Chiral skyrmions in an anisotropy gradient

Abstract

A strategy to drive skyrmion motion by a combination of an anisotropy gradient and spin-Hall effect has recently been demonstrated. Here, we study the fundamental properties of this type of motion by combining micromagnetic simulations and a generalized Thiele's equation. We find that the anisotropy gradient drives the skyrmion mainly along the direction perpendicular to the gradient, due to the conservative part of the torque. There is some slower motion along the direction parallel to the anisotropy gradient due to damping torque. When an appropriate spin-Hall torque is added, the skyrmion velocity in the direction of the anisotropy gradient can be enhanced. This motion gives rise to acceleration of the skyrmion as this moves to regions of varying anisotropy. This phenomenon should be taken into account in experiments for the correct evaluation of the skyrmion velocity. We employ a Thiele-like formalism and derive expressions for the velocity and the acceleration of the skyrmion that match very well with micromagnetic simulation results.

Skyrmion motion induced by voltage-controlled in-plane strain gradients

“…8,9 The traditional way to move skyrmions is by spin-polarized currents, [10][11][12][13] but other approaches that do not involve charge currents and the associated Joule heating effects are being explored, such as electric fields, 14,15 spatially variable magnetic fields, 16,17 temperature gradients, 18,19 spin waves, 20 and voltage-controlled anisotropy gradients. [21][22][23][24] Meanwhile, the possibility of controlling magnetic textures using mechanical stress is currently being investigated, with promising results in domain walls 25,26 and skyrmions. [27][28][29][30][31] In particular, there is experimental evidence that skyrmions can be nucleated and annihilated through stress control of the topological phase transition in bulk single crystals, 27,28 whereas simulations indicate that voltage-controlled strain-mediated switching of skyrmions is possible in heterostructures with the interfacial Dzyaloshinskii-Moriya (DM) interaction on top of a piezoelectric (PZ) substrate.…”

Driven gyrotropic skyrmion motion through steps in magnetic anisotropy

“…This is due to the fact that the skyrmion energy is position-dependent in the simulations due to skyrmion expansion, while in the theoretical calculation it is simplified using a fixed skyrmion radius. If Δ K u > 3 × 10 12 J/m 4 , the inflation of the skyrmion radius breaks this assumption down 24 . Similarly, for large α , the skyrmion radius expands during the simulation.…”

“…Voltage-controlled magnetic anisotropy (VCMA) has been combined with skyrmion devices in various proposals to create static and racetrack-like memories 19–23 . However, these proposals, similarly to proposals concerning current-driven skyrmions, do not exploit the gyrotropic motion created by the topology of skyrmions 9 , with few exceptions 24 .…”

“…In this work, we demonstrate how skyrmion motion can be induced by a step in magnetic anisotropy, which could be created by VCMA, and propose a device implementation exploiting this effect. We start by studying the motion of skyrmions in an anisotropy gradient 24 , comparing the results of micromagnetic simlations with predictions of the analytical Thiele equation 25 which describes the motion of a rigid skyrmion. We investigate how different material parameters affect the skyrmion motion.…”

Bilayer skyrmion dynamics on a magnetic anisotropy gradient

“…The use of a magnetic anisotropy gradient for skyrmion propagation is favoured for its ease of electrical integration and control [27][28][29][30][31]. A magnetic anisotropy gradient is known to exert a force to propagate skyrmions towards the region with lower Ku [32][33][34].…”