction on the frequency and spatial distribution of AFM vectors. Our results show that THz frequency dynamics of noncollinear AFMs is similar to that of collinear AFM, however, the former has the advantage of easier detection by Anamolous Hall effect. 
Tuesday, March 16, 2021 1:54PM  2:06PM Live 
F38.00011: Spintorque switching of the noncollinear antiferromagnetic order in antiperovskites Gautam Gurung, DingFu Shao, Evgeny Y Tsymbal Manganesebased antiperovskite nitrides ANMn_{3} (A = Ga, Ni, etc.) are promising material platforms for antiferromagnetic spintronics due to their noncollinear antiferromagnetic orders supporting the unconventional chargespin conversion through the anomalous and spinHall effects. The efficient control of the noncollinear antiferromagnetic order is the key factor for the application of these properties. Here, we predict an electric control of the noncollinear AFM order in the antiperovskite materials by means of spintransfer torque. Based on the firstprinciples calculations and the atomistic spinmodel simulations, we show that the spintransfer torque induced by a spinpolarized current generates the terahertz dynamics of the magnetization and can switch the noncollinear antiferromagnetic order in ANMn_{3} on a picosecond time scale. We find that the critical current density, which is required for the switching, can be optimized by changing the magnetocrystalline anisotropy of the compound by stoichiometry engineering. The predicted spin dynamical properties of antiperovskites can be useful for device application of these materials. 
Tuesday, March 16, 2021 2:06PM  2:18PM Live 
F38.00012: Spin superfluidity in noncollinear antiferromagnets Bo Li, Alexey Kovalev Spin superfluidity is very promising for applications as it allows longdistance spin transport in magnetic systems. So far superfluidity has been observed in collinear antiferromagnets. We extend the description of spin superfluidity to noncollinear antiferromagnets (nAFMs) and explore various realizations in exchange interaction dominated hexagonal antiferromagnets. We find that the system in the longwavelength regime is described by a nonlinear sigma model with additional corrections from DzyaloshinskiiMoriya interactions and anisotropies. We find that the system approximately holds a U(1)rotational symmetry associated with spin superfluidity. The powerlaw spatial decay signature of spin decay is identified in a nonlocalmeasurement setup. We also discuss the generalization of mixing conductance to noncollinear systems and spin current injection into nAFM. Our simple theory can help in designing and interpreting experiments on spin superfluidity in nAFM. We suggest iron jarosites as promising material candidates for realizing our proposal. 
Tuesday, March 16, 2021 2:18PM  2:30PM Live 
F38.00013: Probing the surface magnetism of antiferromagnets Ivan Schuller, Pavel N. Lapa, Minhan Lee, Igor Roshchin, Kirill Belashchenko Using antiferromagnetferromagnet spin valves, we detected and studied uncompensated magnetization at the boundary of an antiferromagnetic FeF_{2}(110) layer at different temperatures and magnetic fields. It was discovered that, at low temperatures, this magnetization possesses unprecedented stability, and cannot be reversed even in very high magnetic fields (> 90 kOe). Our experimental technique allowed to detect and characterize the reversal of the boundary magnetization at temperatures close to the Néel temperature, thus, demonstrating the possibility of isothermal imprinting of pining into the antiferromagnet. The boundary magnetization was detected only for the valves with the FeF_{2}(110) layer and not for the valves containing FeF_{2}(100) or FeF_{2}(001). This observation is consistent with symmetryrelated theory predicting magnetization at the surface of antiferromagnets with broken timereversal symmetry. We will show and discuss the experimental results on the valves containing antiferromagnetic Mn_{2}Au whose spin structure can be manipulated electrically. 
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