Session V50: Skyrmions

Tunable skyrmionic states by the magnetic field in a nanodisk of chiral magnets

Experimental observations of magnetic skyrmions, have been recently addressed in numerous non-centrosymmetric ferromagnetic materials [1]. In most previous reports, skyrmions are only stabilized in the presence of an external magnetic field. However, even without the magnetic field, skyrmions can be also generated in ultra-small nanodisks by a reasonably structural design. We investigate the spin textures in FeGe nanodisks, and find two types of skyrmions with opposite core polarizations, which can be switched by the external magnetic field. Our results are in good agreement with recent experimental observations. Our numerical results also show the parametric dependences of the different skyrmion states, which exhibit similarities to the localized states and open a new avenue to the development of skyrmionic devices by structural designs. [1] Naoto Nagaosa and Yoshinori Tokura, Nat. nano. 8, 899(2013).   

Skyrmion oscillations in chiral cylindrical nanowires

The occurrence of skyrmions on surfaces due to the competition of symmetric and antisymmetric interactions is a fascinating phenomenon with a promising potential for new technologies. The spatial confinement of spin textures in nanostructures, such as thin films, and the breaking of symmetry by an external or internal field enable the formation of skyrmions and skyrmion lattices. In cylindrical nanowires, the spatial confinement and the symmetry-breaking field are provided by the solid itself due to magnetostatics, i.e., shape anisotropy. Based on high-resolution micromagnetic simulations we will show that in cylindrical nanowires of FeGe non-trivial skyrmionic spin textures occur, which resemble a skyrmion chain. These break the translational symmetry along the wire via an oscillation of the topological charge. We will also discuss how external fields can manipulate the skyrmion-chain state and how magnetization switching occurs via the formation of Bloch points.   

Probing depth-dependent spin textures in artificial skyrmions, magneto-ionic systems and HAMR media

Nanoscale spin textures such as skyrmions and domain walls have received intense interest due to their topological characteristics and potential applications in low power information storage. However, probing their depth-dependent magnetic configurations has been very challenging. Here we report a series of studies on depth-dependent spin textures in technologically important material systems, using a combination of magnetometry, magnetic imaging, polarized neutron reflectometry, x-ray magnetic circular dichroism, and spin transport techniques. In one study, we have achieved room temperature artificial Bloch skyrmion lattices in their ground state [1], as well as a new type of planar skyrmion lattices, without any magnetic nanodots protruding on top. In another study, we have demonstrated magneto-ionic control of metal/oxide interfaces, where the oxygen migration is observed and manifested in an interface-sensitive exchange bias effect [2]. We further show that the magnetoelectric coupling moderated by voltage-driven oxygen migration extends beyond the interface region in relatively thick films [3]. Finally, we have shown an extremely sensitive magnetic yoking effect and tunable interactions in FePt based hard/soft bilayers which are prototype heat-assisted magnetic recording media [4]. Work done in collaboration with J. Olamit, R. K. Dumas, B. J. Kirby, A. J. Grutter, B. B. Maranville, E. Arenholz, A. L. Balk, P. Fischer, D. T. Pierce, J. Unguris, J. A. Borchers, J.W. Liao, M. Winklhofer, C. H. Lai, and Kai Liu. 1. D. A. Gilbert, et al., Nature Commun. 6, 8462 (2015). 2. D. A. Gilbert, et al., Nature Commun. 7, 11050 (2016). 3. D. A. Gilbert, et al., Nature Commun. 7, 12264 (2016). 4. D. A. Gilbert, et al., Sci. Rep. 6, 32842 (2016).   

Creating Skyrmions with Electric Fields: Experiment and Theory

Skyrmions are topologically protected nanoscale spin whirls envisaged as promising information carriers. In this talk, we report both writing and erasing skyrmions with moderate electric fields in ${{\text{Cu}_2 \text{O} \text{Se} \text{O}_3}}$. Using neutron scattering, we demonstrate that the skyrmion pocket either expands or shrinks significantly depending on the direction of electric fields, allowing us to write or erase the skyrmion phase in bulk. To explain the observed phenomena, the effect is addressed theoretically by using the framework of fluctuation-induced phase transitions and the first order perturbation theory in electric fields. As the electric field is almost not heating the insulating ${{\text{Cu}_2 \text{O} \text{Se} \text{O}_3}}$ samples, our study provides further perspectives for dissipation-free electrical control of skyrmions in insulators.   

Switching fixed skyrmions with electrical field in the presence of thermal noise

Switching deterministically and reliably between core pointing up and core pointing down states of magnetic skyrmions could lead to an energy efficient paradigm for the realization of nanomagnetic memory. Perpendicular magnetic anisotropy (PMA) in a ferromagnet/oxide interface can be modulated by employing an electric field and thereby cause core reversal of magnetic skyrmions with an electric field without needing a magnetic field or spin current [1]. Furthermore, in devices that are switched with spin current, voltage control of magnetic anisotropy can reduce the critical current density required for such a reversal [2]. However, switching probability (error) in the presence of thermal noise in these reversal mechanisms is key to their performance. Here, we present stochastic magnetization dynamics simulations to establish switching error at room temperature, how it is impacted by intermediate magnetic states visited and interface properties (i.e. PMA and Dzyaloshinskii-Moriya interaction). 1. D Bhattacharya, MM Al-Rashid, J Atulasimha, Sci. Rep. 6, 31272, 2016 2. D Bhattacharya, MM Al-Rashid, J Atulasimha, arXiv 1611.03138, 2016   

Skyrmion Dynamics in Co/Pt Bilayers Using Spin Transfer Torque

Finding efficient methods to write and read individual skyrmions under ambient conditions is an important first step toward realizing skyrmion-based applications, such as high density information storage and racetrack memory. Of recent interest experimentally are heavy metal /ferromagnet bilayers with a strong interfacial Dzyaloshinskii-Moriya interaction and perpendicular magnetic anisotropy (PMA), which favor the formation of helical spin textures. Micromagnetic simulations of these materials suggest that an out of plane spin polarized current could be used to excite skyrmion dynamics and possibly create isolated skyrmions. In this project, we study a spin-valve-like device consisting of a PMA nanopillar on top of an extended Co/Pt bilayer. The nanopillar generates a spin polarized current, which is expected to locally reverse the magnetization of the film underneath the pillar via spin transfer torque. We report measurements of the DC current and field dependence of the pillar resistance. Effects local to the pillar are isolated by independently monitoring the magnetization state of the extended bilayer. We attempt to measure the emission frequency of dynamical modes and compare it to theory.   

Skyrmion motion induced by plane stress waves

Skyrmions are typically driven by currents and magnetic fields. We propose an alternative method of driving skyrmions using plane stress waves in a chiral ferromagnetic nanotrack. We find that the effective force due to surface acoustic waves couples both to the helicity and the topological charge of the skyrmion. This coupling can be used to probe the helicity of the skyrmion as well as the nature of the Dzyaloshinskii-Moriya interaction. This is particularly important when a ferromagnet lacks both surface- and bulk-inversion symmetry. Plane stress waves can be generated using a pair of interdigital transducers (IDTs). As the nanowire is subject to half-open space boundary conditions, the skyrmion is driven by normal stress in this setup. We find that skyrmions get pinned at the antinodes of the stress wave, much similar to domain walls, which enables skyrmion motion by detuned IDTs. We also consider a nanotrack sandwiched between a piezoelectric layer and a substrate, with electrical contacts placed on top, which results in shear stress in addition to normal stress in nanotrack. We find that unlike domain walls, skyrmions can be driven using shear component of a standing stress wave.   

Reversible Vector Ratchet Effect in Skyrmion Systems

Magnetic skyrmions are topological non-trivial spin textures found in several magnetic materials. Since their motion can be controlled using ultralow current densities, skyrmions are appealing for potential applications in spintronics as information carriers and processing devices. In this work, we studied the collective transport properties of driven skyrmions based on a particle-like model with molecular dynamics (MD) simulation. Our results show that ac driven skyrmions interacting with an asymmetric substrate provide a realization of a new class of ratchet system, which we call a vector ratchet, that arises due to the effect of the Magnus term on the skyrmion dynamics. In a vector ratchet, the dc motion induced by the ac drive can be described as a vector that can be rotated up to 360 degrees relative to the substrate asymmetry direction. This could represent a new method for controlling skyrmion motion for spintronic applications.   

Mapping the skyrmion phase diagram in epitaxial FeGe/Si(111) grown by molecular beam epitaxy

Skyrmions are localized magnetic textures which have particle-like behavior and exist in magnetic materials with broken-inversion symmetry. In addition, the stability and size of the skyrmion are determined by the competition between the exchange interaction and the Dzyaloshinskii-Moriya interaction (DMI). However, in light of these potential applications, the skyrmion phase exists primarily with an external magnetic field and temperature range, the existence of skyrmions in bulk versus thin films can be drastically different. We have grown epitaxial thin films of FeGe/Si(111) by molecular beam epitaxy. Through a combination of reflection high energy electron diffraction and x-ray diffraction, we confirm that we obtain high quality single crystal thin films. The skyrmion phase diagram is mapped out with two techniques: susceptibility measurements from SQUID magnetometry and electrical properties via topological Hall effect. We will discuss how both techniques can be complimentary in identifying the existence of skyrmions in the H vs T phase diagram. We also show a thickness dependence ranging from 20 nm films to 1000 nm films and show that the skyrmion phase can be quite robust in our thickest films.   

First-principles study of anomalous Nernst effect in skyrmion crystals

The magnetic skyrmion, a topological object made up of spins in condensed matter, exhibits many peculiar properties, among which we target the anomalous Nernst effect (ANE), heat-to-electricity conversion in transverse direction, driven by an emergent magnetic field B originating from its spin texture. We have so far found from computations on some models that, in the so-called 2D SkX phase, where skyrmions are crystallized in two dimensions, the crystal-momentum component of B gives rise to the band structure that could generate large ANE when chemical potential μ is properly tuned [1]. Although this behavior was most clearly confirmed in the simplest model of square SkX with single s-orbital per site, our subsequent computations on more realistic models of transition-metal oxides also showed possible large ANE. In this presentation, such intriguing results, the details of our first-principles computational procedures, as well as the origin of large ANE will be discussed. [1] Y. P. Mizuta and F. Ishii, Scientific Reports 6,  28076 (2016)   

Skyrmion Quantum tunneling

Skyrmion is a topological spin-texture of pseudo-two dimensional magnetic systems. Classical nature of skyrmions has been thoroughly understood by the Landau-Lifshitz-Gilbert equation, but its quantum nature by contrast is an intriguing yet poorly understood issue. Here, we investigate the quantum nature focusing on quantum tunnelling, which would be a key component of the overall skyrmion-based electronics, so-called skyrmionics. We determine physical properties such as the size and mass of skyrmions {\it self-consistently}. Characteristic dependences of the physical properties on external magnetic field and Dzyaloshinskii-Moriya interaction are obtained. Armed with the physical properties of skyrmions, we find skyrmion quantum tunneling is observable below 1 K, given typical experimental and material conditions.Several tuning parameters such as external electric-current and the strengths of potential barriers are considered and their intriguing influences on the skyrmion quantum tunnelling through the Berry phase effects are determined. We further propose a plausible experimental setup for the observation of skyrmion quantum tunnelling.