Obstacle course for microscopic whirlpools

Jul 04, 2022

(News from Nanowerk) We know about hurricanes mainly from global weather phenomena, but they have started to occur more frequently also in Europe. However, when researchers use an optical Kerr microscope to zoom in on thin films of magnetic material, they see something related happening in the microcosm, under the right conditions: a sort of small-scale magnetic hurricane. Physicists call these swirling magnetic structures skyrmions.

The idea is to use this phenomenon for data storage or processing devices. For these applications, the motion of mini-vortices, which themselves act as autonomous particles or quasi-particles, must be exploited. Skyrmions can move both due to the effects of temperature and electrical currents. While more powerful “pushes” are needed for some applications, random thermal motion is desirable for others, such as in unconventional computing.

The boundaries of differently shaped skyrmions (left) are at coincident positions. And even the boundaries of the stripe-like structures (right) correspond to these positions. (Image: Raphaël Gruber, JGU)

Pinning: When skyrmions meet the “Obstacle Course”

The films of nanoscale material in which skyrmions can be observed are never perfect. As a result, these small magnetic vortices can get stuck – an effect known as pinning. In most cases they are so trapped that they are unable to escape. It’s like trying to roll a small ball over the surface of an old table covered in scratches and nicks. Its trajectory will be deflected, and if there is a large enough indentation, the ball will simply get stuck.

When skyrmions are trapped in this way, it poses challenges, especially for applications that rely on the thermal motion of quasiparticles. Pinning can lead to a complete stoppage of this movement.

Understand the fundamentals of pinning

“I used a Kerr microscope to study skyrmions as small as a micrometer in size – or, to be more precise, their pinning behavior,” said Raphael Gruber, PhD student and team member of research by Professor Mathias Kläui at Johannes Gutenberg University. Mainz (JGU).

There are already a number of theories about how the effect occurs. Most of them focus on examining skyrmions as a whole; in other words, they concentrate on the movement of their centers. There have even been some experimental studies, but in the presence of strong anchorage where the skyrmions are unable to move at all.

“My investigations are based on a weak anchor allowing the skyrmions to move a bit and keep jumping until they are caught somewhere else, Gruber explained. Its results offer interesting new perspectives. “Skyrmions do not fall like balls into a hole”, summed up the experimental physicist. “What happens is it sticks to something on its surface.”

The corresponding results have been published in Nature Communication (“Skyrmion pinning energetics in thin-film systems”).

Professor Mathias Kläui, head of the research group, is also delighted with the new findings, which are the result of many years of collaboration with theoretical physics groups: “Under the aegis of the Skyrmionics priority program funded by the German Foundation for research and the Spin+X Collaborative Research Center, we studied the dynamics of spin structures with our counterparts working in the field of theoretical physics. I am happy to say that this very productive collaboration, in particular also between students of postgraduate of the groups involved, generated these fascinating results. ”

Dr Peter Virnau, who leads a theoretical physics group in Mainz, added: “Skyrmions are a relatively new aspect of my research. My introduction to them was made possible by funding provided by the state of Rhineland-Palatinate. via the TopDyn – Dynamics and Topology Topology Research Area at JGU I am glad that our numerical methods can contribute to a better understanding of the experimental data.

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