Bulletin of the American Physical Society
APS March Meeting 2018
Monday–Friday, March 5–9, 2018; Los Angeles, California
Session C22: Antiferromagnetic and Topological SpintronicsFocus

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Sponsoring Units: GMAG DMP FIAP Chair: YuanMing Lu, The Ohio State University Room: LACC 402A 
Monday, March 5, 2018 2:30PM  2:42PM 
C22.00001: Spin and Anomalous Hall Conductivity of the Chiral Antiferromagnet PtMn3 Steve Novakov , John Heron Efficient chargetospin current conversion in materials is crucial to the development of spintronic memory or logic devices. A promising and established method of spin current generation is the injection of charge current through a crystal with a strong intrinsic spin Hall conductivity. Recently, a class of antiferromagnets with the composition XMn_{3}, where X={Pt, Ir, Rh}, have been identified as materials with large intrinsic spin Hall conductivities stemming from their nontrivial spin order [1]. The exact role of antiferromagnetic spin texture on the generated spin current, however, is not fully understood. Temperaturedependent triangular AFM  collinear AFM phase transitions in chemically ordered PtMn_{3} can be exploited to probe this directly [2]. Here, we report on the growth and magnetotransport measurement of ordered PtMn_{3} thin films. Harmonic transport measurement techniques utilizing spintransfer torques were performed to determine the spin and anomalous Hall conductivities at select temperatures [3]. 
Monday, March 5, 2018 2:42PM  2:54PM 
C22.00002: Roomtemperature Ferromagnetism of Cobalt Ultrathin Films Covered on a Monolayer MoS_{2} ChunI Lu , ChihHeng Huang , KunTa Lu , TzuHung Chuang , KaiShin Li , YannWen Lan , DerHsin Wei MoS_{2} has been shown a widely application, including spintronics: ferromagnetic (FM) materials/MoS_{2} spinvalve devices [1]. However, a direct observation of the magnetic properties of such interface is still lack. In this report, we focus on the FM behavior of the Co thin films cover on a monolayer (ML) MoS_{2}. First, a series of magnetooptical Kerr effect (MOKE) experiments of Co on an amorphous MoS_{2} surface confirm that the inplane magnetization signal exists when the Co's thickness is above 5 ML. Therefore, we performed a series of insitu experiments with different thicknesses of Co on monolayer MoS_{2} triangular islands at the Xray photoemission electron microscopy (XPEEM) endstation at BL05B2 of Taiwan Light Source. Clear spatiallyresolved magnetic domain images were obtained, and the tendency of thicknessinvariance and the preferred directions of domain boundary are observed. We further covered 3 ML of Fe on the Co surface. The domain images of Fe and Co indicate the ferromagnetically coupled with each other. 
Monday, March 5, 2018 2:54PM  3:06PM 
C22.00003: HighFrequency Spin Pumping from Insulating Antiferromagnet MnF_{2} Priyanka Vaidya , Amit KC , Johan Van Tol , David Lederman , Enrique Del Barco Antiferromagnetic materials, particularly antiferromagnetic insulators provide an alternative to present ferromagnetic spintransfer torque based devices which suffer from limitations in terms of density (e.g., because neighboring units can couple through stray fields) and speed (frequencies are limited to the GHz range). In contrast to ferromagnets, where magnetic anisotropy dominates spin dynamics, in antiferromagnets spin dynamics are governed by the interatomic exchange interaction energies which are orders of magnitude larger than the magnetic anisotropy energy, leading to the potential for ultrafast information processing and communication in the THz frequency range. We will present studies of spin pumping at Manganese Difluoride(MnF_{2}) / Platinum (Pt) interfaces at temperatures below the MnF_{2} Néel temperature (T_{N} = 67.34K). In particular, measurements of the inverse spin Hall effect (ISHE) voltage arising from the interconversion of the dynamically injected spin currents into Pt will be reported. We observe a clear electrostatic potential signal coinciding with the MnF_{2} spinflop transition (H_{SF} = 9T). The signal reverses by switching the polarity of the magnetic field, and displays a marked dependence on the power of the microwave stimuli, as expected from the ISHE. 
Monday, March 5, 2018 3:06PM  3:42PM 
C22.00004: Topological Antiferromagnetic Spintronics Invited Speaker: Jairo Sinova Antiferromagnetic spintronics considers the active manipulation of the antiferromagnetic order parameter in spinbased devices. An additional concept that has emerged is that antiferromagnets provide a unifying platform for realizing synergies among three prominent fields of contemporary condensed matter physics: Dirac quasiparticles and topological phases. Here spintronic devices made of antiferromagnets with their unique symmetries will allow us to control the emergence and to study the properties of Dirac/Weyl fermion topological phases that are otherwise principally immune against external stimuli. In return, the resulting topological magnetotransport phenomena open the prospect of new, highly efficient means for operating the antiferromagnetic memorylogic devices. We discuss how these topological phases emerge and how their robustness depends on the relative orientation of the Neel order parameter that can be manipulated by Neel spinorbit torques. Their natural excitations are in the THz but with the additional consideration that they can now be directly tuned. 
Monday, March 5, 2018 3:42PM  3:54PM 
C22.00005: Prediction of a magnetic Weyl semimetal and strong anomalous Hall effect in the Heusler compensated ferrimagnet Ti_{2}MnAl Yan Sun , Wujun Shi , Lukas Muechler , Kaustuv Manna , Yang Zhang , Klaus Koepernik , Roberto Car , Jeroen Van den Brink , Claudia Felser We predict a magnetic Weyl semimetal in the inverse Heusler Ti_{2}MnAl, a compensated ferrimagnet with a vanishing net magnetic moment and a Curie temperature of over 650 K. Despite the vanishing net magnetic moment, we calculate a large intrinsic anomalous Hall effect (AHE) of about 300 S/cm. It derives from the Berry curvature distribution of the Weyl points, which are only 14 meV away from the Fermi level and isolated from trivial bands. Different from antiferromagnets Mn_{3}X(X= Ge, Sn, Ga, Ir, Rh, and Pt), where the AHE originates from the noncollinear magnetic structure, the AHE in Ti_{2}MnAl stems directly from the Weyl points and is topologically protected. Moreover, because of the large separation between Weyl points of opposite topological charge, the Fermi arcs extent up to 75% of the reciprocal lattice vectors in length. This makes Ti_{2}MnAl an excellent candidate for the comprehensive study of magnetic Weyl semimetals. It is the first example of a material with Weyl points, large anomalous Hall effect and angle despite a vanishing net magnetic moment. 
Monday, March 5, 2018 3:54PM  4:06PM 
C22.00006: Unidirectional Spin Hall Magnetoresistance in Y_{3}Fe_{5}O_{12}/NiO/Pt Structure Egecan Cogulu , Debangsu Roy , Houchen Chang , Mingzhong Wu , Hendrik Ohldag , Andrew Kent The discovery of unidirectional spinHall magnetoresistance (USMR) in heavy metal (HM)/ferromagnetic(FM) bilayers such as Pt/Co and Ta/Co provides a means of studying spin orbit torque switching of the ferromagnetic layer without a third terminal. The USMR depends on the current density J and the magnetization direction m, as (Jxz).m implying that this effect is nonlinear, in contrast to spinHall magnetoresistance(SMR) which depends on m^{2}. In this work, we demonstrate that USMR is associated with antiferromagnetHM interfaces by studying Y_{3}Fe_{5}O_{12 }(YIG)/(x) NiO/5nm Pt structures. The USMR is measured with the YIG magnetized in the xy, xz and yz planes, using a lockin technique that probes the response at 2ω. Unlike Ta/Co or Pt/Co bilayers where a current of amplitude of 10^{8}A/cm^{2} is required, the USMR signal appears at only 10^{6}A/cm^{2}. The contribution of USMR has been separated from the anomalous Nernst and spin Seebeck effect by varying the current amplitude and magnetic field. We have carried out temperature dependent USMR measurements as well. We note that USMR may prove a useful probe of spindynamics and spintorque switching in antiferromagnetic layers. 
Monday, March 5, 2018 4:06PM  4:18PM 
C22.00007: Large Intrinsic Spin Hall Effect in the A15 Superconductor Family: A Topology and Symmetry based Design Strategy Yan Sun , Elena Derunova , Stuart S Parkin , Binghai Yan , Mazhar Ali The spin Hall effect (SHE) is the conversion of a charge current to a spin current and nonmagnetic metals with a large SHE are useful for a variety of spintronic applications, but their rarity has stifled their widespread use. Here we predict the presence of a large intrinsic SHE in the A15 family of superconductors: Ta_3Sb, Nb_3Au, and Cr_3Ir having spin hall conductivities of 1400, 1060, and 1210 hbar/e*(Omega*cm)^1, respectively. Combining concepts from topological physics with the dependence of the SHE on the Berry curvature of the Fermi surface, we propose a simple strategy to design materials with a large intrinsic SHE based on the following ideas: that high symmetry combined with heavy atoms can give rise to multiple Dirac/Weyl crossings, that these crossings can gap due to spin orbit coupling without sufficient symmetry protection, and that gapped Dirac/Weyl crossings can create a large Berry curvature. We use these to explain why betaW and Pt have a large intrinsic SHE and present a family of sputterable, large SHE compounds and alloys (e.g. W_3Ta, 2250). This represents the first application of topological physics in state of the art technology and, with tuning of extrinsic parameters, will represent a new direction in the route to efficient charge/spin conversion. 
Monday, March 5, 2018 4:18PM  4:30PM 
C22.00008: Independence of spinorbit torques from the exchange bias direction in Ni_{81}Fe_{19}/IrMn bilayers Hilal Saglam , Juan Carlos Rojas Sanchez , Sebastien PetitWatelot , Michel Hehn , Wei Zhang , John Pearson , Stephane Mangin , Axel Hoffmann It has recently been demonstrated that metallic antiferromagnets have sizeable spin Hall effects [1,2] and therefore can act as active components in spintronics devices. At the same time, interfacial exchange coupling in ferromagnet (FM)/antiferromagnet (AF) bilayers are commonly used for generating reference magnetization directions. Here we investigated whether there is a direct correlation between exchange bias and spin orbit torques, since both are effects that rely on interfacial magnetic interactions. We fabricated Py (Ni_{81}Fe_{19})/IrMn bilayers and used spin transfer torque ferromagnetic resonance measurements with different relative orientations between applied fields and exchange bias. These measurements revealed no significant dependence of spin orbit torques on the exchange bias field directions. 
Monday, March 5, 2018 4:30PM  4:42PM 
C22.00009: Transport Properties of Antiferromagnetic Tunneling Junctions Kei Yamamoto , Georg Schwiete , Olena Gomonay , Jairo Sinova We study a ferromagneticantiferromagneticnormal metal tunneling junction using nonequilibrium Green's function techniques and derive microscopic formulae for various types of spin torque as well as charge and spin conductance. Relative strength of the torques and their dependence on control parameters such as the tunneling probabilities to the two leads are presented. We also discuss dynamical consequences of the different torques. Charge and spin transmission is shown to depend on the angle between the ferromagnetic and antiferromagentic order parameters. We suggest potential applications of these angular dependences. 
Monday, March 5, 2018 4:42PM  4:54PM 
C22.00010: Electronic structure calculations of intrinsic magnetic properties in antiferromagnetic CuMnAs Ivan Zhuravlev , Anil Adhikari , Kirill Belashchenko A lot of attention has been recently directed towards storing and manipulating information in the order parameter of an antiferromagnetic metal. An antiferromagnet creates no stray fields and is stable against demagnetization. Antiferromagnets with broken space inversion symmetry like tetragonal CuMnAs are of interest because their order parameter can be manipulated by currentinduced torques [1,2,3]. The magnetocrystalline anisotropy (MCA) of CuMnAs is easyplane [1], which is unfavorable for storing information. Here we predict, using firstprinciples calculations in the coherent potential approximation, that CuMnAs experiences a spinreorientation transition under a small hole doping, which can be achieved by substituting 23% of Ge, Si, B, or Al for As. The maximum easyaxis MCA is reached at 1015% substitution, while the exchange coupling is insensitive to this substitution. Calculations using the disordered local moment method reveal a monotonic temperature dependence of MCA with no anomalies. 
Monday, March 5, 2018 4:54PM  5:06PM 
C22.00011: Antiferromagnetic resonance excited by oscillating electric currents Volker Sluka

Monday, March 5, 2018 5:06PM  5:18PM 
C22.00012: Planar Hall effect induced by spinorbit coupling in a thinfilm MnTe Gen Yin , JieXiang Yu , Yizhou Liu , Roger Lake , Jiadong Zang , Kang Wang Recent developments in antiferromagnetic spintronics have demonstrated that a special case of anomalous Hall effect: planar Hall effect (PHE) or anisotropic Hall effect can be employed as a reliable readout scheme. This has been known to originate from the spinorbit coupling effect in many bulk antiferromagnetic crystals. Here we theoretically show that the PHE in MnTe thin films comes from the Fermi rings at the gamma point induced by a unique type of spinorbital coupling. Firstprinciple calculations and grouptheory analysis demonstrate that the rotation symmetry of a fourthorder k dot p model has a period of π, which explains the behavior of the PHE signal in a rotating inplane magnetic field. Furthermore, an analytical solution to the Boltzmann transport equation explicitly demonstrates that the Hall effect may originate from the spindependent scattering off spinpolarized defects in the antiferromagnetic order. 
Monday, March 5, 2018 5:18PM  5:30PM 
C22.00013: Magnon drag thermopower of the antiferromagnetic semiconductor Li doped MnTe Yuanhua Zheng , Tianqi Lu , Morteza Rasoulianboroujeni , Huaizhou Zhao , Daryoosh Vashaee , Joseph P Heremans In antiferromagnetic semiconductors, magnons enhance thermopower by interacting with electrons, creating an advective transport process called the magnon drag thermpower. The hydrodynamic model of magnon drag effect (1) predicts that the magnon drag thermopower is proportional to the magnon specific heat and inversely proportional to the total number of free electrons. We report here specific heat and thermopower data of Li doped MnTe. The specific heat is analyzed in terms of an electron, phonon and magnon contribution. The thermopower is analyzed in terms of a diffusion and a magnon drag contribution. Based on the results of specific heat and Hall measurements, we calculate the diffusion thermopower and magnon drag thermopower using the hydrodynamic model. The calculation agrees well with the experiment, and shows that the magnon drag contribution is one order of magnitude larger than the diffusion thermopower. Surprisingly, the magnondrag contribution seems to extend in the paramagnetic regime above the Neel temperature of 307 K. 
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