Bulletin of the American Physical Society
APS March Meeting 2019
Volume 64, Number 2
Monday–Friday, March 4–8, 2019; Boston, Massachusetts
Session K41: Chiral Magnetism and Structures IFocus

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Sponsoring Units: GMAG DMP Chair: Daniele Pinna, Johannes Gutenberg Universität Mainz Room: BCEC 209 
Wednesday, March 6, 2019 8:00AM  8:36AM 
K41.00001: Rise of Chiral Spintronics Invited Speaker: SeeHun Yang Chirality is one of the fundamental asymmetries in nature. Recently chiral nature of magnetic and nonmagnetic structures have been enormously highlighted and arisen to be very useful for potential application to not only spintronics but biology and pharmaceutical industries For example, it has been reported that the combination with spinorbit interaction such as spin Hall effect can be very efficient in manipulation of magnetic elements [1], and spin currents in chiral molecules induced by chirality interacts with magnetic structures [2]. I will present recent remarkable progress in emergent novel phenomena associated with chiral properties from not only emergent magnetic nanostructures but chiral molecules: spinorbit torques from perpendicularly magnetized ultrathin films [2], exchange coupling torque [3] and entirely new phase of chiral exchange drag [4] from synthetic antiferromagnets, and separation of chiral molecules by magnetic structure: enantiomer [5]. In the end I will discuss potential applications and promising outlooks from these new findings. 
Wednesday, March 6, 2019 8:36AM  8:48AM 
K41.00002: Unit cell thick ferrimagnetic Mn_{3}Z Heusler Domain Wall motion Panagiotis Ch. Filippou, Jaewoo Jeong, Yari Ferrante, SeeHun Yang, Teya Topuria, Mahesh G. Samant, Stuart S Parkin Antiferromagnets and ferrimagnets are of particular interest for spintronic devices. In this talk, we present the Domain Wall (DW) motion of Mn_{3}Z (Z=Ge, Sn, Sb) Heuslers, using the chemical templating layer (CTL) concept. Unit cell thick, ferrimagnetic, binary Heusler compounds, have low magnetization and high perpendicular magnetic anisotropy in their tetragonally distorted forms, can sustain current driven DW motion in nanowire racetracks of 129 ms^{1} with the lowest current density to initiate motion of 2.8 10^{6} A cm^{2}. The direction of the DW motion is determined by the bulk spin polarization of the Heusler. Moreover, the DW motion shows contribution by chiral spin orbit torques when studied under inplane fields. We discuss the complex mechanism of the DW motion in Heusler films and identify a bulk DzyaloshinskiiMoriya interaction and a spin Hall effect deriving from the CTLs. 
Wednesday, March 6, 2019 8:48AM  9:00AM 
K41.00003: Binding a Hopfion in Chiral Magnet Nanodisk Yizhou Liu, Roger Lake, Jiadong Zang Hopfions are threedimensional topological textures characterized by the integer Hopf invariant $Q_H$. Here, we present the realization of a zerofield, stable hopfion spin texture in a magnetic system consisting of a chiral magnet nanodisk sandwiched by two films with perpendicular magnetic anisotropy. The preimages of the spin texture and numerical calculations of $Q_H$ show that the hopfion has $Q_H=1$. Furthermore, another nontrivial state that includes a monopoleantimonopole pair (MAP) is also stabilized in this system. By applying an external magnetic field, hopfion and MAP states with the same polarization can be switched between each other. The topological transition between the hopfion and the MAP state involves a creation (annihilation) of the MAP and twist of the preimages. Our work paves the way to study nontrivial 3D topological spin textures and stimulates more investigations in the field of 3D spintronics. 
Wednesday, March 6, 2019 9:00AM  9:12AM 
K41.00004: Visualizing Chiral Ferrimagnetism in Amorphous GdCo Films Robert Streubel, CharlesHenri Lambert, Noah Kent, Peter Ercius, Alpha N'Diaye, Colin Ophus, Sayeef Salahuddin, Peter Fischer Inversion symmetry breaking has become a thriving research in modern magnetism with phenomena including Rashba effect, spin Hall effect and DzyaloshinskiiMoriya interaction (DMI). The latter one may stabilize chiral spin textures with topologically nontrivial properties, such as Skyrmions. So far, chiral spin textures have mainly been studied in helimagnets and thin ferromagnets with heavyelement capping. 
Wednesday, March 6, 2019 9:12AM  9:24AM 
K41.00005: Beyond Baby Skyrmions: principle chiral model in noncollinear magnets Cristian Batista, Mikhail A Shifman, Zhentao Wang, ShangShun Zhang We discuss noncollinear magnetic phenomena whose local order parameter is characterized by more than one spin vector. By focusing on the simple cases of 2D triangular and 3D pyrochlore lattices, we demonstrate that their lowenergy theories can be described by a oneparametric class of sigma models continuously interpolating between the classical Heisenberg model and the principal chiral model. A natural consequence of our derivation is that stable Skyrmion of type π_{3}(SO(3)) can be generically induced in noncollinear magnets. 
Wednesday, March 6, 2019 9:24AM  9:36AM 
K41.00006: Boundary twists, instabilities, and creation of skyrmions and antiskyrmions Alexey Kovalev, Aldo Raeliarijaona, Rabindra Nepal We formulate and study the general boundary conditions dictating the magnetization profile in the vicinity of an interface between magnets with dissimilar properties. We show that in general case the boundary conditions lead to the magnetization profile corresponding to the N\'eel, Bloch, or intermediate twist due to DzyaloshinskiiMoriya interactions. We explore how such twists can be utilized for creation of skyrmions and antiskyrmions, e.g., in a view of magnetic memory applications. To this end, we study various scenarios how skyrmions and antiskyrmions can be created from interface magnetization twists due to local instabilities. We also show that a judicious choice of DzyaloshinskiiMoriya tensor (hence a carefully designed material) can lead to local instabilities generating certain types of skyrmions or antiskyrmions. The local instabilities are shown to appear in solutions of the BogoliubovdeGennes equations describing ellipticity of magnon modes bound to interfaces. In one considered scenario, a skyrmionantiskyrmion pair can be created due to instabilities at an interface between materials with properly engineered DzyaloshinskiiMoriya interactions. We use micromagnetics simulations to confirm our analytical predictions. 
Wednesday, March 6, 2019 9:36AM  9:48AM 
K41.00007: Transport signatures of interfacial exchange coupling and chiral spin textures in magnetic insulator thin films Qiming Shao, Yawen Liu, Alexander Grutter, Guoqiang Yu, Se Kwon Kim, Xiaoyu Che, Chi Tang, Yaroslav Tserkovnyak, Jing Shi, Kang L. Wang Ferrimagnetic insulators (FMIs) attract tremendous interest for spintronic applications due to high characteristic frequency, low Gilbert damping, and absence of Ohmic loss. First, we demonstrate the critical role of dimensionality on the SOT efficiency by systematically studying the FMI layer thickness dependent SOT efficiency in tungsten/thulium iron garnet (W/TmIG) bilayers. Currentinduced switching in FMI thin films is demonstrated with a thickness up to 15 nm [1]. Second, we use temperature dependent Hall measurements to identify contributions of spin Hall, magnetic proximity, and sublattice effects to the anomalous Hall signal in various heavy metal/ferrimagnetic insulator heterostructures. This approach enables detection of both the magnetic proximity effect onset temperature and magnetization compensation temperature and provides essential information regarding the interfacial exchange coupling. At last, we show the transport signatures of chiral spin textures, topological Hall effect, in the platinum/ferrimagnetic insulator bilayer. [1] Q. Shao, et al., Nat. Commun., 9, 3612 (2018) 
Wednesday, March 6, 2019 9:48AM  10:00AM 
K41.00008: Z_{2} topological invariant for magnon spin Hall systems Hiroki Kondo, Yutaka Akagi, Hosho Katsura The classification and characterization of different phases of matter based on the topology of band structures has recently attracted much attention. It is known that some of the topological phenomena in electron systems are carried over to bosonic systems [1]. However, the topological invariant of bosonic systems with timereversal symmetry has not yet been identified, as Kramers' theorem cannot be applied to them. To address this issue, we first introduce the pseudotimereversal operator which ensures the existence of ``Kramers pairs'' of bosons. Then, we define the Z_{2} topological invariant for magnon spin Hall systems [2] using the Berry connection and curvature for bosons, which are different from those of electrons [3]. Furthermore, we propose two magnetic models with magnon bands carrying the nontrivial Z_{2} topological invariant. We also demonstrate that the presence (absence) of helical edge states corresponds to the nontrivial (trivial) value of the Z_{2} topological invariant. 
Wednesday, March 6, 2019 10:00AM  10:12AM 
K41.00009: Observation of longwavelength modulation in chiral Mn_{1/3}NbS_{2} Sunil Karna, David P Young, Frank N. Womack, Yan Wu, Huibo Cao, Lisa DeBeerSchmitt, Philip W Adams, John Ditusa We have investigated the magnetic properties of Mn_{1/3}NbS_{2}, through neutron diffraction and magnetic susceptibility measurements. Mn_{1/3}NbS_{2} crystallizes in the noncentrosymmetric hexagonal space group P6_{3}22 and is built from NbS_{2} layers intercalated by Mn ions. The Mn atoms are situated in the octahedral holes between trigonal prismatic layers of 2HNbS_{2}. The magnetic susceptibility and magnetization indicate a magnetic transition at T_{N} ~ 45 K with an anisotropy that is similar to that observed in Cr_{1/3}NbS_{2}. Neutron diffraction reveals increased scattering near the structural Bragg peaks. However, our data indicate that the magnetic scattering had a wider qdependence than the nuclear Bragg peaks suggesting a long wavelength modulation of the magnetic ordering along the caxis. 
Wednesday, March 6, 2019 10:12AM  10:24AM 
K41.00010: Investigating the stability of incommensurate spin textures in GaV_{4}S_{8} Eleanor Clements, Ganesh Pokharel, David George Mandrus, Hariharan Srikanth, ManhHuong Phan The polar noncentrosymmetric GaV_{4}S_{8} hosts a Néel skyrmion lattice (SkL) below T_{C} ~ 13 K. The Néel SkL extends over a broad range of temperature and magnetic field relative to the SkL in the chiral B20 helimagnets. It has been proposed that the modulated phases evolve into a ferromagnetic ground state at temperatures below ~ 5 K. By exploiting the magnetocaloric effect, we analyze the temperature and field evolution of the magnetic entropy change to gain insight into the stabilization mechanism of the magnetic phases close to T_{C}, which are thought to be governed by thermal fluctuations. We also investigate the behavior of the magnetic entropy change at low temperatures approaching the magnetic ground state. 
Wednesday, March 6, 2019 10:24AM  10:36AM 
K41.00011: Nontrivial topology in 2D magnetic systems without skyrmions JieXiang Yu, Wentao Hou, Morgan A Daly, Jiadong Zang The magnetic skyrmion is a kind of twodimensional(2D) topological nontrivial spin textures which can be identified by an integer number, called topological charge. However, our study on 2D chiral magnets, however, showed that one topological charge does not necessarily correspond to one skyrmion and the nontrivial topology can be found in a disordered phase. Here, we further investigate nontrivial topology in various systems where noskyrmion is expected in low magnetic field region. In 2D spinfrustrated systems, such thermally driven topology is dominated in a wide range of magnetic fields; In the antiferromagnetic system with staggered DzyaloshinskiiMoriya interactions, giant response of topological charge is found in a canted spinordering phase. Both two systems are related to real systems so that we expect the topological properties can be observed by thermal magnon and/or anomalous Hall effect. 
Wednesday, March 6, 2019 10:36AM  10:48AM 
K41.00012: Direct imaging of topological phase discontinuities in chiral spin textures via electron ptychography beyond diffractionlimited resolution Zhen Chen, Emrah Turgut, Krishnan Chander, Kayla Nguyen, Matthew Stolt, Song Jin, Dan Ralph, Gregory Fuchs, David Anthony Muller Realspace imaging of magnetic structures can provide crucial information for understanding many magnetic phenomena. Here, utilizing a new electron microscope detector, EMPAD, we resolve the internal fine structure of skyrmions in single crystal FeGe using Lorentz STEM and electron ptychography. We have unambiguously uncovered the topological phase discontinuities near the skyrmion lattice dislocation and boundaries. We also demonstrate a significant resolution improvement using electron ptychography, allowing magnetic imaging beyond the diffraction limit imposed by the electron optics. Magnetic domain images from ultrathin metallic multilayer films show the outstanding detection limit of our new imaging method. This new direct imaging technique can be applied to determine complex chiral spin structures in a wide variety of materials. 
Wednesday, March 6, 2019 10:48AM  11:00AM 
K41.00013: Multiple magnetic phases in chiral Mn$_{1/3}$NbS$_2$ Bing Shen The chirality in materials always leads to the intriguing functionalities. In a magnetic system, the spin texture can be chiral due to the competition of DzyaloshinskiiMoriya (DM) interaction and ferromagnetic coupling. The unique magnetic structure could be modulated into a particlelike order by applying field such as a magnetic soliton. In this talk, we present the systematic magnetotransport study on the chiral magnetic Mn1/3NbS2single crystal, a proposed candidate of chiral magnetic solation semimetal. With tuning temperature and applied field, the system undergoes several magnetic states. An anisotropic magnetic phase diagram is revealed in this the layered materials. 
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