Bulletin of the American Physical Society
APS March Meeting 2015
Volume 60, Number 1
Monday–Friday, March 2–6, 2015; San Antonio, Texas
Session A28: Focus Session: Skyrmions I |
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Sponsoring Units: GMAG DMP FIAP Chair: John Xiao, University of Delaware Room: 205 |
Monday, March 2, 2015 8:00AM - 8:36AM |
A28.00001: Magnetic droplets and dynamical skyrmions Invited Speaker: Johan Akerman Nanocontact spin-torque oscillators (NC-STOs) provide an excellent environment for studying nano-magnetic phenomena such as localized and propagating auto-oscillatory spin wave (SW) modes. The recent experimental observation of magnetic droplet solitons in NC-STOs with perpendicular magnetic anisotropy (PMA) free layers [1], and the numerical [2] and experimental [3] demonstrations of spin transfer torque (STT) nucleated skyrmions in similar magnetic thin films add two interesting and useful nanoscale magnetic objects. Due to the competition between exchange, anisotropy, and, in the case of skyrmions, the Dzyaloshinskii-Moriya interaction (DMI), the droplet and the skyrmion are extremely compact, on the order of 10-100 nm. One of the main differences between a magnetic dissipative droplet soliton and a skyrmion is that the former is a dynamical object with all its spins precessing around an effective field and stabilized by STT, exchange, and PMA, while the latter has static spins and an internal structure stabilized by DMI, exchange, and PMA. The dissipative droplet is furthermore a non-topological soliton, while the skyrmion is topologically protected. In this work I will report on our most recent droplet experiments, including droplet collapse at very high fields, droplets excited in nano-wire based NC-STOs, and studies of the field-current droplet nucleation boundary. I will also demonstrate numerically and analytically that STT driven precession can stabilize so-called dynamical skyrmions even in the absence of DMI, and I will describe their very promising properties in detail. From a more fundamental perspective, precession is hence a third independent possibility to stabilize a skyrmion, without the need for the conventional stabilization from either dipolar energy or DMI. \\[4pt] [1] S. M. Mohseni et al, Science 339, 1295 (2013).\\[0pt] [2] J. Sampaio et al, Nature Nanotechn. 8, 839 (2013).\\[0pt] [3] N. Romming et al, Science 341, 636 (2013) [Preview Abstract] |
Monday, March 2, 2015 8:36AM - 8:48AM |
A28.00002: Physical Mechanisms and Limits of Skyrmions for Information Processing and Storage Gen Yin, Roger Lake, Chia-Ling Chien, Jiadong Zang Magnetic Skyrmions have been proposed for applications in future information storage because of their small size, their stability and their facial movement with low current. For such purposes the ability to create single Skyrmions is required and an understanding of the process of Skyrmion creation and decay is highly desirable. Here we numerically show that the location and the moment of Skyrmion creation or annihilation can be precisely controlled by a nano second unpolarized current pulse. To analyze the microscopic process we employ a lattice version of the topological charge on a tight-binding 2D plate. It provides a clear picture of the spin trajectories and orientations that locally trigger a topological transition, and it reviews the topological origin of a Skyrmion's stability at finite temperatures. The robustness and experimental feasibility of the proposed mechanism are numerically examined. [Preview Abstract] |
Monday, March 2, 2015 8:48AM - 9:00AM |
A28.00003: Electrical transport in three-dimensional cubic Skyrmion crystal Xiao-Xiao Zhang, Naoto Nagaosa Two-dimensional magnetic Skyrmions have been well confirmed via various experimental techniques in the bulk or on epitaxial thin films. Besides, a topologically nontrivial three-dimensional cubic Skyrmion crystal in the bulk, which is essentially a hedgehog-antihedgehog pair texture predicted theoretically, has also been tentatively observed. Equipped with a sophisticated spectral analysis program, we adopt Matsubara Green's function technique to study electrical transport, especially diagonal conductivity, in such system. We consider conduction electrons interacting with spinwaves via the strong Hund's rule coupling, wherein fluctuation of monopolar emergent electromagnetic field exits within adiabatic approximation. We describe in detail the influence of temperature and Skyrmion number on both dc and ac conductivities. Possible deviation from Fermi liquid behavior will also be discussed. [Preview Abstract] |
Monday, March 2, 2015 9:00AM - 9:12AM |
A28.00004: Statics and dynamics of zero field stable skyrmions in magnetic thin layers Nikolai S. Kiselev, Changhoon Heo, Ashis Kumar Nandy, Theo Rasing, Stefan Bl\"ugel ~ We present a comprehensive theoretical study of the statics and dynamics of magnetic skyrmions stabilized in zero magnetic field. The essential energy contributions for the stability of such magnetic skyrmions are the Dzyaloshinskii-Moriya interaction (DMI) and the uniaxial anisotropy. We define a finite range of the strength of these parameters corresponding to the stability of isolated skyrmions at zero magnetic field. Within this range there are two metastable skyrmion solutions characterized by opposite polarity and opposite topological charge. Such skyrmions can lead to conceptually new approaches in data storage provided by field or current induced switching between two of such skyrmion states. We discuss in detail various aspects of the problem connected with the switching between two skyrmion states driven by an applied magnetic field pulse, including the role of magnetic pulse width, intensity and direction. The role of damping and dependencies on magnetic layer thickness, size and shape are also discussed. Presented results are obtained by the method of stochastic spin dynamics simulation applied to an extended Heisenberg model for localized magnetic spins. [Preview Abstract] |
Monday, March 2, 2015 9:12AM - 9:24AM |
A28.00005: Collective Transport Properties of Driven Skyrmions with Random Disorder Dipanjan Ray, Cynthia Olson Reichhardt, Charles Reichhardt We use particle-based simulations to examine the static and driven collective phases of skyrmions interacting with random quenched disorder. We show that non-dissipative effects due to the Magnus term reduce the depinning threshold and strongly affect the skyrmion motion and the nature of the dynamic phases. The quenched disorder causes the Hall angle to become drive-dependent in the moving skyrmion phase, while different flow regimes produce distinct signatures in the transport curves. For weak disorder, the skyrmions form a pinned crystal and depin elastically, while for strong disorder the system forms a pinned amorphous state that depins plastically. At high drives the skyrmions can dynamically reorder into a moving crystal, with the onset of reordering determined by the strength of the Magnus term. [Preview Abstract] |
Monday, March 2, 2015 9:24AM - 9:36AM |
A28.00006: Dynamics of monopole-like excitation in chiral magnets under a current drive Shi-Zeng Lin, Avadh Saxena Skyrmions in chiral magnets are hedgehog-like spin textures, which wrap the sphere once. Skyrmion lines in metallic chiral magnets produce an emergent magnetic field that couples to the orbital motion of the conduction electrons. The inflow and outflow of this field around a closed surface is not necessary equal, i.e. the divergence of this field is not necessary zero, and thus it allows for the existence of emergent monopoles. One example is a finite segment of a skyrmion line inside crystal. The two ends of the segment produce a monopole and antimonopole, which are connected by lines of the emergent magnetic flux. Here we study the monopole dynamics induced by an electric current injected in the chiral magnet. We reveal that skyrmion segments are nucleated via the creation of monopoles and antimonopoles. Then these segments merge to form longer skyrmion lines via annihilation of monopoles and antimonopoles. Finally these skyrmion lines span the whole system, where all monopoles and antimonopoles disappear. The skyrmion lines are destroyed via the proliferation of monopoles and antimonopoles. We also propose to create the monopoles in a controlled way by applying current to surfaces. The existence of monopoles can be inferred from transport or imaging measurements. [Preview Abstract] |
Monday, March 2, 2015 9:36AM - 9:48AM |
A28.00007: Detection of helical spin structures by magnetotransport Ali C. Basaran, Rafael Morales, Stefan Guenon, Ivan K. Schuller We have developed a method which allows determining the magnetic helicity in thin films by magnetotransport measurements. A helical spin configuration occurs during magnetization reversal in exchange coupled Ni/FeF$_{2}$ heterostructures. Both longitudinal and transverse components of the magnetization are concurrently detected using magnetotransport through a lithographically patterned cross junction. Small angle deviations around the well-defined unidirectional anisotropy axis reveal the change in the helicity of in-depth spin configuration. The simulations obtained using an incomplete domain wall model are in excellent agreement with the experimental data. Thus, we show that the complex spin formations in nanomagnetic systems can be studied with a lab based, well known, and rather simple experimental technique. [Preview Abstract] |
Monday, March 2, 2015 9:48AM - 10:00AM |
A28.00008: Micromagnetic modeling of skyrmion injection in an inhomogeneous geometry Olle Heinonen, Wanjun Jiang, Suzanne te Velthuis, Axel Hoffmann Magnetic systems with broken inversion symmetry can support skyrmion structures stabilized by the Dzyaloshinskii-Moriya interaction (DMI). This can be realized in thin magnetic films, such as CoFe, on a substrate with appreciable spin-orbit interactions, e.g., Ta or Pt. Recent works have explored the creation and manipulations in such thin films using spin transfer torque or domain wall injection, which is challenging. We recently demonstrated that an inhomogeneous spin-Hall current can efficiently create and inject skyrmions, which can then be manipulated and transported. Based on this observation we will here present micromagnetic modeling of the injection dynamics of skyrmions in a geometry similar to the ones used experimentally. For small current densities, the system will inject meandering stripes that will arrange themselves along equipotential lines. For large enough current densities, a domain wall forms in the narrow part of the system and much complicated dynamical structures are sprayed into the wider part of the system. When the driving current is subsequently turned off, these structures coalesce into skyrmions. A key ingredient here is the inhomogeneous spin-Hall torque resulting from the geometry of the system. [Preview Abstract] |
Monday, March 2, 2015 10:00AM - 10:12AM |
A28.00009: Annihilation of skyrmions by spin wave in crossbar geometry Yizhou Liu, Jiadong Zang, Roger Lake Magnetic Skyrmions are possible candidates for future information storage applications due to their small size and topological protection. Understanding of the creation and annihilation process is necessary for developing a Skyrmion-based technology. We theoretically study the annihilation process within a crossbar geometry. By micromagnetic simulation, we find the annihilation of a single Skyrmion can be achieved by injecting a spin wave. The influence of a defect in this system is also discussed. Our method could be applied to individually address arrays of Skyrmions in a memory architechture. [Preview Abstract] |
Monday, March 2, 2015 10:12AM - 10:24AM |
A28.00010: Magnetization pumping and dynamics in a uniform Dzyaloshinskii-Moriya magnet Alexey Kovalev, Utkan G\"ung\"ord\"u We formulate a phenomenological description of thin ferromagnetic layers with inversion asymmetry where the long-wavelength magnetic dynamics experiences magnon current-induced torques and leads to magnon-motive forces. We first construct a phenomenological theory based on irreversible thermodynamics, taking into account the symmetries of the system. Furthermore, we confirm that these effects originate from Dzyaloshinskii-Moriya interactions from the analysis based on the stochastic Landau-Lifshitz-Gilbert equation. Our phenomenological results can be generalized to other systems such as pyrochlore crystals and chiral magnets. Possible applications include spin current generation, magnetization reversal and magnonic cooling. [Preview Abstract] |
Monday, March 2, 2015 10:24AM - 10:36AM |
A28.00011: Current-Driven Dynamics of Skyrmions Stabilized in MnSi Nanowires Revealed by Topological Hall Effect Dong Liang, John DeGrave, Matthew Stolt, Yoshinori Tokura, Song Jin Skyrmions, novel topologically stable spin vortices, hold promise for next-generation high-density magnetic storage technologies due to their nanoscale domains and ultralow energy consumption. One-dimensional (1D) nanowires are ideal hosts for skyrmions since they not only serve as a natural platform for magnetic racetrack memory devices but also can potentially stabilize skyrmions. We use the topological Hall effect (THE) to study the phase stability and current-driven dynamics of the skyrmions in MnSi nanowires. The THE was observed in an extended magnetic field-temperature window (15 to 30 K), suggesting stabilization of skyrmion phase in nanowires compared with the bulk (27 to 29.5 K). Furthermore, we study skyrmion dynamics in this extended skyrmion phase region and found that under the high current-density of 10$^{\mathrm{8}}$-10$^{\mathrm{9}}$Am$^{\mathrm{-2}}$ enabled by nanowire geometry, the THE decreases with increasing current densities, which demonstrates the current-driven motion of skyrmions generating the emergent electric field. These results open up the exploration of nanowires as an attractive platform for investigating skyrmion physics in 1D systems and exploiting skyrmions in magnetic storage concepts. [Preview Abstract] |
Monday, March 2, 2015 10:36AM - 10:48AM |
A28.00012: Towards Skyrmions: Magnetic and electrical properties of Mn$_x$Rh$_y$Sn Heusler compounds Kumari Gaurav Rana, E. Benedikt, O. Meshcheriakova, A. K\"ohler, D. Ebke, C. Felser A variety of possible ground states and tunable magnetic and electronic properties makes Mn $_2$-based Heuslers one of the most relevant materials for spintronic devices. Mn $_2$ RhSn, a non-centrosymmetric tetragonal Heusler with enhanced spin-orbit coupling, is recently found to possess a strong spin canting of its magnetic sublattices and the found strong Dzyaloshinskii-Moriya exchange encourages studies of Skyrmions in this promising Heusler compound. We report on the evolution of magnetic and electronic properties of ordered Mn$_x$Rh$_y$Sn films grown on single crystalline MgO (100) substrates using DC co-sputtering. For tetragonally ordered films, a curie temperature of up to 281 K and coercive field of up to 630 Oe (out of plane) is obtained. However, an in-plane component sets in for temperatures below 100 K. The structural ordering obtained for the films grown at different substrate temperatures influences the magnetic as well as the electronic properties. The exchange energies, magneto-transport properties, hall effect will be discussed. Our work gives an insight to tailor the properties of Mn$_x$Rh$_y$Sn films and highlights it as a potential candidate for future spintronic devices. [Preview Abstract] |
Monday, March 2, 2015 10:48AM - 11:00AM |
A28.00013: Large non-collinearity and spin-reorientation in the novel Mn$_{2}$RhSn Heusler magnet O. Meshcheriakova, S. Chadov, A. Nayak, U.K. Roessler, J. Kuebler, G. Andre, A.A. Tsirlin, C. Felser Heusler compounds is a large class of materials, which exhibits diverse fundamental phenomena, together with the possibility of their specific tailoring for various engineering tasks. Present work discusses the magnetic noncollinearity in the family of noncentrosymmetric Mn$_{2}$-based Heusler compounds. According to the experimental and theoretical results, Mn$_{2}$\textit{YZ} Heusler family is suspected to provide promising candidates for the formation of the skyrmion lattice. The work is focused on Mn$_{2}$RhSn bulk polycrystalline sample, which serves as a prototype. It crystallizes in the tetragonal noncentrosymmetric structure (No. 119,\textit{I-4m2}), which enables the anisotropic DM coupling. Additional short-range modulation, induced by the competing nearest and next-nearest interplanes Heisenberg exchange, is suppressed above the 80 K. This allows to develop the long-range modulations in the ideal ferrimagnetic structure within the \textit{ab} crystallographic planes. [Preview Abstract] |
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