Skyrmions are vortices in magnetic materials that spread over a few nanometers and are found in extremely thin magnetic films with a thickness of only a few atoms. Skyrmions have a certain property, the so-called topological charge, which plays a similar role to electrical charges. For example, if an applied force deflects the skyrmions to the left, the same force would deflect anti-skyrmions, the corresponding antiparticle, to the right. Since the first experimental observations in 2009, skyrmions have been the focus of intensive research because they open up new possibilities for data storage and information processing.
Surprise at High Currents
Now scientists from Uppsala University, Kiel University, Johannes Gutenberg University in Mainz (JGU), and the University of Paris-Saclay have shown that far more complex phenomena can occur in ferromagnetic nano-layers in which both skyrmions and anti-skyrmions are present. They used state-of-the-art simulation techniques to calculate the magnetic properties and dynamics in such films and investigated how skyrmions and anti-skyrmions behave when electric currents are applied that exert a force on the particles.
The expected behavior can is observable at low currents: Opposite topological charges are deflected in opposite directions by the same force. However, if the current is gradually increased, the movements are no longer mirror-inverted. While skyrmions continue to move in a straight line, anti-skyrmions take curved trajectories, initially only for a short time, then permanently when the electric current increases further. The track then resembles the curve of the pedal on a bicycle on a straight path. These striking results show that opposite topological charges can indeed behave very differently.
Skyrmions + Anti-skyrmions = More of Both
But there were other surprises. When the energy introduced into the system by the applied currents is increased, the trochoidal movement can lead to the periodic formation of skyrmion-anti-skyrmion pairs. Because of their different types of movement, the resulting skyrmions move away, while anti-skyrmions with their curvilinear movement tend to remain in the area in which they were created. Remarkably, each anti-skyrmion produced becomes a new source of skyrmion-anti-skyrmion pairs, leading to particle multiplication.
The scope of this theoretical work may be very far-reaching. With regard to future technologies, the study suggests that anti-skyrmions could serve as a constant source of skyrmions. This would be crucial for all future applications that use skyrmions to transfer and store data. In addition, the pedal movement determines the absolute speed limitation of such topological charges - an important parameter if circuits are to be developed with the aid of skyrmions in the future.
The results have now been published in the journal Nature Electronics.