Bulletin of the American Physical Society
APS March Meeting 2020
Volume 65, Number 1
Monday–Friday, March 2–6, 2020; Denver, Colorado
Session U42: Antiferromagnetic SpintronicsFocus
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Sponsoring Units: GMAG FIAP Chair: Xin Fan, Univ of Denver Room: 709/711 |
Thursday, March 5, 2020 2:30PM - 3:06PM |
U42.00001: Time-resolved ultrafast oscillation of octupole polarization in a chiral antiferromagnet Mn3Sn Invited Speaker: Shinji Miwa Understanding of spin-precession dynamics forms the basis of spintronic application. For the ultrafast THz control of staggered moments in antiferromagnets, the time-resolved observation of spin precession is essential for developing the associated technology. However, such observation has been limited to insulators to date [1]. Recently, the metallic antiferromagnets Mn3X have attracted significant attention for its strong response (e.g. anomalous Hall effect) comparable to ferromagnets owing to the hidden ferroic order, which configures large Berry curvature originated from Weyl points in momentum space [2]. Such ferroic order can be characterized by cluster octupoles based on neighboring magnetic moments [3], and thus it is highly important to clarify the dynamics of cluster octupole for designing the spintronic application. |
Thursday, March 5, 2020 3:06PM - 3:18PM |
U42.00002: Evidence of Electrical Switching of the Antiferromagnetic Néel Vector Chih-Chieh Chiang, Ssu Yen Huang, Danru Qu, Po-Hsun Wu, Chia-Ling Chien Recently, field-free switching of antiferromagnetic (AFM) Néel vector via spin-orbit torque (SOT) attracting worldwide attention. By applying a writing current in the AFM or the normal metal (NM)/AFM bilayer, in a patterned multi-terminal structures, the measured resistance shows recurring signals due to supposedly electrical switching of the AFM Néel vector. However, in this work, we demonstrate that similar signals can be observed in such patterned structures, with and without the AFM layer [1]. The strength of the signal is also greatly affected by different metals and substrates. We show that under a large writing current density beyond the Ohmic regime, the multi-terminal devices can generate unintended anisotropic thermal gradient and voltages. Therefore, this switching signal may not be the conclusive evidence of the SOT switching of AFM, but the thermal artifacts of patterned metal structures on substrate. |
Thursday, March 5, 2020 3:18PM - 3:30PM |
U42.00003: Spin-torque control of the noncollinear antiferromagnetic order in antiperovskites Gautam Gurung, Ding-Fu Shao, Evgeny Y Tsymbal Antiferromagnetic (AFM) spintronics exploits the AFM order to control the spin-dependent transport properties. Recent theoretical studies suggest that the AFM order parameter can be switched by a spin torque, while the related experimental realizations are limited to a few collinear antiferromagnets. There is, however, a large group of high temperature noncollinear antiferromagnets, which are suitable for such switching. Here, we predict that the spin torque can be efficiently used to control the noncollinear AFM order in antiperovskite materials. Based on first-principles calculations and atomistic spin model simulations, we show that in antiperovskites ANMn3 (A = Ga, Ni, etc) with the Γ4g AFM ground state, the AFM order parameter can be switched on the picosecond scale by a spin-transfer torque generated by a spin polarized current. The threshold switching current density can be tuned by the ANMn3 stoichiometry engineering, changing the magnetocrystalline anisotropy. The Γ4g AFM phase supports the anomalous Hall effect, which can be used to detect the spin-torque switching of the AFM order. The predicted ultrafast switching dynamics and the efficient detection of AFM order parameter make noncollinear AFM antiperovskites promising material platforms for AFM spintronics. |
Thursday, March 5, 2020 3:30PM - 3:42PM |
U42.00004: Nonlinear anomalous Hall effect for Néel vector detection Ding-Fu Shao, Shuhui Zhang, Gautam Gurung, Wen Yang, Evgeny Y Tsymbal The manipulation and detection of the Néel vector are cores of antiferromagnetic (AFM) spintronics. Recent studies have shown that the field-like and antidamping spin-orbit torques (SOT) can be used to switch the Néel vector in antiferromagnets with proper symmetries. On the other hand, the efficient detection of the AFM order parameter remains a challenging problem. Here, we predict that the nonlinear anomalous Hall effect (AHE) can be used to detect the Néel vector in most compensated antiferromagnets supporting the antidamping SOT. The magnetic crystal group symmetry of these antiferromagnets combined with spin-orbit coupling results in a sizable Berry curvature dipole and hence the nonlinear AHE. As a specific example, we consider half Heusler alloy CuMnSb, which Néel vector can be switched by the antidamping SOT. Based on density functional theory calculations, we show that the nonlinear AHE in CuMnSb produces a measurable Hall voltage under conventional experimental conditions. The strong dependence of the Berry curvature dipole on the Néel vector orientation provides a new detection scheme of the AFM order parameter based on the nonlinear AHE. |
Thursday, March 5, 2020 3:42PM - 3:54PM |
U42.00005: Rotating Néel Order to Probe Crystalline and Non-Crystalline AMR in FeRh Joseph Sklenar, Soho Shim, HIlal Saglam, Kisung Kang, Junseok Oh, Greg A Hamilton, Wei Zhang, Matthew Gilbert, Andre Schleife, Axel Hoffmann, Nadya Mason Anisotropic magnetoresistance (AMR) effects are a promising starting point for the electrical readout of antiferromagnetic memory in a spintronic device. We characterized both crystalline and non-crystalline AMR in thin films of FeRh, a material which undergoes a ferro- to antiferromagnetic transition near room temperature. The resistance is measured as an external field is rotated in the sample plane. In the antiferromagnetic phase we observe a striking dependence of the AMR signal on both field magnitude, and current orientation relative to the FeRh [100] crystalline axis. We confirm that AMR arises from rotating Neel order two ways: 1) We developed a procedure combining rotating and linearly swept fields to demonstrate an angular hysteresis effect; 2) We use density functional theory for a first-principles description of the evolution of the AMR signal, representing the external magnetic field by canted spins and the sweeping field by Néel vector rotation from the [100] direction. |
Thursday, March 5, 2020 3:54PM - 4:06PM |
U42.00006: WITHDRAWN ABSTRACT Jiahao Han, Pengxiang Zhang, Yabin Fan, Taqiyyah S Safi, Junxiang Xiang, Ran Cheng, Luqiao Liu WITHDRAWN ABSTRACT. |
Thursday, March 5, 2020 4:06PM - 4:18PM |
U42.00007: Intrinsic spin Nernst effect of magnons in a noncollinear antiferromagnet Bo Li, Shane Sandhoefner, Alexey Kovalev We investigate the intrinsic magnon spin current in a noncollinear antiferromagnetic insulator. To this end, we derive a magnon spin current in a noncollinear antiferromagnet from continuity equation and find that in general it is not conserved, but for certain symmetries and spin polarizations, the averaged effect of non-conserving terms can vanish. We formulate a general linear response theory for magnons in noncollinear antiferromagnets subject to a temperature gradient and analyze the effect of symmetries on the response tensor. We apply this theory to single-layer potassium iron jarosite KFe3(OH)6(SO4)2 and predict a measurable spin current response. According to the symmetry analysis, many other material candidates should exhibit the spin Nernst effect, including quasi-two- and three-dimensional noncollinear antiferromagnets and antiferromagnets with magnetic textures such as skyrmions. Our proposal can be useful for spin generation in spintronics applications. Furthermore, this effect can also serve as a probe for the materials that can hold multiple noncollinear states. |
Thursday, March 5, 2020 4:18PM - 4:30PM |
U42.00008: Electric-field control of magnon spin current in an antiferromagnetic insulator Changjiang Liu, Yongming Luo, Deshun Hong, Shulei Zhang, john pearson, Brandon Fisher, Axel Hoffmann, Anand Bhattacharya A fundamental challenge in the development of spintronics is the effective control of spin currents by electrical means. This is because the spin degree of freedom is often decoupled from external electric fields, making its control difficult. Over the last few years, the spin Seebeck effect has shown its versatility in generating pure spin currents in a diverse class of magnetic systems. Here we first show how a magnon spin current can be produced in an antiferromagnetic insulator by means of the spin Seebeck effect. The polarity of the spin current is determined by the orientation of the magnetic sublattice in this antiferromagnet. Owning to the response of the magnetic ions to an external electric field, the anisotropy energy of this material may be effectively tuned. As a result, we show that the spin-flop transition in this material can be switched on or off by varying only an electrical voltage. Furthermore, we demonstrate how thermally generated spin currents in this antiferromagnet can be effectively controlled by sweeping the control voltage. |
Thursday, March 5, 2020 4:30PM - 4:42PM |
U42.00009: Phenomenological theory of coherent and incoherent spin currents in antiferromagnetic spin pumping Ran Cheng Magnetic resonance driven by coherent microwaves can pump spin currents into a neighboring nonmagnetic system. Owing to non-linear effects, however, coherentlly driven spin dynamics inevitably generates incoherent magnons through thermalization, which makes it complicated to distinguish between coherent and incoherent contributions in real experiments. Recent measurements have demonstrated spin pumping effects in antiferromagnets at the sub-THz frequency, where coherent and incoherent magnons contribute oppositely to the spin current and are thus distinguishable. We formulate a phenomenological theory to capture the coupled dynamics of coherent and incoherent magnons in collinear antiferromagnets driven by sub-THz sources, which can fit very well with the experimental observation and explain why the competition between coherent and incoherent magnons leads to a sign change in the spin current at a turning temperature. Our phenomenological theory predicts that using magnetic thin films can substantially suppress the undesirable incoherent contribution and magnify coherent spin pumping. |
Thursday, March 5, 2020 4:42PM - 4:54PM |
U42.00010: Coherent ac spin current transmission across an antiferromagnetic CoO insulator Qian Li, Mengmeng Yang, Christoph Klewe, Padraic Shafer, Alpha T. N'Diaye, Dazhi Hou, Tianye Wang, Nan Gao, Eiji Saitoh, Chanyong Hwang, Robert J Hicken, Jia Li, Elke Arenholz, Zi Q. Qiu The recent discovery of spin-current transmission through antiferromagnetic insulating materials opens up vast opportunities for fundamental physics and spintronics applications. The question currently surrounding this topic is: whether and how could THz antiferromagnetic magnons mediate a GHz spin current? This mis-match of frequencies becomes particularly critical for the case of coherent ac spin-current, raising the fundamental question of whether a GHz ac spin-current can ever keep its coherence inside an antiferromagnetic insulator and so drive the spin precession of another ferromagnet layer coherently? Utilizing element- and time-resolved x-ray pump-probe measurements on Py/Ag/CoO/Ag/Fe75Co25/MgO(001) heterostructures, here we demonstrate that a coherent GHz ac spin current pumped by the Py ferromagnetic resonance can transmit coherently across an antiferromagnetic CoO insulating layer to drive a coherent spin precession of the Fe75Co25 layer. Further measurement results favor thermal magnons rather than evanescent spin waves as the mediator of the coherent ac spin current in CoO. |
Thursday, March 5, 2020 4:54PM - 5:06PM |
U42.00011: Spin Current from sub-Terahertz-generated Antiferromagnetic Magnons Junxue Li, Blake Wilson, Ran Cheng, Mark I Lohmann, Marzieh Kavand, Wei Yuan, Mohammed Aldosary, Nikolay I Agladze, Peng Wei, Mark S Sherwin, Jing Shi Spin dynamics in antiferromagnets offers attractive benefits for potential ultrafast device applications. To date, spin current generation via antiferromagnetic resonance and simultaneous electrical detection by the inverse spin Hall effect in heavy metals have not been explicitly demonstrated. Here we report sub-terahertz spin pumping in heterostructures of a uniaxial antiferromagnetic Cr2O3 crystal and a heavy metal of Pt or Ta (Beta-phase). The magnetic resonances in Cr2O3 are excited both below and above spin flop transitions. Both resonances generate pure spin currents in the heterostructures, which are detected by the heavy metal as an open-circuit voltage peak or dip. The pure spin current nature of the electrically detected signals is unambiguously confirmed by the reversal of voltage polarity under two circumstances: one when switching the detector metal from Pt to Ta which reverses the sign of spin Hall angle, and the other when flipping the magnetic field direction which reverses the magnon chirality. |
Thursday, March 5, 2020 5:06PM - 5:18PM |
U42.00012: Quantitative Study on Current-Induced Effects in an Antiferromagnet Insulator/Pt Bilayer Film Pengxiang Zhang, Luqiao Liu Quantitative investigation on the current-induced torque in antiferromagnets represents a great challenge, due to the lack of an independent method for controlling Néel vectors. Here by utilizing an antiferromagnetic insulator with Dzyaloshinskii-Moriya interaction, α-Fe2O3, we show that the Néel vector can be controlled with a moderate external field, which is further utilized to calibrate the current-induced magnetic dynamics. In a Pt/α-Fe2O3(0001) bilayer film, we see that the current-induced magnetoresistance change in antiferromagnets can be complicated by resistive switching that is non-magnetic. By excluding non-magnetic switching, we reveal two significant magnetic contributions, the magnetoelastic effect and the field-like spin-orbit torque, and quantify their magnitudes. Our approach in principle is applicable to many other easy plane antiferromagnets, and would be beneficial for future systematic studies of current-induced antiferromagnetic dynamics. |
Thursday, March 5, 2020 5:18PM - 5:30PM |
U42.00013: Resonant ultrasound studies of Fex=0.33NbS2 Sylvia Lewin, Vikram Nagarajan, Gabriel Perko-Engel, Eran Maniv, Shannon Haley, James Analytis The intercalated transition metal dichalcogenide Fex=0.33NbS2 appears to exhibit frustrated antiferromagnetism. The orientation of its antiferromagnetic order can be switched by the application of electrical currents [1]. Similar behavior at room temperature could form the basis of a new generation of magnetic memory storage. However, the specific magnetic order of Fex=0.33NbS2 and how it evolves with temperature have not been exactly determined, leaving open questions about the microscopic mechanism of switching. Resonant Ultrasound Spectroscopy—a study of the material’s mechanical resonances—can shed light on the magnetic order in Fex=0.33NbS2. |
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