Topological Domain Walls in Helimagnets
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Chiral Transport Along Magnetic Domain Walls in the Quantum Anomalous Hall Effect
www.nature.com/npjquantmats ARTICLE OPEN Chiral transport along magnetic domain walls in the quantum anomalous Hall effect Ilan T. Rosen1,2, Eli J. Fox3,2, Xufeng Kou 4,5, Lei Pan4, Kang L. Wang4 and David Goldhaber-Gordon3,2 The quantum anomalous Hall effect in thin film magnetic topological insulators (MTIs) is characterized by chiral, one-dimensional conduction along the film edges when the sample is uniformly magnetized. This has been experimentally confirmed by measurements of quantized Hall resistance and near-vanishing longitudinal resistivity in magnetically doped (Bi,Sb)2Te3. Similar chiral conduction is expected along magnetic domain walls, but clear detection of these modes in MTIs has proven challenging. Here, we intentionally create a magnetic domain wall in an MTI, and study electrical transport along the domain wall. In agreement with theoretical predictions, we observe chiral transport along a domain wall. We present further evidence that two modes equilibrate while co-propagating along the length of the domain wall. npj Quantum Materials (2017) 2:69 ; doi:10.1038/s41535-017-0073-0 2 2 INTRODUCTION QAH effect, ρyx transitions from ∓ h/e to ±h/e over a substantial The recent prediction1 and subsequent discovery2 of the quantum range of field (H = 150 to H = 200 mT for the material used in the 15–20 anomalous Hall (QAH) effect in thin films of the three-dimensional work), and ρxx has a maximum in this field range. Hysteresis μ magnetic topological insulator (MTI) (CryBixSb1−x−y)2Te3 has loops of the four-terminal resistances of a 50 m wide Hall bar of opened new possibilities for chiral-edge-state-based devices in MTI film are shown in Fig. -
Deciphering Structural and Magnetic Disorder in the Chiral Skyrmion Host Materials Coxznymnz (X + Y + Z = 20)
Deciphering structural and magnetic disorder in the chiral skyrmion host materials CoxZnyMnz (x + y + z = 20) Joshua D. Bocarsly,1, ∗ Colin Heikes,2 Craig M. Brown,2 Stephen D. Wilson,1 and Ram Seshadri1 1Materials Department and Materials Research Laboratory, University of California, Santa Barbara, California 93106, United States 2Center for Neutron Research, National Institute of Standards and Technology, Gaithersburg, Maryland 20899, United States (Dated: January 9, 2019) CoxZnyMnz (x + y + z = 20) compounds crystallizing in the chiral β-Mn crystal structure are known to host skyrmion spin textures even at elevated temperatures. As in other chiral cubic skyrmion hosts, skyrmion lattices in these materials are found at equilibrium in a small pocket just below the magnetic Curie temperature. Remarkably, CoxZnyMnz compounds have also been found to host metastable non-equilibrium skyrmion lattices in a broad temperature and field range, including down to zero-field and low temperature. This behavior is believed to be related to disorder present in the materials. Here, we use neutron and synchrotron diffraction, density functional theory calculations, and DC and AC magnetic measurements, to characterize the atomic and magnetic disorder in these materials. We demonstrate that Co has a strong site-preference for the diamondoid 8c site in the crystal structure, while Mn tends to share the geometrically frustrated 12d site with Zn, due to its ability to develop a large local moment on that site. This magnetism-driven site specificity leads to distinct magnetic behavior for the Co-rich 8c sublattice and the Mn on the 12d sublattice. The Co-rich sublattice orders at high temperatures (compositionally tunable between 210 K and 470 K) with a moment around 1 µB /atom and maintains this order to low temperature. -
Mean Field Theory of Phase Transitions 1
Contents Contents i List of Tables iii List of Figures iii 7 Mean Field Theory of Phase Transitions 1 7.1 References .............................................. 1 7.2 The van der Waals system ..................................... 2 7.2.1 Equationofstate ...................................... 2 7.2.2 Analytic form of the coexistence curve near the critical point ............ 5 7.2.3 History of the van der Waals equation ......................... 8 7.3 Fluids, Magnets, and the Ising Model .............................. 10 7.3.1 Lattice gas description of a fluid ............................. 10 7.3.2 Phase diagrams and critical exponents ......................... 12 7.3.3 Gibbs-Duhem relation for magnetic systems ...................... 13 7.3.4 Order-disorder transitions ................................ 14 7.4 MeanField Theory ......................................... 16 7.4.1 h = 0 ............................................ 17 7.4.2 Specific heat ........................................ 18 7.4.3 h = 0 ............................................ 19 6 7.4.4 Magnetization dynamics ................................. 21 i ii CONTENTS 7.4.5 Beyond nearest neighbors ................................ 24 7.4.6 Ising model with long-ranged forces .......................... 25 7.5 Variational Density Matrix Method ................................ 26 7.5.1 The variational principle ................................. 26 7.5.2 Variational density matrix for the Ising model ..................... 27 7.5.3 Mean Field Theoryof the PottsModel ........................ -
Dynamics of a Ferromagnetic Domain Wall and the Barkhausen Effect
VOLUME 79, NUMBER 23 PHYSICAL REVIEW LETTERS 8DECEMBER 1997 Dynamics of a Ferromagnetic Domain Wall and the Barkhausen Effect Pierre Cizeau,1 Stefano Zapperi,1 Gianfranco Durin,2 and H. Eugene Stanley1 1Center for Polymer Studies and Department of Physics, Boston University, Boston, Massachusetts 02215 2Istituto Elettrotecnico Nazionale Galileo Ferraris and GNSM-INFM, Corso M. d'Azeglio 42, I-10125 Torino, Italy (Received 2 July 1997) We derive an equation of motion for the dynamics of a ferromagnetic domain wall driven by an external magnetic field through a disordered medium, and we study the associated depinning transition. The long-range dipolar interactions set the upper critical dimension to be dc 3, so we suggest that mean-field exponents describe the Barkhausen effect for three-dimensional soft ferromagnetic materials. We analyze the scaling of the Barkhausen jumps as a function of the field driving rate and the intensity of the demagnetizing field, and find results in quantitative agreement with experiments on crystalline and amorphous soft ferromagnetic alloys. [S0031-9007(97)04766-2] PACS numbers: 75.60.Ej, 68.35.Ct, 75.60.Ch The magnetization of a ferromagnet displays discrete Here, we present an accurate treatment of magnetic in- jumps as the external magnetic field is increased. This teractions in the context of the depinning transition, which phenomenon, known as the Barkhausen effect, was first allows us to explain the experiments and to give a micro- observed in 1919 by recording the tickling noise produced scopic justification for the model of Ref. [13]. We study by the sudden reversal of the Weiss domains [1]. -
Room-Temperature Helimagnetism in Fege Thin Films S
www.nature.com/scientificreports OPEN Room-temperature helimagnetism in FeGe thin films S. L. Zhang1, I. Stasinopoulos 2, T. Lancaster3, F. Xiao3, A. Bauer4, F. Rucker4, A. A. Baker1,5, A. I. Figueroa 5, Z. Salman 6, F. L. Pratt7, S. J. Blundell1, T. Prokscha 6, A. Suter6, J. 8 8,9 2,10 5 4 1 Received: 8 November 2016 Waizner , M. Garst , D. Grundler , G. van der Laan , C. Pfleiderer & T. Hesjedal Accepted: 14 February 2017 Chiral magnets are promising materials for the realisation of high-density and low-power spintronic Published: xx xx xxxx memory devices. For these future applications, a key requirement is the synthesis of appropriate materials in the form of thin films ordering well above room temperature. Driven by the Dzyaloshinskii- Moriya interaction, the cubic compound FeGe exhibits helimagnetism with a relatively high transition temperature of 278 K in bulk crystals. We demonstrate that this temperature can be enhanced significantly in thin films. Using x-ray scattering and ferromagnetic resonance techniques, we provide unambiguous experimental evidence for long-wavelength helimagnetic order at room temperature and magnetic properties similar to the bulk material. We obtain αintr = 0.0036 ± 0.0003 at 310 K for the intrinsic damping parameter. We probe the dynamics of the system by means of muon-spin rotation, indicating that the ground state is reached via a freezing out of slow dynamics. Our work paves the way towards the fabrication of thin films of chiral magnets that host certain spin whirls, so-called skyrmions, at room temperature and potentially offer integrability into modern electronics. -
Quantum Control of Topological Defects in Magnetic Systems
Quantum control of topological defects in magnetic systems So Takei1, 2 and Masoud Mohseni3 1Department of Physics, Queens College of the City University of New York, Queens, NY 11367, USA 2The Physics Program, The Graduate Center of the City University of New York, New York, NY 10016, USA 3Google Inc., Venice, CA 90291, USA (Dated: October 16, 2018) Energy-efficient classical information processing and storage based on topological defects in magnetic sys- tems have been studied over past decade. In this work, we introduce a class of macroscopic quantum devices in which a quantum state is stored in a topological defect of a magnetic insulator. We propose non-invasive methods to coherently control and readout the quantum state using ac magnetic fields and magnetic force mi- croscopy, respectively. This macroscopic quantum spintronic device realizes the magnetic analog of the three- level rf-SQUID qubit and is built fully out of electrical insulators with no mobile electrons, thus eliminating decoherence due to the coupling of the quantum variable to an electronic continuum and energy dissipation due to Joule heating. For a domain wall sizes of 10 100 nm and reasonable material parameters, we estimate qubit − operating temperatures in the range of 0:1 1 K, a decoherence time of about 0:01 1 µs, and the number of − − Rabi flops within the coherence time scale in the range of 102 104. − I. INTRODUCTION an order of magnitude higher than the existing superconduct- ing qubits, thus opening the possibility of macroscopic quan- tum information processing at temperatures above the dilu- Topological spin structures are stable magnetic configura- tion fridge range. -
Investigation of Magnetic Barkhausen Noise and Dynamic Domain Wall Behavior for Stress Measurement
19th World Conference on Non-Destructive Testing 2016 Investigation of Magnetic Barkhausen Noise and Dynamic Domain Wall Behavior for Stress Measurement Yunlai GAO 1,3, Gui Yun TIAN 1,2,3, Fasheng QIU 2, Ping WANG 1, Wenwei REN 2, Bin GAO 2,3 1 College of Automation Engineering, Nanjing University of Aeronautics and Astronautics, Nanjing 211106, P.R. China; 2 School of Automation Engineering, University of Electronic Science and Technology of China, Chengdu 611731, P.R. China 3 School of Electrical and Electronic Engineering, Newcastle University, Newcastle upon Tyne, NE1 7RU, United Kingdom Contact e-mail: [email protected]; [email protected] Abstract. Magnetic Barkhausen Noise (MBN) is an effective non-destructive testing (NDT) technique for stress measurement of ferromagnetic material through dynamic magnetization. However, the fundamental physics of stress effect on the MBN signals are difficult to fully reveal without domain structures knowledge in micro-magnetics. This paper investigates the correlation and physical interpretation between the MBN signals and dynamic domain walls (DWs) behaviours of an electrical steel under applied tensile stresses range from 0 MPa to 94.2 MPa. Experimental studies are conducted to obtain the MBN signals and DWs texture images as well as B-H curves simultaneously using the MBN system and longitudinal Magneto-Optical Kerr Effect (MOKE) microscopy. The MBN envelope features are extracted and analysed with the differential permeability of B-H curves. The DWs texture characteristics and motion velocity are tracked by optical-flow algorithm. The correlation between MBN features and DWs velocity are discussed to bridge the gaps of macro and micro electromagnetic NDT for material properties and stress evaluation. -
The Barkhausen Effect
The Barkhausen Effect V´ıctor Navas Portella Facultat de F´ısica, Universitat de Barcelona, Diagonal 645, 08028 Barcelona, Spain. This work presents an introduction to the Barkhausen effect. First, experimental measurements of Barkhausen noise detected in a soft iron sample will be exposed and analysed. Two different kinds of simulations of the 2-d Out of Equilibrium Random Field Ising Model (RFIM) at T=0 will be performed in order to explain this effect: one with periodic boundary conditions (PBC) and the other with fixed boundary conditions (FBC). The first model represents a spin nucleation dynamics whereas the second one represents the dynamics of a single domain wall. Results from these two different models will be contrasted and discussed in order to understand the nature of this effect. I. INTRODUCTION duced and contrasted with other simulations from Ref.[1]. Moreover, a new version of RFIM with Fixed Boundary conditions (FBC) will be simulated in order to study a The Barkhausen (BK) effect is a physical phenomenon single domain wall which proceeds by avalanches. Dif- which manifests during the magnetization process in fer- ferences between these two models will be explained in romagnetic materials: an irregular noise appears in con- section IV. trast with the external magnetic field H~ ext, which is varied smoothly with the time. This effect, discovered by the German physicist Heinrich Barkhausen in 1919, represents the first indirect evidence of the existence of II. EXPERIMENTS magnetic domains. The discontinuities in this noise cor- respond to irregular fluctuations of domain walls whose BK experiments are based on the detection of the motion proceeds in stochastic jumps or avalanches. -
The Dynamics of Domain Wall Motion in Spintronics
magnetochemistry Communication The Dynamics of Domain Wall Motion in Spintronics Diego Bisero Dipartimento di Fisica e Scienze della Terra, Università degli studi di Ferrara, Via Saragat 1, I-44122 Ferrara, Italy; [email protected] Received: 26 May 2020; Accepted: 24 June 2020; Published: 2 September 2020 Abstract: A general equation describing the motion of domain walls in a magnetic thin film in the presence of an external magnetic field has been reported in this paper. The equation includes all the contributions from the effects of domain wall inertia, damping and stiffness. The effective mass of the domain wall, the effects of both the interaction of the DW with the imperfections in the material and damping have been calculated. Keywords: magnetic thin films; domain walls; magnetic nanostructures; theoretical models and calculations 1. Introduction Domain wall (DW) motion can be induced by external magnetic fields or by spin polarized currents. Domain wall (DW) propagation [1–4] in laterally patterned magnetic thin films [5–8] holds promise for both fundamental research and technological applications and has attracted much attention because of its potential in data storage technology and logic gates, where data can be encoded nonvolatilely as a pattern of magnetic DWs traveling along magnetic wires [9]. Up to now, a remarkable part of measurements with injected spin polarized currents have been performed on systems with the magnetization direction in-plane. Nanosized wires containing DWs driven by spin-transfer torque are the basis, at the moment, of the racetrack memory concept development. However, the induction of DW displacement requires high spin-current densities, unsuitable to realize low power devices. -
Antiferromagnetic Domain Wall As Spin Wave Polarizer and Retarder
ARTICLE DOI: 10.1038/s41467-017-00265-5 OPEN Antiferromagnetic domain wall as spin wave polarizer and retarder Jin Lan 1, Weichao Yu1 & Jiang Xiao 1,2,3 As a collective quasiparticle excitation of the magnetic order in magnetic materials, spin wave, or magnon when quantized, can propagate in both conducting and insulating materials. Like the manipulation of its optical counterpart, the ability to manipulate spin wave polar- ization is not only important but also fundamental for magnonics. With only one type of magnetic lattice, ferromagnets can only accommodate the right-handed circularly polarized spin wave modes, which leaves no freedom for polarization manipulation. In contrast, anti- ferromagnets, with two opposite magnetic sublattices, have both left and right-circular polarizations, and all linear and elliptical polarizations. Here we demonstrate theoretically and confirm by micromagnetic simulations that, in the presence of Dzyaloshinskii-Moriya inter- action, an antiferromagnetic domain wall acts naturally as a spin wave polarizer or a spin wave retarder (waveplate). Our findings provide extremely simple yet flexible routes toward magnonic information processing by harnessing the polarization degree of freedom of spin wave. 1 Department of Physics and State Key Laboratory of Surface Physics, Fudan University, Shanghai 200433, China. 2 Collaborative Innovation Center of Advanced Microstructures, Nanjing 210093, China. 3 Institute for Nanoelectronics Devices and Quantum Computing, Fudan University, Shanghai 200433, China. Jin Lan and -
Fractional Charge and Quantized Current in the Quantum Spin Hall State
LETTERS Fractional charge and quantized current in the quantum spin Hall state XIAO-LIANG QI, TAYLOR L. HUGHES AND SHOU-CHENG ZHANG* Department of Physics, McCullough Building, Stanford University, Stanford, California 94305-4045, USA *e-mail: [email protected] Published online: 16 March 2008; doi:10.1038/nphys913 Soon after the theoretical proposal of the intrinsic spin Hall spin-down (or -up) left-mover. Therefore, the helical liquid has half effect1,2 in doped semiconductors, the concept of a time-reversal the degrees of freedom of a conventional one-dimensional system, invariant spin Hall insulator3 was introduced. In the extreme and thus avoids the doubling problem. Because of this fundamental quantum limit, a quantum spin Hall (QSH) insulator state has topological property of the helical liquid, a domain wall carries been proposed for various systems4–6. Recently, the QSH effect charge e/2. We propose a Coulomb blockade experiment to observe has been theoretically proposed6 and experimentally observed7 this fractional charge. As a temporal analogue of the fractional in HgTe quantum wells. One central question, however, remains charge effect, the pumping of a quantized charge current during unanswered—what is the direct experimental manifestation each periodic rotation of a magnetic field is also proposed. This of this topologically non-trivial state of matter? In the case provides a direct realization of Thouless’s topological pumping13. of the quantum Hall effect, it is the quantization of the We can express the effective theory for the edge states of a non- Hall conductance and the fractional charge of quasiparticles, trivial QSH insulator as which are results of non-trivial topological structure. -
Magnetic Susceptibility and Chemical Shift
A Magnetic susceptibility and chemical shift Susceptibility and chemical shift both deal with the interaction of electrons with a mag- netic field. Therefore, it is often difficult to distinguish these phenomena in MR images. The major difference between these two Zeemanlike influences is in the locality of their action and in their orientational dependence in an external polarizing field [1]. Chemical shift is a local phenomenon, acting on a single nucleus, while magnetic susceptibility acts on a larger scale. In this chapter, both phenomena are described and some consequences for MR imaging are illustrated by an infinite coaxial cylinder. Magnetic susceptibility The magnetic moments associated with atoms in materials are mainly determined by fac- tors originating from electrons and include the electron spin, electron orbital motion and the change in electron orbital motion caused by an imposed magnetic field. The existence and interaction of one or more of these phenomena in a material, determine the magnetic behavior. Also nuclear magnetism contributes to the magnetic moment, but it is weak and has a negligible effect on the bulk susceptibility [2]. Therefore, it will not be described in this appendix. Traditionally, classification of materials in groups according to their mag- netic behavior resulted in a division into groups according to the bulk susceptibility. The first group includes materials with negative and small susceptibility (χ = O( 10−5)) and includes e.g. copper, silver, bismuth and gold. These materials are called diamagnetic.− Paramagnetic materials have a small, positive susceptibility (χ = O(10−3) O(10−5)) and include aluminum, manganese, platinum and titanium.