Bulletin of the American Physical Society
APS March Meeting 2018
Volume 63, Number 1
Monday–Friday, March 5–9, 2018; Los Angeles, California
Session P22: Novel Chiral Spin Textures and MaterialsFocus

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Sponsoring Units: GMAG DMP Chair: Felix Buttner, Massachusetts Institute of TechnologyMIT Room: LACC 402A 
Wednesday, March 7, 2018 2:30PM  2:42PM 
P22.00001: Direct Observation of Chiral Bobbers in Nanostructured FeGe Nikolai S. Kiselev, Fengshan Zheng, Filipp Rybakov, Aleksandr Borisov, Dongsheng Song, Shasha Wang, ZiAn Li, Haifeng Du, Jan Caron, Mingliang Tian, Yuheng Zhang, Stefan Bluegel, Rafal DuninBorkowski Chiral magnets represent a special class of magnetically ordered crystals that are characterized by broken inversion symmetry and strong spinorbit coupling, giving rise to the DzyaloshinskiiMoriya interaction (DMI). The presence of the DMI usually prevents the stability and coexistence of topological excitations of different types. However, recently a new type of localized particlelike object  the chiral bobber (ChB)  was predicted theoretically in such materials^{1}. Such hybrid particlelike states are composed of continuum magnetization vector fields and singularities that are known as Bloch points. Here, we report the experimental observation of ChBs in thin films of B20type FeGe by means of quantitative offaxis electron holography^{2}, which allowed us to identify the part of the temperaturefield magnetic phase diagram, in which ChBs exist and to identify two distinct mechanisms for their nucleation. We show that ChBs are able to coexist with skyrmions over a wide range of parameters, suggesting their possible practical application in novel magnetic solid state memory devices, in which a stream of binary data bits can be encoded by a sequence of skyrmions and bobbers^{2}. 
Wednesday, March 7, 2018 2:42PM  2:54PM 
P22.00002: Chiral bobbers and skyrmions in MBE grown FeGe on Si(111) films Adam Ahmed, James Rowland, Bryan Esser, Sarah Dunsiger, David McComb, Mohit Randeria, Roland Kawakami Novel topological spin textures such as skyrmions have been studied in a wide class of materials. They are typically categorized by either having interfacial or bulk DzyaloshinskiiMoriya interactions (DMI). However, systems containing both interfacial and bulk DMI have not been explored. We report experimental and theoretical evidence for the formation of chiral bobbers in FeGe films grown by molecular beam epitaxy (MBE). After establishing the presence of skyrmions in FeGe/Si(111) thin film samples through Lorentz transmission electron microscopy and topological Hall effect, we perform magnetization measurements that reveal an inverse relationship between film thickness and the slope of the susceptibility. We present evidence for the evolution as a function of film thickness, L, from a skyrmion phase for L < L_{D}/2 to a cone phase with chiral bobbers at the interface for L > L_{D}/2, where L_{D} ~70 nm is the FeGe pitch length. We show using micromagnetic simulations that chiral bobbers, earlier predicted to be metastable, are in fact the stable ground state in the presence of an additional interfacial DMI. 
Wednesday, March 7, 2018 2:54PM  3:06PM 
P22.00003: Skyrmions and chiral bobbers: evolution from 2D to 3D with film thickness James Rowland, Adam Ahmed, Roland Kawakami, Mohit Randeria Theoretical studies of chiral magnets have focused on either 3D systems with broken bulk inversion symmetry or quasi2D systems with broken surface inversion. Building on our earlier theoretical results [1] and motivated by recent experiments [2] of B20 thin films grown on substrates, we model the behavior of materials with two types of DzyaloshinskiiMoriya interaction (DMI) that arise from broken bulk inversion and broken surface inversion. We investigate the T=0 phase diagram as a function of field H, anisotropy K, and sample thickness L. We highlight the importance of surface twists in quasi2D systems. We also show how skyrmion tubes in very thin films evolve into chiral bobbers confined to the interface in thicker samples. The role of interfacial DMI in stabilizing chiral bobbers and stacked spirals is emphasized. The important role of magnetocrystalline anisotropy and demagnetization factors will also be discussed and comparison with experiments presented. 
Wednesday, March 7, 2018 3:06PM  3:18PM 
P22.00004: Abstract Withdrawn

Wednesday, March 7, 2018 3:18PM  3:30PM 
P22.00005: Skyrmion phases in thin films: a finitetemperature Monte Carlo study PoKuan Wu, James Rowland, YingJer Kao, Mohit Randeria We present the results of largescale Monte Carlo simulations of the fieldtemperature phase diagram of thin films of chiral magnets with ferromagnetic exchange and DzyaloshinskiiMoriya interaction. We will show results for the specific heat, spin structure factor S(Q), average skyrmion density, and skyrmion density structure factor S_{sk}(Q) to characterize the various phases: helical, skyrmion crystal, skyrmion liquid and fieldpolarized ferromagnet. The melting of the skyrmion crystal to a liquid state is characterized using the orientational order parameter. A significant portion of the phase diagram at high temperatures harbors a skyrmion liquid which has a nonzero average skyrmion density, but exhibits shortrange orientational order and "rings" in both S(Q) and S_{sk}(Q). This is distinct from a state with fluctuating helices, at say H = 0, which only exhibits a "ring" in S(Q), but not in S_{sk}(Q). We will show that the phase diagram changes in interesting ways with increasing number of layers as the system evolves from the strict 2D limit of a monolayer. 
Wednesday, March 7, 2018 3:30PM  3:42PM 
P22.00006: Topology for Magnetization Control in Complex Magnets JanPhilipp Hanke, Frank Freimuth, Chengwang Niu, Stefan Bluegel, Yuriy Mokrousov The topological properties of magnets as encoded in the Berry phase have revolutionized our understanding of elementary transport effects. The discovery that the nontrivial geometry of reciprocal space relates to orbital properties of electrons allows us to predict from theoretical arguments pronounced orbital magnetism in various situations ranging from Rashba systems to Chern insulators [13]. We demonstrate that the combination of complex geometry in real and momentum spaces manifests in the emergence of topological orbital magnetism in noncollinear magnets, which opens new vistas in large currentinduced orbital response and magnetization manipulation in antiferromagnets [1,2]. Finally, we predict that in insulating systems with complex topologies the magnitude of magnetoelectric phenomena in terms of spinorbit torques and DzyaloshinskiiMoriya interaction can exceed significantly that of conventional metallic magnets, which lays out promising perspectives for the dissipationless magnetization control in nanomagnets [4]. 
Wednesday, March 7, 2018 3:42PM  3:54PM 
P22.00007: Topology and surface states of spin waves Fuyan Lu, YuanMing Lu While the topology of electron band structures is well understood, a systematic framework is still missing from the topology of spin waves. By establishing a map from a spin wave (or magnon) system to an electron system, we classify the topology of magnon bands according to the unbroken symmetries of the magnetic orders. We demonstrate it in various magnetic orders, including colinear, coplanar and chiral ones, and discuss its application to certain magnetic materials. 
Wednesday, March 7, 2018 3:54PM  4:06PM 
P22.00008: Magnetic skyrmion bubble driven by surface acoustic waves Rabindra Nepal, Utkan Güngördü, Alexey Kovalev We study the dynamics of magnetic skyrmion bubble induced by surface acoustic waves (SAWs). We estimate the bubble mass and derive the force due to SAWs acting on a magnetic bubble using Thiele's method. We then study bubble dynamics due to counterpropagating SAWs applied across a nanowire. The strain force is proportional to the strain gradient for dominant strain component and pushes the bubble towards the antinode of the standing wave where the force vanishes. We find that a magnetic bubble can be pinned to the antinode of a standing wave and therefore, can be driven by introducing a small detuning between the counterpropagating SAWs. Compared to magnetic domain walls, such pinning is weaker and leads to smaller velocities. In a disk geometry, we propose a SAWdriven skyrmion bubble oscillator with two resonant frequencies. This effect can be potentially applied for the generation of microwaves. 
Wednesday, March 7, 2018 4:06PM  4:18PM 
P22.00009: Stability and Lifetime of Antiferromagnetic Skyrmions Oleg Tretiakov, P. F. Bessarab, D. Yudin, D. R. Gulevich, P. Wadley, M. Titov The twodimensional Heisenberg exchange model with outofplane anisotropy and DzyaloshinskiiMoriya interaction is employed to investigate the lifetime and stability of antiferromagnetic (AFM) skyrmion as a function of temperature and external magnetic field. An isolated AFM skyrmion is shown to be metastable at zero temperature in a certain parameter range set by two boundaries separating the skyrmion state from the uniform AFM phase and a stripe domain state. The distribution of the energy barriers for the AFM skyrmion decay into the uniform AFM state complements the zerotemperature stability diagram and demonstrates that the skyrmion stability region is significantly narrowed at finite temperatures. We show that the AFM skyrmion stability can further be enhanced by an application of magnetic field, which strength is comparable with the spinflop field. This stabilization of AFM skyrmions in external magnetic fields is in sharp contrast to the behavior of their ferromagnetic counterparts. Furthermore, we demonstrate that the AFM skyrmions are stable on the timescales of milliseconds below 50 K for realistic material parameters, making it feasible to observe them in modern experiments. 
Wednesday, March 7, 2018 4:18PM  4:30PM 
P22.00010: Novel perspectives for the control of antiferromagnetic solitons Ricardo Zarzuela, Se Kwon Kim, Yaroslav Tserkovnyak Magnetic solitons have been object of an intense study in recent years due to their topological robustness and potential use as building blocks for logic devices and information storage. Antiferromagnetic (AFM) materials appear to be perfect platforms for their manipulation, since they do not produce stray fields and typically exhibit ultrafast spin dynamics. However, the Néel order is largely hidden from magnetic fields and, therefore, AFM solitons are difficult to drive by conventional magnetic techniques. In this talk we will discuss novel approaches to the stabilization and control of AFM solitons (in particular, domain walls and skyrmions) in thin films based on the magnetoelectric effect and on fieldlike torques at the bulk level. In the first case, we will show that domain walls and skyrmions behave as massive particles moving in a viscous medium subjected to a gyrotropic force, and that the film thickness can be used as a control parameter for their motion. In the second case, we will show that the skyrmion crystal phase can arise as the ground Néel state and that its dynamics can be triggered via spintransfer effects. 
Wednesday, March 7, 2018 4:30PM  4:42PM 
P22.00011: Topological stability of the double vertical Bloch line in a canted antiferromagnetic system Inhyeok Choi, Jongseok Lee Topological defects in the magnetic system have been attracting a great attention, and the skyrmion, in particular, has been considered as an important candidate for a nanoscale digital information carrier in the spintronic applications. In this work, we investigate the topological defects in canted antiferromagnetic materials with the orthoferrite as a model system, and demonstrate that the vertical Bloch line (VBL) can couple to the other VBL to form a stable antibonding state. Whereas such a double VBL is a topologically nontrivial object with a skyrmion number equivalent to that of the skyrmion itself, we demonstrate that a transition to other topological states is possible only when a strong enough magnetic field is applied. As long as the double VBL is in a stable condition, it is found that that the magnetic field can drive it to move straight with a high speed, 3000 m/s without experiencing either a walkerbreakdown or a skyrmion Hall effect. 
Wednesday, March 7, 2018 4:42PM  4:54PM 
P22.00012: Magnetic chains with twisted long range interactions in transition metal dichalcogenide flakes Oscar Avalos Ovando, Barbara Bravo, Diego Mastrogiuseppe, Sergio Ulloa Achieving tunable magnetic order in 1D magnetic chains could have important implications in systems which require custommade order such as spinwire bits, Majorana bound states, and time crystals. We present a study of an ensemble of magnetic impurities (MI) set close to the edges of MoS_{2} flakes, showing that order is possible and tunable. It has been shown that these MIs interact effectively through the RKKY interaction, leading to large DzyaloshinskiiMoriya and longranged interactions terms [1]. This behavior and the experimental availability of such flakes, make these systems a unique platform for the study of MI chains with longrange stable phases. We present here studies of the ground state of the system using a classical Hamiltonian with different parameters and compare them to fully quantum DMRG calculations for the same system. The ensemble of MIs along the edges results indeed in stable magnetic ordering with helical arrangements, with unique properties and defined phase transition points. We show that the ordering is tunable, via the concentration of MIs, their separation and/or the Fermi level of the flake, all experimentally achievable parameters. 
Wednesday, March 7, 2018 4:54PM  5:06PM 
P22.00013: Chiral Magnetic Interactions in Zigzag Graphene Nanoribbons Deposited on Topological Insulators Wei Zhang, Farideh Hajiheidari, Riccardo Mazzarello It is known that the interplay between strong spinorbit coupling and broken inversion symmetry can lead to chiral configurations in lowdimensional magnetic systems deposited on heavyelement surfaces. This phenomenon is due to the DzyaloshinskiiMoriya interaction, which favours noncollinear magnetic structures. In this work we investigate zigzag graphene nanoribbons on the (111) surface of the topological insulator Sb_{2}Te_{3} by firstprinciples simulations based on Density Functional Theory. We consider unpassivated nanoribbon edges, which bind strongly to the substrate. We show that the edge magnetism is preserved, in spite of the strong chemical interaction between the edge carbon atoms and the substrate. Furthermore, we show that the DzyaloshinskiiMoriya interaction leads to a twisting of the two antiferromagneticallycoupled edge states of the graphene nanoribbon. We also study the effects of this magnetic configuration on the surface states of the topological insulator. We find that the resulting net magnetization of the nanoribbon shifts the Dirac point of the surface state and induces a small gap. 
Wednesday, March 7, 2018 5:06PM  5:18PM 
P22.00014: Abstract Withdrawn

Wednesday, March 7, 2018 5:18PM  5:30PM 
P22.00015: DMRG Study of a TimeReversal Symmetry Breaking Phase without Magnetism ShunChiao Chang Invariably, states of matter that break timereversal symmetry (TRS) are associated with some form of magnetism. Recently, a phase was conjectured as an exception  it breaks TRS via chiral ordering of spins along a particular direction, but is spinrotation symmetric (Ref: Hosur, arXiv:1510.00975). In this work, we prove the existence of such a state by performing a density matrix renormalization group (DMRG) study of a spin1 chain with biquadratic interactions at zero temperature, and show that the ground state breaks TRS via antiferromagnetic chiral ordering of the spins, but is spinrotation symmetric. 
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