A research team led by Masahito Mochizuki and Xichao Zhang at Waseda University, working with collaborators from the Hong Kong University of Science and Technology, Los Alamos National Laboratory, the Chinese University of Hong Kong, Nanjing University, and the University of York, has shown that magnetic skyrmions can diffuse asymmetrically when confined to a structured environment, moving more readily in one direction than the other even though their underlying thermal motion is random. The work proposes topology and geometry as a new lever for controlling diffusion, with potential relevance to unconventional, physics-based computing hardware.
Skyrmions are particle-like, topologically protected spin textures that can be nudged into motion by tiny thermal fluctuations, and prior work has documented behaviors, such as wall-guided "Brownian gyromotion," that ordinary particles don't exhibit. Directional or asymmetric diffusion of particle-like systems has drawn growing interest for unconventional AI hardware, where geometry rather than circuitry could shape how information propagates. Until now, though, how skyrmions diffuse in structured, chamber-like environments, as opposed to open thin films, had been largely unexplored.
To study this, the team built a theoretical model of Néel-type skyrmions confined to a nanostructured magnetic thin film divided into two chambers connected by a narrow, off-center asymmetric gate, then confirmed the model's predictions with computational simulations of multiple repulsive skyrmions released on either side of the gate. Skyrmions starting in one chamber passed through the gate more readily than those approaching from the other side. The asymmetry traces back to skyrmion topology rather than the gate shape alone: when a skyrmion nears a chamber wall, the wall's repulsive force drives it into a gyrating, wall-guided motion, and because the off-center gate exposes the skyrmion to different sections of the wall depending on which direction it's arriving from, that gyromotion ends up favoring one crossing direction over the other. A centered, symmetric gate, by contrast, produced nearly symmetric diffusion in both directions.
The simulations also turned up two related effects. Diffusing skyrmions can transiently pair up and rotate around one another through their mutual repulsion, forming short-lived "binary" skyrmion systems, and at high enough density, skyrmions can be pushed out of a chamber without meaningfully interacting with the gate at all. Gate width mattered too: an opening much wider than a skyrmion's diameter let everything through regardless of direction, an opening too narrow blocked passage entirely, and only a gate width in between produced the directional asymmetry.
"When many repulsive skyrmions diffuse thermally inside two connected chambers separated by an off-center gate, they can pass more easily in one direction than the other within a finite time interval," said Mochizuki. "This establishes a new principle for controlling thermal diffusion using topology and geometry." Zhang framed the result as part of a broader shift in the field: "Over the past decade, the community has primarily focused on the conventional dynamics of a single skyrmion or a solid lattice of skyrmions... It is therefore of great significance to explore the physics of interacting skyrmions, where richer interactions may give rise to odd dynamics, especially when they are coupled with structured environments." The authors describe the results as a starting point for a new research direction in nonequilibrium statistical mechanics for magnetic textures, and as a step toward physical computing platforms that would combine randomness, geometry, dissipation, and topology.