Aug 26: Diffusion is a fundamental natural phenomenon that can be observed across a wide range of length and time scales. It plays a key role in many different fields, including physics, biology, and economics. In particular, asymmetric or directional diffusion of particle systems has attracted growing interest for practical applications, including the development of unconventional artificial-intelligence hardware, where it could enable nonlinear, geometry-controlled information processing.
Magnetic skyrmions are a type of topological spin textures that can behave as particle-like objects with chiral dynamic nature. Interestingly, recent reports have shown that even tiny thermal fluctuations can drive effective diffusion of skyrmions in ultrathin magnetic films and layered heterostructures. Some experiments have also revealed a topology-dependent sideways, wall-guided motion known as the Brownian gyromotion of skyrmions when they interact in a confined space. Magnetic skyrmions can also exhibit exotic dynamic behaviors that cannot be reproduced by common particles. Particularly, their diffusive properties have immense potential in novel information processing applications. However, these properties, especially in structured environments, remain largely unexplored.
In a breakthrough study, a research team led by Professor Masahito Mochizuki and Associate Professor Xichao Zhang from the Department of Applied Physics at Waseda University, Japan, has shown that magnetic skyrmions can exhibit asymmetric diffusion in a structured environment.
“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,” explains Prof. Masahito Mochizuki. “This establishes a new principle for controlling thermal diffusion using topology and geometry.”
Their study was published in npj Spintronics on July 16, 2026.
To study the diffusion of skyrmions in a structured environment, the team designed a theoretical model, consisting of Néel-type skyrmions confined in a nanostructured magnetic thin-film system containing two chambers linked by a narrow off-center asymmetric gate. Their analysis showed that despite thermally induced random motion of magnetic skyrmions, the off-center geometry caused a clear directional imbalance in their diffusion. Skyrmions placed in the left chamber were more likely to pass through the OAG than skyrmions approaching from the opposite side.
Computational simulations with multiple repulsive skyrmions initially placed on either side of the OAG confirmed this theoretical finding. The team found that this key mechanism was not simply due to the asymmetric configuration of the gate but instead emerged from the interplay between the structured environment and the unique topology-dependent dynamics of skyrmions.
When a skyrmion approaches a chamber wall, the wall exerts a repulsive force. Because of the nontrivial topology of the skyrmion, this force guides it into Brownian gyromotion. Since the OAG exposes the skyrmion to different sections of the wall in the direction from which it arrives, the resulting motion favors one crossing direction over the other. In contrast, in the case of a centered symmetric gate, the simulations showed a nearly symmetric diffusion for skyrmions on both sides.
The results further revealed that two diffusive skyrmions can transiently rotate around one another, forming a short-lived binary skyrmion system, highlighting the importance of interactions between repulsive skyrmions and their density. The researchers also found that a high initial skyrmion density may push skyrmions out of the chamber without effective interaction with the OAG. Moreover, the diffusion asymmetry depends strongly on the gate opening width. An OAG with a much wider width than the skyrmion diameter can allow all skyrmions to pass through, while a narrow OAG can prevent any skyrmion from passing. Only a reasonable OAG width can lead to asymmetric diffusion.
“Topological magnetic textures, such as skyrmions, are spatial patterns formed by localized magnetic moments residing on the atomic lattice sites of a magnetic crystal. It is remarkable that such a mere magnetization pattern can demonstrate particle behavior and exhibit thermal diffusion. Even more intriguing is the fact that its thermal diffusion becomes asymmetric due to the topological geometric origin,” remarks Prof. Mochizuki. “Our findings uncover novel physics beyond that of conventional particle systems and are expected to open up a new research field in nonequilibrium statistical mechanics focused on magnetic textures.”
“Over the past decade, the community has primarily focused on the conventional dynamics of a single skyrmion or a solid lattice of skyrmions. In both scenarios, skyrmion–skyrmion and skyrmion–environment interactions are either absent or trivial. 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,” remarks Dr. Zhang. “This study deepens our understanding of skyrmion diffusion in confined and structured environments, paving the way for novel physical computing platforms, where randomness, geometry, dissipation, and topology work together.”