Bulletin of the American Physical Society
APS March Meeting 2016
Volume 61, Number 2
Monday–Friday, March 14–18, 2016; Baltimore, Maryland
Session V18: Skyrmion Dynamics and MotionFocus
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Sponsoring Units: GMAG DMP Chair: Vincent Cros, Unite Mixte de Physique CNRS/Thales Room: 317 |
Thursday, March 17, 2016 2:30PM - 2:42PM |
V18.00001: Room-temperature creation and spin-orbit torque-induced manipulation of skyrmions in thin film Guoqiang Yu, Pramey Upadhyaya, Xiang Li, Wenyuan Li, Se Kwon K im, Yabin Fan, Kin L. Wong, Yaroslav Tserkovnyak, Pedram Khalili Amiri, Kang L. Wang Magnetic skyrmions, which are topologically protected spin texture, are promising candidates for ultra-low energy and ultra-high density magnetic data storage and computing applications1, 2. To date, most experiments on skyrmions have been carried out at low temperatures. The choice of materials available is limited and there is a lack of electrical means to control of skyrmions. Here, we experimentally demonstrate a method for creating skyrmion bubbles phase in the ferromagnetic thin film at room temperature. We further demonstrate that the created skyrmion bubbles can be manipulated by electric current. This room-temperature creation and manipulation of skyrmion in thin film is of particular interest for applications, being suitable for room-temperature operation and compatible with existing semiconductor manufacturing tools. 1. Nagaosa, N., Tokura, Y. Nature Nanotechnology 8, 899-911 (2013). 2. Fert, A., \textit{et al.}, Nature Nanotechnology 8, 152-156 (2013). [Preview Abstract] |
Thursday, March 17, 2016 2:42PM - 2:54PM |
V18.00002: Spin Torque induced anti-vortex excitations Kaan Ozbozduman, Vedat Karakas, Sevdenur Arpaci, Ali Taha Habibioglu, Aisha Gokce, Anna Giordano, Federica Celegato, Paula Tiberto, Giovanni Finocchio, Gulen Aktas, Ozhan Ozatay Nanodevices that are designed to stimulate the formation of unique magnetic configurations (vortex, anti-vortex, skyrmion etc.) are applicable to spin based technologies, namely, microwave oscillators and magnetic sensors. In this talk, we report the observed dynamic behavior of an anti-vortex, which had not been thoroughly studied due to the complexity in stabilization of the structure, by analyzing its interaction with magnetic field and DC current. Permalloy (Ni$_{\mathrm{81}}$Fe$_{\mathrm{19}})$ based 2x2\textmu m$^{\mathrm{2}}$ asteroid geometry devices, consisting of four tangent circles of equal radii, facilitate the nucleation of an anti-vortex pair at the center with the application of an in-plane AC demagnetizing field and an out of plane magnetic saturation field. Magnetic force microscopy (MFM) data shows that an external magnetic field can rearrange the positions of diagonally located anti-vortex pair. Spin torque effect induces an anti-vortex pair circular motion, known as gyration. The resulting RF signal is measured using the anisotropic magneto-resistance effect (AMR) which indicates a \textasciitilde 250-300 m$\Omega $ change in the resistance of our samples. This study will help develop our understanding of the anti-vortex, current and magnetic field interactions for practical on-chip microwave oscillator applications. [Preview Abstract] |
Thursday, March 17, 2016 2:54PM - 3:06PM |
V18.00003: Observation of spin transfer torques in the transverse magnetic susceptibility of the Skyrmion lattice phase of MnSi Felix Rucker, Christoph Schnarr, Andreas Bauer, Christian Pfleiderer In the Skyrmion lattice phase of MnSi the observation of sizeable spin transfer torques [1-3] promises easy experimental access to the precise qualitative and quantitative form of the Landau Lifshitz Gilbert equation. We report measurements of the transverse magnetic susceptibility, $\chi_{\perp}$, in the skyrmion lattice phase of MnSi. Our measurements show a distinct increase of $\chi_{\perp}$ with increasing current density around the critical current density $j_\mathrm{c}$. We further find a siszable dissipative part of $\chi_{\perp}$ evolving above $j_\mathrm{c}$. We discuss the broader implications of our experimental findings, which provide, for the first time, a direct link between a thermodynamic property and the effects of spin transfer torques in skyrmion lattices. \newline [1] F. Jonietz et al., Science \textbf{330}, 1648 (2010) \newline [2] T. Schulz et al., Nat. Phys. \textbf{8}, 301 (2012) \newline [2] K. Everschor et al., Phys. Rev. B \textbf{86}, 054432 (2012) [Preview Abstract] |
Thursday, March 17, 2016 3:06PM - 3:18PM |
V18.00004: Chiral Skyrmion Hall effect in Antiferromagnets Matthew Daniels, Ran Cheng, Jiang Xiao, Di Xiao We study the interaction between magnetic skyrmions and spin wave currents in antiferromagnetic (AFM) insulators. Micromagnetic simulations reveal that magnon-skyrmion scattering in AFMs is dependent on the chirality of the spin wave, a degree of freedom unique to easy-axis AFMs. We also find nontrivial dynamical differences between circularly and linearly polarized waves incident upon AFM skyrmions in simulation. We characterize the resulting chiral magnon Hall effect using the O(3) nonlinear sigma model, and we elucidate the corresponding chiral skrymion Hall effect as arising from certain magnon spin currents. [Preview Abstract] |
Thursday, March 17, 2016 3:18PM - 3:30PM |
V18.00005: Ratchet Effects, Negative Mobility, and Phase Locking for Skyrmions on Periodic Substrates Charles Reichhardt, Dipanjan Ray, Cynthia Olson Reichhardt We examine the dynamics of skyrmions interacting with 1D and 2D periodic substrates in the presence of dc and ac drives. We find that the Magnus term strongly affects the skyrmion dynamics and that new kinds of phenomena can occur which are absent for overdamped ac and dc driven particles interacting with similar substrates. We show that it is possible to realize a Magnus induced ratchet for skyrmions interacting with an asymmetric potential, where the application of an ac drive can produce quantized dc motion of the skyrmions even when the ac force is perpendicular to the substrate asymmetry direction. For symmetric substrates it is also possible to achieve a negative mobility effect where the net skyrmion motion runs counter to an applied dc drive. Here, as a function of increasing dc drive, the velocity-force curves show a series of locking phases that have different features from the classic Shapiro steps found in overdamped systems. In the phase locking and ratcheting states, the skyrmions undergo intricate 2D orbits induced by the Magnus term. [Preview Abstract] |
Thursday, March 17, 2016 3:30PM - 3:42PM |
V18.00006: FMR study of thin film FeGe skyrmionic material Vidya P. Bhallamudi, Michael R. Page, James Gallagher, Carola Purser, Joseph Schulze, Fengyuan Yang, P. Chris Hammel Magnetic Skyrmions have attracted intense interest due to their novel topological properties and the potential for energy efficient computing. Magnetic dynamics play an important part in enabling some of these functionalities. Understanding these dynamics can shed light on the interplay of the various magnetic interactions that exist in these materials and lead to a rich magnetic phase diagram, including the Skyrmion phase. We have grown phase-pure FeGe epitaxial films on Si (111) and studied them using ferromagnetic resonance (FMR). FeGe has one of the highest recorded skyrmion transition temperatures, close to room temperature, and thin films are known to further stabilize the Skyrmion phase in the magnetic field-temperature space. We have performed cavity-based single frequency FMR from liquid nitrogen to room temperature on 120 nm thick films in both in-plane and out-of-plane geometries. The resulting complex spectra are consistent with those reported in literature for the bulk material and can be understood in terms of a conical model for the magnetism. Variable temperature broadband spectroscopy and measurements on thinner films, to better identify the various magnetic phases and their dynamic behavior, are ongoing and their progress will be discussed. [Preview Abstract] |
Thursday, March 17, 2016 3:42PM - 3:54PM |
V18.00007: Magnetic excitations of the skyrmion host Cu$_2$OSeO$_3$ G S Tucker, J S White, J Romh\'anyi, D Szaller, I K\'ezsm\'arki, B Roessli, U Stuhr, A Magrez, F Groitl, P Babkevich, P Huang, I \v{Z}ivkovi\'c, H M R\o{}nnow Inelastic neutron scattering (INS) has been used to measure the magnetic excitation spectrum along high-symmetry directions of the first Brillouin zone of the magnetic skyrmion hosting compound Cu$_2$OSeO$_3$. The INS data are mostly consistent with the predictions of a recently proposed model for the magnetic excitations in Cu$_2$OSeO$_3$, for which best-fit parameters will be reported. As will be shown, differences exist between the model predictions and the experimental findings in the form of two energy scales that likely arise due to neglected anisotropic interactions. Thus highlighting the need for the inclusion of anisotropy in future theoretical works aimed at a full microscopic understanding of the emergence of the skyrmion state in this material. [Preview Abstract] |
Thursday, March 17, 2016 3:54PM - 4:06PM |
V18.00008: Staggered magnetization and low-energy magnon dispersion in the multiferroic skyrmion host Cu2OSeO3 Guy G. Marcus, Benjamin A. Trump, Jonas Kindervater, Lacy L. Jones, Matthew B. Stone, Tyrel M. McQueen, Collin L. Broholm We present neutron diffraction and inelastic scattering of the insulating helimagnet, Cu$_2$OSeO$_3$ which provide evidence for staggered magnetization and elucidate the associated low-energy magnon spectrum. The modulation wavelength of approximately $\lambda \approx 50$ nm detected at antiferromagnetic Bragg points is of the same length scale as previously reported for the skyrmion lattice. This superstructure evidences the composite nature of the spin-$1$ tetrahedra that form the topological magnetic structure of the material. To understand the interplay of ferrimagnetism and long wavelength modulated magnetism, we have performed inelastic neutron scattering on a co-aligned sample of chemical vapor transport grown single crystals. We shall present the low-energy magnon dispersion and infer an effective spin Hamiltonian to account for the long-wavelength, low-energy magnetism of Cu$_2$OSeO$_3$. [Preview Abstract] |
Thursday, March 17, 2016 4:06PM - 4:18PM |
V18.00009: Neutron scattering study of the field-induced tricritical point in MnSi J. Kindervater, A. Bauer, M. Garst, M. Janoschek, N. Martin, S. M\"uhlbauer, W. H\"aussler, P. B\"oni, C. Pfleiderer The intermetallic compound MnSi attracts great scientific interest due to two unusual phase transitions, namely the transition from the conical phase to a skyrmion lattice in small fields and the transition from the helical to the paramagnetic phase without external magnetic field that was recently identified to be a fluctuation induced first-order transition, i.e. a so called Brazovskii-transition. Recent measurements of the specific heat provide striking evidence for a tricritical point (TCP), were the first order transition alters to second order. We report neutron spin echo measurements using the MIEZE technique. The recorded quasi elastic linewidth shows a change of the characteristic spin fluctuations at the TCP. The combination with additional SANS measurements and a generalized Brazovskii theory establishes a consistent picture of the statics and dynamics of the transition. [Preview Abstract] |
Thursday, March 17, 2016 4:18PM - 4:54PM |
V18.00010: Controlling and imaging chiral spin textures Invited Speaker: Gong Chen Chirality in magnetic materials is fundamentally interesting and holds potential for logic and memory applications [1,2,3]. Using spin-polarized low-energy electron microscopy at National Center for Electron Microscopy, we recently observed chiral domain walls in thin films [4,5]. We developed ways to tailor the Dzyaloshinskii-Moriya interaction, which drives the chirality, by interface engineering [6] and by forming ternary superlattices [7]. We find that spin-textures can be switched between left-handed, right-handed, cycloidal, helical and mixed domain wall structures by controlling uniaxial strain in magnetic films [8]. We also demonstrate an experimental approach to stabilize skyrmions in magnetic multilayers without external magnetic field [9]. These results exemplify the rich physics of chirality associated with interfaces of magnetic materials. [1] A. Fert et al. \textit{Nat. Nanotechnol.} \textbf{8}, 152 (2013). [2] N. Nagaosa et al. \textit{Nat. Nanotechnol.} \textbf{8}, 899 (2013). [3] W. Jiang et al. \textit{Science} \textbf{349}, 283 (2015). [4] G. Chen, et al. \textit{Phys. Rev. Lett}. \textbf{110}, 177204 (2013). [5] G. Chen, et al. \textit{Adv. Mater}. \textbf{27}, 5738 (2015). [6] G. Chen, et al. \textit{Nat. Commun.} \textbf{4}, 2671 (2013). [7] G. Chen, et al. \textit{Appl. Phys. Lett.} \textbf{106}, 062404 (2015). [8] G. Chen, et al. \textit{Nat. Commun.} \textbf{6}, 6598 (2015). [9] G. Chen, et al. \textit{Appl. Phys. Lett.} \textbf{106}, 242404 (2015). [Preview Abstract] |
Thursday, March 17, 2016 4:54PM - 5:06PM |
V18.00011: Observation of room-temperature skyrmion Hall effect. W. Jiang, X. Zhang, P. Upadhyaya, W. Zhang, G. Yu, M. Jungfleisch, F. Fradin, J. Pearson, Y. Tserkovnyak, K. Wang, O. Heinonen, Y. Zhou, Suzanne te Velthuis, A. Hoffmann The realization of room-temperature magnetic skyrmions is key to enabling the implementation of skyrmion-based spintronics. In this work, we present the efficient conversion of chiral stripe domains into N\'{e}el skyrmions through a geometrical constriction patterned in a Ta/CoFeB/TaO$_{\mathrm{x}}$ trilayer film at room temperature. This is enabled by an interfacial Dzyaloshinskii-Moriya interaction, and laterally divergent current-induced spin-orbit torques [1]. We further show the generation of magnetic skyrmions solely by the divergent spin-orbit torques through a nonmagnetic point contact. By increasing the current density, we observe the skyrmion Hall effect -- that is the accumulation of skyrmions at one side of the device. The related Hall angle for skyrmion motion is also revealed under an \textit{ac} driving current. Financial support for the work at Argonne came from Department of Energy, Office of Science, Basic Energy Science, Materials Sciences and Engineering Division, work at UCLA was supported by TANMS. Reference: [1] W. Jiang, et al., Science, 349, 283 (2015). [Preview Abstract] |
Thursday, March 17, 2016 5:06PM - 5:18PM |
V18.00012: Controlling skyrmion helicity via engineered Dzyaloshinskii-Moriya interactions Sebastian Diaz, Roberto Troncoso Single magnetic skyrmion dynamics in chiral magnets with a spatially inhomogeneous Dzyaloshinskii-Moriya interaction (DMI) is considered. Based on the relation between DMI coupling and skyrmion helicity, it is argued that the latter must be included as an extra degree of freedom in the dynamics of skyrmions. An effective description of the skyrmion dynamics for an arbitrary inhomogeneous DMI coupling is obtained through the collective coordinates method. The resulting generalized Thiele's equation\footnote{Tretiakov et al., Phys. Rev. Lett. {\bf 100}, 127204 (2008).} is a dynamical system for the center of mass position and helicity of the skyrmion. It is found that the dissipative tensor and hence the Hall angle become helicity dependent. The skyrmion position and helicity dynamics are fully characterized by our model in two particular examples of engineered DMI coupling: half-planes with opposite-sign DMI and linearly varying DMI. In light of a recent experiment\footnote{Shibata et al., Nature Nanotech. {\bf 8}, 723 (2013).} on the magnitude and sign of the DMI, our results constitute the first step toward a more complete understanding of the skyrmion helicity as a new degree of freedom that could be harnessed in future high-density magnetic storage and logic devices. [Preview Abstract] |
Thursday, March 17, 2016 5:18PM - 5:30PM |
V18.00013: Quantum Anomalous Hall effect in a Topological Insulator coupled to a Skyrmion Lattice. Tonmoy Bhowmick, Yafis Barlas, Gen Yin, Roger Lake A Skyrmion is a topologically protected spin texture characterized by a topological charge that has been experimentally observed in both bulk B20 compounds and thin films. In a quantum anomalous Hall phase, a material develops a topologically nontrivial electronic structure giving rise to quantized hall conductivity without any external magnetic field. We predict that a conventional bulk topological insulating material (e.g. Bi$_{\mathrm{2\thinspace }}$Se$_{\mathrm{3}}$, Bi$_{\mathrm{2\thinspace }}$Te$_{\mathrm{3,\thinspace }}$Sb$_{\mathrm{2\thinspace }}$Te$_{\mathrm{3}})$ in proximity with a Skyrmion crystal, with a weak exchange coupling, will be driven into an anomalous Hall insulating phase characterized by a nonzero integer chern number in the gap. We have calculated band structure, identified the gaps, and calculated the chern number at those gaps. The calculations show that the non trivial topological properties of the Skyrmion spin texture can be imprinted on the Dirac electrons of the topological insulator. [Preview Abstract] |
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