Researchers at JGU have visualized the interaction of magnetic vortices and shown that antiferromagnetic skyrmions move in line with an applied current

Skyrmions – essentially magnetic vortices – represent a promising approach in spintronics; in the future, they could serve as components in storage media or computers, potentially complementing established CMOS technologies. Researchers at Johannes Gutenberg University Mainz (JGU) have now visualized the interaction of antiferromagnetic skyrmions for the first time and shown that antiferromagnetic skyrmions move reproducibly along straight trajectories that are aligned with the driving electrical current. "Our results establish a quantitative framework for the interactions of antiferromagnetic skyrmions. In doing so, they pave the way for spintronic devices based on large numbers of skyrmions," said Mona Bhukta from the research group of Professor Mathias Kläui at the JGU Institute of Physics. The researchers published their findings today in the renowned scientific journal Nature Physics.

Skyrmions offer numerous advantages

Skyrmions offer numerous advantages: they are extremely small (on the nanometer scale), highly stable, and can be manipulated using electric currents. They have already enabled groundbreaking new concepts in ferromagnets – for instance, as racetrack memory, logic gates, and in unconventional computing. However, their practical utility is limited by the so-called skyrmion Hall effect: when an electric current is used to drive skyrmions in a specific direction, they do not directly follow the current direction; instead, they are deflected laterally – at angles of up to 30 degrees.

Theoretical studies by other research groups had predicted that the skyrmion Hall effect is absent in antiferromagnetic systems. The team with Bhukta has now directly confirmed this behavior in time-resolved measurements of an interacting antiferromagnetic skyrmion lattice. "We have demonstrated – reproducibly and within the experimental uncertainty – that skyrmions in antiferromagnetic systems move along the current direction, meaning the skyrmion Hall effect does not come into play," said Bhukta. This reproducibility of motion is crucial for skyrmion-based devices, as high-frequency applications require reliable, repeatable motion of many skyrmions.

For their investigations, the researchers led by Bhukta created a dense lattice of interacting skyrmions whose relative positions remain fixed during coherent motion. "We moved the entire lattice using short electric-current pulses and were able to demonstrate that all the skyrmions moved in a straight line along the direction of the current," said Bhukta. To visualize the skyrmions and their motion, the researchers examined them using time-resolved X-ray microscopy at the BESSY II facility of the Helmholtz-Zentrum Berlin.

A skyrmion lattice like a crowded array of soft balls

In a second experiment, the researchers again excited the skyrmion lattice using very short current pulses, however at less current density and captured the resulting motion. "Since this sequence was repeated identically billions of times, we were able to assemble a movie of the motion with nanosecond time resolution," said Dr. Robert Frömter of JGU, who participated in the study. The motion observed after the current is switched off is particularly revealing: some mobile skyrmions are driven toward neighboring skyrmions that remain pinned by local material inhomogeneities, defects, or grain boundaries; after the current is switched off, the mobile skyrmions recoil. "A useful analogy is a crowded array of soft balls in which some balls are locally anchored: when the mobile balls are pressed against these anchored ones they deform and will bounce back as soon as they are no longer pushed," said Bhukta. From this recoil motion – observed in real time and in real space – the researchers can reconstruct the strength of the repulsion between skyrmions and determine how this interaction diminishes as the distance between them increases. A key contribution to the quantitative analysis came from Kilian Leutner, Ph.D. student in the research group of Mathias Kläui, who developed and refined the physical model, fitted it to the measured trajectories, and performed the micromagnetic simulations used to validate the results.

This skyrmion-skyrmion interaction potential is particularly relevant when large numbers of them are to be integrated into devices. How do these magnetic vortices interact with one another? What distances and timescales are involved? At what separation distance, then, must the interaction between skyrmions be taken into account? "Our results provide a foundation for answering these questions and for realizing devices that incorporate numerous antiferromagnetic skyrmions," said Bhukta.