Bulletin of the American Physical Society
APS March Meeting 2018
Volume 63, Number 1
Monday–Friday, March 5–9, 2018; Los Angeles, California
Session K22: Control and Detection of Skyrmions: from Fundamentals to ApplicationsFocus
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Sponsoring Units: GMAG DMP Chair: Kab-Jin Kim, KAIST Room: LACC 402A |
Wednesday, March 7, 2018 8:00AM - 8:36AM |
K22.00001: Structure, Energetics, and Deterministic Writing of Skyrmions in Thin Film Ferromagnets Invited Speaker: Felix Buettner Skyrmions are the smallest non-trivial entities in magnetism with great potential for data storage applications. They were recently observed at room temperature in magnetic multilayer systems [1-4], most of them in materials with sizable Dyzaloshinskii-Moriya interaction (DMI). Despite this experimental breakthrough, our understanding of skyrmions is still limited because existing theories cannot analytically predict how the skyrmion energy changes as a function of its size. In particular, for many decades, the 6-fold integral of the stray field energies was considered unsolvable. |
Wednesday, March 7, 2018 8:36AM - 8:48AM |
K22.00002: Tailoring Magnetic Skyrmions by Geometric Confinement of Magnetic Structures Shulei Zhang, Charudatta Phatak, Amanda Petford-Long, Olle Heinonen There is growing interest in systems that can host magnetic skyrmions at room temperature (RT) without Dzyaloshinskii-Moriya interaction (DMI), which enable novel ways to manipulate properties of skyrmions. In this work [1], we show theoretically that, in ferromagnetic thin films with inverse symmetry, skyrmion lattices (SkX) can be stabilized by spatially varying uniaxial magnetic anisotropy with easy axis periodically rotating from in-plane to out-of-plane. Phase diagrams in the plane of magnetic field and temperature were calculated by using a Ginzburg-Landau approach, which demonstrate that SkX is energetically favorable at RT with small magnetic field of the order of 10 Oe. Remarkably, the size of the skyrmions is determined by the ratio of the exchange length and the period of the spatial modulation of the anisotropy, at variance with those stabilized by dipolar interaction and DMI. We will also apply our theory to centrosymmetric multiferroics where SkXs were recently observed at RT. |
Wednesday, March 7, 2018 8:48AM - 9:00AM |
K22.00003: Skyrmion nucleation via localized spin current injection in confined nanowire geometry in low chirality magnetic materials Sourav Dutta, Dmitri Nikonov, George Bourianoff, Sasikanth Manipatruni, Ian Young, Azad Naeemi Magnetic skyrmions have been the focus of intense research with promising applications in memory, logic and interconnect technology. Several schemes have been proposed to nucleate skyrmions. However, they either result in an uncontrolled skyrmion bubble production or are mostly targeted towards integration with racetrack memory device. We propose a novel scheme for a controlled single skyrmion nucleation in a confined nanowire geometry with sub-100 nm width using a generalized approach of “localized spin current injection” technique in material systems exhibiting low Dzyaloshinskii–Moriya interaction (DMI). Our proposed nucleation mechanism follows a pathway involving the creation of a reversed magnetic domain containing vertical Bloch line (VBL) pairs that form an edge domain wall as the VBLs get annihilated at the edge of the nanowire. However, pinning of the edge domain walls within a narrow gap using notches or anti-notches results in the creation of a magnetic bubble with defect-free domain wall that relaxes into a skyrmion structure. Our simulations predict that the proposed mechanism allows skyrmion nucleation on sub-ns timescale, shows robustness to variations like local pinning sites and is applicable for any skyrmion-based logic, memory and interconnect application. |
Wednesday, March 7, 2018 9:00AM - 9:12AM |
K22.00004: Generating and Manipulating Hybrid Skyrmions in Thin DMI Magnetic Films Noah Kent, Robert Streubel, Charles-Henri Lambert, Scott Dhuey, Peter Fischer Skyrmions are promising candidates for stable and highly mobile spin textures in novel magnetic storage applications. Originally found only in exotic materials, at low temperatures, recently RT skyrmions have been confirmed in thin film magnetic materials. However, controlling the actual location of the skyrmion nucleation in such a film has so far been elusive. Hybrid skyrmions that can be generated by the spin transfer torque of a current pulse sent through a nanomagnetic disk in a vortex state coupled to a magnetic film with DMI, could solve this problem. Using four step nanolithography, and three different types of thin film deposition we report first results on hybrid skyrmions generated in Ir/Co/Pt multilayers with large DMI connected to 30nm thin Permalloy (Py) disks with diameters from 250-1000nm. Characterization of hybrid skyrmions using element-specific magnetic soft x-ray microscopy allows us to image the magnetic structure of the Py nanomagnets and the DMI film independently. We found that hybrid skyrmions exist over a larger range of applied magnetic fields and nucleate at the location of the Py disks. Future studies will focus on the dynamics of hybrid skyrmions by running current pulses through the DMI film. |
Wednesday, March 7, 2018 9:12AM - 9:24AM |
K22.00005: Robust Pathway for Voltage Controlled Switching of Ferromagnets Via an Intermediate Skyrmion State Dhritiman Bhattacharya, Jayasimha Atulasimha Traditional voltage control of magnetic anisotropy can be used to reverse the magnetization of a perpendicular nanomagnet by utilizing magnetization precession around a bias magnetic field. This is energy efficient as electric field is used instead of spin current but suffer from switching error due to incoherence in the presence of defects[1]. We show with extensive micromagnetic simulation that the presence of Dzyaloshinskii-Moriya Interaction can drive the magnetization through an intermediate skyrmionic state while switching between two stable perpendicular states thereby avoiding precessional magnetization dynamics as well as the need for a bias magnetic field[2]. We investigate the switching physics that involves alternative skyrmion mediated pathway in detail in the presence of thermal noise, defects and perturbative spin current. We show our proposed mechanism leads to extremely low error switching while also being potentially 2 orders of magnitude more energy efficient than state of the art spin transfer torque based switching. |
Wednesday, March 7, 2018 9:24AM - 9:36AM |
K22.00006: Purely Electrical Manipulation of Magnetic Domain and Skyrmion Bubbles Yufan Li, Qinli Ma, Daniel Gopman, Yury Kabanov, Robert Shull, Chia-ling Chien In ferromagnetic materials, the exchange interaction, magnetic anisotropy, and dipolar forces dictate the domain walls (DWs), such as the Bloch- or Néel-type. An enclosed DW with a set chirality carries a topological charge of Q = ±1, in equivalence to a magnetic skyrmion. It has been long speculated that DW spin structures with greater Q could exist [1], about which, however, experimental evidences remain elusive. We demonstrate experimental evidences of a novel type of DWs which possesses a high topological charge of Q = 2. Using MOKE microscopy, we show that the chiral winding of the core magnetization of DW can be revealed by its dynamical response to the electrical current induced spin-orbit torque (SOT). The breaking of rotational symmetry as a consequence of the Q = 2 chiral DW dictates that the DW responses differently when the current flows along different directions. We demonstrate two distinctive examples. The deterministic SOT switching of a magnetic layer can be achieved by applying the current in one direction, while skyrmion bubbles are generated by applying the current to another direction. Both are realized without any magnetic field, as a benefit of the novel DW topology. |
Wednesday, March 7, 2018 9:36AM - 9:48AM |
K22.00007: Theoretical study of current-driven skyrmions in disordered magnets Wataru Koshibae, Naoto Nagaosa The magnetic skyrmion, an emergent topological particle in magnets, has several unique features distinct from the other spin textures such as domain wall, helical structure, and vortex. One of the features is the small critical current density for the current-driven motion in terms of spin transfer torque effect. The critical current density indicates the pinning effect due to the disorder such as impurities and defects, it is always relevant in real systems. |
Wednesday, March 7, 2018 9:48AM - 10:00AM |
K22.00008: Skyrmion Creep and Avalanches Charles Reichhardt, Cynthia Reichhardt, Sebastian Diaz, Avadh Saxena We numerically examine skyrmion motion in systems with random disorder. We find that with increasing temperature, the depinning threshold decreases and a distinct creep regime emerges in which the skyrmion motion occurs in avalanches or jumps. Within the creep regime the skyrmion Hall angle is zero, while at higher drives when the skyrmions start to move continuously, the skyrmion Hall angle becomes finite and increases with increasing drive until it saturates at the intrinsic value. We show that the skyrmion avalanches exhibit a power law size distribution indicative of critical behavior near a depinning point. For systems with moderate to low intrinsic skyrmion Hall angles, the time-averaged avalanche signatures have a symmetric shape, while for high intrinsic skyrmion Hall angles, the avalanche signatures become strongly asymmetric due to the non-dissipative nature of the Magnus force. |
Wednesday, March 7, 2018 10:00AM - 10:12AM |
K22.00009: Proposal of a Skyrmion-Antiskyrmion racetrack memory in rank-1 DMI magnets Markus Hoffmann, Bernd Zimmermann, Gideon Müller, Nikolai S. Kiselev, Christof Melcher, Stefan Bluegel Recently, we extended the scope of skyrmions and antiskyrmions and introduced a classification scheme of chiral magnets [1]. Typically investigated Bloch-type skyrmions in B20 alloys and Néel-type skyrmions at (111) oriented interfaces belong to isotropic rank-three Dzyaloshinskii-Moriya (DM) bulk and rank-two DM film magnets with a DM interaction described by a single spiralization constant. Within this class, antiskyrmions are stable only for bulk crystals with certain point group symmetries. New are the anisotropic rank-two DMI film magnets where skyrmions and antiskyrmions can coexist while the determinant of the spiralization tensor determines which of them has lower energy. Finally, zero determinant indicates a rank-one DMI material in which they have the same energy. |
Wednesday, March 7, 2018 10:12AM - 10:24AM |
K22.00010: Reservoir Computing with Random Skyrmion Fabrics Daniele Pinna, Karin Everschor-Sitte The many nanoscale properties of skyrmions have shown promise in applications ranging from non-volatile memory and spintronic logic devices, to enabling the implementation of unconventional computational standards such as Stochastic and Reservoir Computing. Particularly, Reservoir Computing is a type of recursive neural network capable of recognizing and predicting spatio-temporal events. No knowledge of its topology or node weights are required for its basic functioning and, therefore, one can utilize naturally existing networks formed by a wide variety of physical processes. Here we will discuss how random skyrmion clusters can effectively implement a functional reservoir. This is achieved by leveraging the nonlinear resistive response of the individual skyrmions arising from their current dependent AMR. Complex time-varying current signals injected via contacts are modulated nonlinearly by the fabric’s AMR as the current flow distributes along paths of least resistance as it traverses the geometry. By tracking resistances across multiple input and output contacts, we show how the instantaneous current distribution effectively carries temporally correlated information about the injected signal allowing for a numerical demonstration of simple pattern recognition. |
Wednesday, March 7, 2018 10:24AM - 10:36AM |
K22.00011: Electrical detection of a skyrmion in a magnetic tunneling junction Keita Hamamoto, Naoto Nagaosa A magnetic skyrmion, the swirling spin texture in magnets, is expected to be a future information carrier. Many efforts, such as creation/deletion, control, and detection, are made to implement skyrmions into devices, however, the detection seems to be much difficult. In this study, we investigated the magnetoresistance (TMR) ratio in the 2 dimensional skyrmion/ferromagnet bilayer system using the tunneling Hamiltonian approach and the double exchange model. As a result, we show that the TMR ratio can be larger than 30%, which is determined sorely by the spin profile of the skyrmion, if only the spectral broadening is smaller than the exchange coupling strength. This criterion is quite simple and easily achievable in many realistic systems, therefore the magnetic tunneling junction geometry is shown to be a good candidate for the readout part of the future skyrmionics memory devices. |
Wednesday, March 7, 2018 10:36AM - 10:48AM |
K22.00012: Magnetic skyrmion resonance and characterization in magnetic tunnel junctions Nicholas Penthorn, Rajapaksayalage Rajapakse, HongWen Jiang We explore a spectroscopic method of characterizing individual Neel-type magnetic skyrmions through their resonant dynamics within a magnetic tunnel junction structure. Using micromagnetic simulations and taking thermal effects into account, we show that magnetic skyrmion resonances and their evolution in the presence of an external magnetic field can be efficiently probed with a current-induced spin-transfer torque and measured via tunnel magnetoresistance, with a magnetic noise level comparable to that found in standard ferromagnetic resonance measurements. We find that the skyrmion resonance dispersion is an accurate probe for skyrmion size, with potentially higher resolution than standard magnetic imaging techniques. This result could lead to non-invasive, on-chip skyrmion detection and measurement in skyrmion-based memory and logic architectures. |
Wednesday, March 7, 2018 10:48AM - 11:00AM |
K22.00013: Huge Differential Magneto Optic Kerr Effects of Interfacial Skyrmions on Topological Insulators Tonmoy Bhowmick, Amritanand De, Roger Lake Magneto optic memory devices typically suffer from low Kerr rotation angles and long write times. We show |
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