Current-Induced Dynamics of Chiral Magnetic Structures : Skyrmions, Emergent Electrodynamics and Spin-Transfer Torques

In magnets without inversion symmetry weak spin-orbit coupling leads to the formation of smooth twisted magnetic structures like helices with a long period. In 2009, a new chiral magnetic phase was observed in the helimagnet manganese silicide (MnSi) within a certain temperature and magnetic field range. It turned out that this phase is a lattice of skyrmions which are topologically stable whirl-lines. In the first Part of this Thesis, we review the discovery of the skyrmion lattice in MnSi, its manifestation in other chiral magnets and in thin films. Furthermore, we review the Ginzburg-Landau theory for chiral magnetic structures describing their equilibrium properties, followed by a discussion of additional terms that orient and distort the skyrmion lattice. Finally, we analyze the crystalline nature of the skyrmion lattice. In the second Part of this Thesis, we investigate the interplay of electric currents and magnetic structures in bulk materials, in particular the skyrmion phase. Electrons traversing a spatially or temporally inhomogeneous magnetization configuration pick up a Berry phase which, rewritten as an Aharonov-Bohm phase arising from emergent magnetic and electric fields, leads to an effective Lorentz force acting on the electrons. For the skyrmion lattice these emergent fields are particularly interesting as the emergent magnetic field is quantized due to the topology of the skyrmions. On the other hand, the electric current induces forces on the magnetic texture via spin-tranfer torques, describing the transfer of angular momentum from the conduction electrons to the local magnetic structure. We show that skyrmions act very sensitively to electric currents, and we study their current-induced dynamics, i.e. the translational motion as well as rotations of the topologically stable knots. This research was, is and will be done in cooperation with recent experiments performed by the group of Prof. Dr. Christian Pfleiderer from the Technical University of Munich. The very efficient coupling of skyrmions to electric currents was experimentally confirmed by an ultra-low threshold current density of about 10^6 A/m^2 above which spin-transfer torque effects were observed. It is about five order of magnitude smaller compared to that of other present-day spin-torque effects like domain wall motion. Hence, skyrmions are expected to be excellent systems to study the interplay of magnetism and electric currents, thereby advancing the general understanding of spin-transfer torque effects. We further think that the gained knowledge from such studies might be useful for future spintronic devices

Twists in Ferromagnetic Monolayers With Trigonal Prismatic Symmetry

Two-dimensional materials such as graphene or hexagonal boron nitride are indispensable in industry. The recently discovered 2D ferromagnetic materials also promise to be vital for applications. In this work, we develop a phenomenological description of non-centrosymmetric 2D ferromagnets with trigonal prismatic crystal structure. We chose to study this special symmetry group since these materials do break inversion symmetry and therefore, in principle, allow for chiral spin structures such as magnetic helices and skyrmions. However, unlike all non-centrosymmetric magnets known so far, we show that the symmetry of magnetic trigonal prismatic monolayers neither allow for an internal relativistic Dzyaloshinskii-Moriya interaction (DMI) nor a reactive spin-orbit torque. We demonstrate that the DMI only becomes important at the boundaries, where it modifies the boundary conditions of the magnetization and leads to a helical equilibrium state with a helical wavevector that is inherently linked to the internal spin orientation. Furthermore, we find that the helical wavevector can be electrically manipulated via dissipative spin-torque mechanisms. Our results reveal that 2D magnets offer a large potential for unexplored magnetic effects.Comment: 5 pages, 3 figure

Potential implementation of Reservoir Computing models based on magnetic skyrmions

Reservoir Computing is a type of recursive neural network commonly used for recognizing and predicting spatio-temporal events relying on a complex hierarchy of nested feedback loops to generate a memory functionality. The Reservoir Computing paradigm does not require any knowledge of the reservoir topology or node weights for training purposes and can therefore utilize naturally existing networks formed by a wide variety of physical processes. Most efforts prior to this have focused on utilizing memristor techniques to implement recursive neural networks. This paper examines the potential of skyrmion fabrics formed in magnets with broken inversion symmetry that may provide an attractive physical instantiation for Reservoir Computing.Comment: 11 pages, 3 figure