Brownian motion of magnetic domain walls and skyrmions, and their diffusion constants (original) (raw)

Diffusion of magnetic skyrmions in 2-dimensional space

2021

Two-dimensional magnetic skyrmions are particle-like magnetic domains in magnetic thin films. The kinetic property of the magnetic skyrmions at finite temperature is well described by the Thiele equation, including a stochastic field and a finite mass. In this paper, the validity of the constant-mass approximation is examined by comparing the Fourier spectrum of Brownian motions described by the Thiele equation and the Landau-Lifshitz- Gilbert equation. Then, the 4-dimensional Fokker-Planck equation of the skyrmion is derived from the Thiele equation with a mass-term. Finally, an expression of the diffusion flow and diffusion constant in a tensor form are derived under a local thermalization approximation. The result is also applicable for the chiral diffusions, such as a diffusion of the classical electron gas in a magnetic field.

Gate-controlled skyrmion and domain wall chirality

Nature Communications

Magnetic skyrmions are localized chiral spin textures, which offer great promise to store and process information at the nanoscale. In the presence of asymmetric exchange interactions, their chirality, which governs their dynamics, is generally considered as an intrinsic parameter set during the sample deposition. In this work, we experimentally demonstrate that a gate voltage can control this key parameter. We probe the chirality of skyrmions and chiral domain walls by observing the direction of their current-induced motion and show that a gate voltage can reverse it. This local and dynamical reversal of the chirality is due to a sign inversion of the interfacial Dzyaloshinskii-Moriya interaction that we attribute to ionic migration of oxygen under gate voltage. Micromagnetic simulations show that the chirality reversal is a continuous transformation, in which the skyrmion is conserved. This control of chirality with 2–3 V gate voltage can be used for skyrmion-based logic devices, ...

Experimental observation of magnetic domain wall skyrmions

arXiv: Materials Science, 2020

Topological magnetic excitations called skyrmions exhibit striking spin configurations with deterministic chirality stabilized by the Dzyaloshinskii-Moriya interaction (DMI). These objects come in many forms depending on the symmetry and dimensionality of the system under consideration. Here, for the first time, we experimentally observe a new topological excitation called a magnetic domain wall (DW) skyrmion using Lorentz transmission electron microscopy (LTEM). LTEM contrast matches images simulated from micromagnetic calculations and their expected pinning behavior is observed through \textit{in situ} application of a perpendicular magnetic field. Calculations of the energy barrier to DW skyrmion annihilation using the micromagnetic geodesic nudged elastic band (GNEB) model support this observed metastability of DW skyrmions at room temperature.

Current-driven dynamics of magnetic skyrmions in an ultrathin film: experiments and modelling

2019

Magnetic skyrmions are chiral spin textures which hold great promise as nanoscale information carriers. Their recent observation at room temperature and their fast current-induced manipulation in multiple repetitions of heavy metal/ferromagnetic stacks have lifted an important bottleneck towards the practical realisation of skyrmion-based devices. However, the complex spin textures and large power dissipation in these multilayers limit their practical implementation as well as the fundamental understanding of the skyrmion dynamics. Here, we report on the current-driven motion of skyrmions in an ultrathin Pt/Co/MgO model system. We find that skyrmions with diameters in the 100 nm range can move at speeds up to 100 m.s^-1. Our experiments also reveal that the skyrmion Hall effect is markedly drive-dependent. These observations are well substantiated both by a simple analytical model and micromagnetic simulations, which highlight the important role of pinning in the skyrmion dynamics.

Thermal stability of isolated skyrmions in an ultrathin magnetic film

2016

The thermal stability of isolated skyrmions is studied in a Co/Pt(111) monolayer, using atomic scale simulations. Langevin dynamics around 80~K is first used to simulate the thermal collapse, with lifetimes of a few tens of nanoseconds under a destabilizing field of 0.25 T. A path method is then employed, to generally and precisely describe the skyrmion collapse. Two mechanisms are found, and discussed in relation with the change of topology between the skyrmion and uniform states. It appears that, for the lowest energy barrier path, skyrmion destabilization occurs much before any topology change, suggesting that topology plays a minor role in the skyrmion stability. On the contrary, an important role appears devoted to the Dzyaloshinskii-Moriya interaction, establishing a route towards improved skyrmion stability.