Bulletin of the American Physical Society
APS March Meeting 2020
Volume 65, Number 1
Monday–Friday, March 2–6, 2020; Denver, Colorado
Session D47: Spin Dynamics in Low Dimensional MaterialsFocus

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Sponsoring Units: GMAG Chair: Se Kwon Kim, Univ of Missouri  Columbia Room: 710/712 
Monday, March 2, 2020 2:30PM  2:42PM 
D47.00001: Planar Hall driven torque in a FM/NM/FM system Christopher Safranski, JunWen XU, Andrew Kent, Jonathan Z Sun Electrical manipulation of magnetization is an intensely studied topic with goals of producing energy efficient nanodevices. The charge to spin current conversion in bilayers of magnetic and nonmagnetic materials is one area of investigation. Typically studied materials have been mostly limited to the generation of inplane polarized spin currents. We investigate spin currents produced by the planar Hall effect in Co/Ni multilayers, which carry a polarization dictated by the FM magnetization direction. In a sample based on CoNi/Au/CoFeB, spin torque ferromagnetic resonance is used to measure the dampinglike torque on the CoFeB layer. The response as a function of the applied field angle and current is consistent with the symmetry expected for a torques produced by the planar Hall effect. We find the strength of this effect to be comparable to that of the spin Hall effect. However, unlike the spin Hall effect, it can produce a partially out of plane spin polarization. Our results indicate that the planar Hall effect holds potential as a spin current source with a controllable polarization direction. 
Monday, March 2, 2020 2:42PM  2:54PM 
D47.00002: Excitonphonon damping in the two and threedimensional anisotropic XY model Leonardo dos Santos Lima

Monday, March 2, 2020 2:54PM  3:06PM 
D47.00003: Probing Spinwave Excitations in Layered Chromium Trihalides with NitrogenVacancy Magnetometers in Diamond Ruolan Xue, Mark JenHao Ku, Dahlia Klein, David MacNeill, Takashi Taniguchi, Kenji Watanabe, Ronald L Walsworth, Pablo JarilloHerrero, Amir Yacoby Nitrogenvacancy (NV) centers in diamond are highly sensitive, nanoscale magnetic sensors. NV magnetometry has demonstrated detection of spinwave excitations in bulk magnets and has recently been used as a local probe to detect static magnetization in layered magnets. While monolayer and fewlayer van Waals magnets have been discovered and their static properties investigated, spinwave excitations in these materials remains unexplored. Understanding lowenergy excitations in layered magnets is crucial to further elucidate dynamical properties of 2D spin systems and to explore applications of these materials. In this work, we report progress towards probing spinwave excitations in chromium trihalides with NV magnetometry. 
Monday, March 2, 2020 3:06PM  3:18PM 
D47.00004: Exceptional nonreciprocity in nonHermitian spin dynamics Alexey Galda, Valerii Vinokur We investigate topological structure of complex energy surfaces of nonequilibrium magnetic systems. We consider spintransfer torquedriven singlespin model described by a nonHermitian Hamiltonian. The spectrum of the Hamiltonian admits branch point singularities that appear as chiral exceptional points where both eigenvalues and eigenstates coalesce. The emergence of exceptional spectral points provides access to a wealth of topological effects unique to nonHermitian spin systems, e.g. nonreciprocal spin dynamics. An implementation of an asymmetric spinfilter device based on this effect is proposed. 
Monday, March 2, 2020 3:18PM  3:30PM 
D47.00005: Quantized ΔS=2 excitation spectra by confinement in an S=1 Ising spin chain under magnetic fields Seiichiro Suga, Takafumi Suzuki We calculate the dynamical spinstructure factor of the S = 1 Ising spin chain with negative singleion anisotropy in transverse and longitudinal magnetic fields using the infinite timeevolvingblockdecimation algorithm [1]. We show that when only a transverse magnetic field is applied, both the ΔS = 2 excitation continuum and onemagnon mode appear in the lowlying excitation. When a longitudinal magnetic field is further applied, the excitation continuum changes into quantized spectra. The quantized ΔS = 2 excitation spectra originate from the confinement of two domain walls, each of which carries ΔS = 1. The quantized excitation energies are explained by the zeros in the Airy function. [1] T. Suzuki and S. Suga, J. Phys. Soc. Jpn. 88, 053702 (2019). 
Monday, March 2, 2020 3:30PM  3:42PM 
D47.00006: Spin dynamics in interacting ferromagnetic discs arranged on a Kagome lattice Mojtaba Taghipour Kaffash, Sergi Lendinez, Wonbae Bang, Axel Hoffmann, John B. Ketterson, Matthias Benjamin Jungfleisch Artificial spin ice (ASI) consists of periodic arrays of nanomagnets in the shape of elongated elements where competing interactions between the elements lead to geometric frustration. Recently, nonIsinglike ASI have attracted great attention due to their exotic phase diagrams. One example is an array of ferromagnetic nanodiscs. 
Monday, March 2, 2020 3:42PM  4:18PM 
D47.00007: Magnetization reversal driven by low dimensional chaos in a nanoscale ferromagnet Invited Speaker: Eric Montoya Energyefficient magnetization switching is an essential problem in the realization of practical nonvolatile magnetic storage [1] and magnetic neuromorphic computing [2]. In the past two decades, several efficient methods of magnetic switching were demonstrated including spin torque, magnetoelectric, and microwaveassisted switching mechanisms. In this talk, we experimentally demonstrate that low dimensional magnetic chaos [3] induced by alternating spin torque can strongly increase the rate of thermallyactivated magnetic switching of the free layer in a magnetic tunnel junction (MTJ)[4]. This mechanism exhibits a wellpronounced threshold character in spin torque amplitude and its efficiency increases with decreasing spin torque frequency. We present analytical and numerical calculations that quantitatively explain these experimental findings and reveal the crucial role played by low dimensional magnetic chaos near saddle equilibria in enhancement of the switching rate. This work shows that ac spin torque driven chaos can facilitate thermallyassisted switching of magnetization in a MTJ [5] and provides a new path towards improved energy efficiency of spin torque memory based on thermally stable MTJs. Furthermore, MTJs with superparamagnetic free layers are attractive for neuromorphic computing. Our results show that low dimensional can be used to tune the switching rate of such systems, and therefore, may lead to computing schemes that simultaneously harness stochasticity and deterministic chaos. 
Monday, March 2, 2020 4:18PM  4:30PM 
D47.00008: Effects of LowAtomicNumber Dopants on Magnetic Relaxation in Epitaxial Fe Alloys David Smith, Youngmin Lim, Michael Clavel, Zijian Jiang, Tim Hartnett, Mantu Hudait, Jean J Heremans, Prasanna V Balachandran, Dwight D Viehland, Satoru Emori We investigate the dependence of magnetic relaxation on dopant concentration in epitaxial thin films of Fe_{1x}V_{x} and Fe_{1x}Al_{x} grown by magnetron sputtering. By substituting Fe with lighter elements, one can expect to observe a reduction in the Gilbert damping parameter due to reduced spinorbit coupling. The magnetic properties of these alloys were determined using broadband ferromagnetic resonance. For low concentrations of V, we observe a reduction in magnetic relaxation (e.g., effective Gilbert damping), consistent with previous experimental [1] and theoretical [2] results. In contrast, introducing Al causes a monotonic increase in magnetic relaxation. We attribute these behaviors to a modification in the density of states at the Fermi level, as shown by our density functional theory calculations. Our results serve as an avenue for predicting how to reduce Gilbert damping for applications in energyefficient spintronic devices. 
Monday, March 2, 2020 4:30PM  4:42PM 
D47.00009: Spin dynamics in La_{0.67}Sr_{0.33}MnO3/YBa_{2}Cu_{3}O_{7δ} heterostructures Jacob Wisser, Lauren Riddiford, Satoru Emori, Yuri Suzuki Spin polarization in superconductors has largely been studied via injection of spinpolarized electrical currents, with relatively little attention paid to spin currents generated via spin wave excitations. To this end, we have fabricated heterostructures of the cuprate superconductor yttrium barium copper oxide (YBCO) with the halfmetallic ferromagnet lanthanum strontium manganite (LSMO) via pulsed laser deposition on (LaAlO_{3})_{0.3}(Sr_{2}TaAlO_{6})_{0.7} (001) substrates. In this study, we fix the LSMO thickness at 30 nm and vary the YBCO thickness from 217 nm. We have verified that the YBCO superconducts with T_{c} varying from 5588 K depending on YBCO thickness (with a minimum thickness of 5 nm required for superconductivity). To characterize spin wave transport, we perform ferromagnetic resonance (FMR) as a function of temperature and YBCO thickness. We find that the resonance linewidth of bare LSMO increases with decreasing temperature, which is attributed to increased twomagnon scattering at lower temperatures. Additionally, with the addition of 17 nm of YBCO, we see a near doubling of the FMR linewidth at all temperatures, indicative of spin pumping into the YBCO and possibly the coexistence of spin polarization with superconductivity. 
Monday, March 2, 2020 4:42PM  4:54PM 
D47.00010: Tuning the dynamic behavior of a bicomponent artificial spin ice Sergi Lendinez, Mojtaba Taghipour Kaffash, Matthias Benjamin Jungfleisch Artificial spin ices (ASI) are magnetic metamaterials initially designed to mimic the frustrated behavior of crystalline spin ice systems such as pyrochlore crystals. They show complex magnetic ordering and can exhibit exotic phase diagrams. Unlike their crystalline counterparts, the ASI geometry can be designed and the state of their constituent elements can be directly probed. Recent studies have focused on their dynamics at high frequencies (GHz to tens of GHz), as they can be used as magnonic metamaterials to modify the spinwave properties by creating band gaps in the resonance spectra. Previous works have shown that different geometries produce unique dynamic spectra. Here, we performed angulardependent broadband ferromagnetic resonance measurements on a square ASI composed of different materials for each sublattice (NiFe and FeCo). Our experiments show that the interaction between the sublattices results in unique spectra attributed to the sublattices. By performing micromagnetic simulations, we identify the modes observed in the experiment. Our results show that the interaction in the ASI can be tuned not only by the geometry of the lattice, but also by the proper choice of the materials. 
Monday, March 2, 2020 4:54PM  5:06PM 
D47.00011: Abinitio spinlattice dynamics based on the tightbinding method Simon Streib, Danny Thonig, Manuel Pereiro, Anders Bergman, Erik Sjöqvist, Anna Delin, Olle Eriksson Recent experiments on ultrafast demagnetization have shown that the magnetization dynamics on ultrafast timescales involves both magnetic and lattice degrees of freedom. We present a method for the quasiclassical simulation of coupled spin and lattice dynamics in metallic magnets based on a tightbinding (TB) model. The TB parameters are fitted to abinitio data and take the structure dependence of the material into account. From the TB model we calculate both magnetic and lattice forces for the integration of the equations of motion. We use the CAHMD package [1], which implements the TB model and the atomistic spinlattice dynamics. Our approach opens the door for a better understanding of the transfer of angular momentum between magnetization and lattice already on the electronic structure level. We demonstrate the feasibility of our method by simulations of chains and clusters. 

D47.00012: Enhancement of damping factor in Fe/Bi_{2}Se_{3} heterostructures via topological surface states modification li na, Rui Sun, Yang Xu, Yunbin Sun, Wei He, Xiangqun Zhang, Zhaohua Cheng Owing to the spinmomentumlocked surface state of topological insulators (TIs), the spin dynamics in ferromagnetic materials(FM)/TI heterostructures is expected to be significantly different with that in FM/nonmagnetic heavy metals. Previous studies suggested that TI can modulate the spin dynamics of FM layer, however, the temperature dependence of damping factors is controversial. Here, we report spin dynamics in Fe/Bi_{2}Se_{3} heterostructures by changing thickness of Bi_{2}Se_{3} with and without topological surface state (TSS). Compared with that in Fe/Bi_{2}Se_{3}(3QL), where Bi_{2}Se_{3} containing only bulk state without TSS, a significant enhancement of damping factor was achieved in Fe/Bi_{2}Se_{3}(9QL) by TSS of Bi_{2}Se_{3}. On the basis of ab initio electronic structure calculations, we confirm that the enhancement of damping factor is originated from the strong band hybridizations of Fe and Bi_{2}Se_{3}(9QL) near the Fermi level. Furthermore, by inserting Cu layer between Fe and Bi_{2}Se_{3}, the contribution of spin pumping and magnetic proximity effect to the overall damping factor was separated. Our work not only reveals the mechanism of TSSinduced damping factor, but provides a new freedom to tailor the damping factor of FM/TI spintronic devices as well. 
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