Direct imaging of magnetic field-driven transitions of skyrmion cluster states in FeGe nanodisks

Abstract

Magnetic skyrmion is a nanosized magnetic whirl with nontrivial topology, which is highly relevant for applications on future memory devices. To enable the applications, theoretical efforts have been made to understand the dynamics of individual skyrmions in magnetic nanostructures. However, directly imaging the evolution of highly geometrically confined individual skyrmions is challenging. Here, we report the magnetic field-driven dynamics of individual skyrmions in FeGe nanodisks with diameters on the order of several skyrmion sizes by using Lorentz transmission electron microscopy. In contrast to the conventional skyrmion lattice in bulk, a series of skyrmion cluster states with different geometrical configurations and the fielddriven cascading phase transitions are identified at temperatures far below the magnetic transition temperature. Furthermore, a dynamics, namely the intermittent jumps between the neighboring skyrmion cluster states, is found at elevated temperatures, at which the thermal energy competes with the energy barrier between the skyrmion cluster states.T he complex spin configurations in helimagnets have attracted considerable attention recently, with the topologically stable particle-like spin texture with a size down to the nanoscale, namely magnetic skyrmion, as the focus of interest (1). Magnetic skyrmion is characterized as a nanoscale topological particle producing unconventional spin electronic phenomena (2, 3) that holds great promise for future spintronic devices, including racetrack memory (4), magnetic random access memory, and magnetic sensors (1). Essentially, such schemes rely on the controllable formation and manipulation of individual skyrmions at nanostructured elements with various shapes such as disks, stripes, or wires (5-7). Investigation of skyrmions in confined geometries has therefore become one of the major topics in the field of skyrmion physics (5-10).Unlike ordinary magnetic vortices in microsized soft magnetic disks due to the minimization of the dipolar energy (11), the key ingredient of stabilizing skyrmions in helimagnets is the antisymmetry Dzyaloshinskii−Moriya (DM) interactions originating from the broken inversion symmetry (12). The competition of the DM coupling with ferromagnetic exchange interaction results in periodic helical ground state in helimagnets. Under the action of a magnetic field and temperature, these magnetic helices transfer into skyrmion crystal with triangular lattice configuration, and finally to the field-polarized ferromagnetic state. Notably, both ferromagnetic and DM couplings occur among the neighboring spins and belong to short-range interaction. Thus, it was demonstrated theoretically that skyrmions cluster states, characterized by certain arrangements of limited skyrmions, still persist even in submicrometer objects (8, 13). There, the longrange lattice form of skyrmions in 2D or bulk helimagnets is broken, but a short-ranged ordering with specific geometrical symmetries still remains, and the number of skyrmions in the cluster...

Control of morphology and formation of highly geometrically confined magnetic skyrmions

“…According to theoretical analysis, such an edge spin configuration can be regarded as a type of surface state in a chiral magnet, which preserves the magnetic chirality of the spin texture around the edge14151617 and has been anticipated to play a key role in current-induced skyrmion motion in nanostripes1213. Experimentally, such chiral edge twists have been inferred in our previous Lorentz TEM imaging of magnetic structure in both nanostripes18 and nanodisks19. However, the contrast around the edge associated with Fresnel fringes from the rapid change in specimen thickness, as discussed in the introduction, complicated the observed images, especially for sample sizes of below 100 nm.…”

Characteristics of ferroelectric-ferroelastic domains in Néel-type skyrmion host GaV4S8

“…Recently, intensive research is focused on the formation and stability of skyrmions in confined geometries, such as nanowires and nanodisks3132333435 for potential memory applications. We expect that engineering of the ferroelectric domain patters in polar skyrmion host compounds may provide a new route to create geometrical confinement for skyrmions in bulk 3D materials as well.…”

An Improved Racetrack Structure for Transporting a Skyrmion

“…Many progresses have been made on the dynamics of current-induced skyrmion motion2381423, however, there is still a long way for putting the skyrmion-based racetrack memory to practice. For example, to create, drive and annihilate/destroy the skyrmions or the skyrmion cluster2425 at will is still a challenge. In particular, current-driven skyrmions will drift from the racetrack direction due to the presence of Magnus force142026 if the velocity is high enough, which leads to their annihilation at the racetrack edge and the loss of information.…”