Physicists observe “swirling rings” in a magnetic material for the first time

Wherever you have fluid, you can also find vortex rings there.

Now, scientists have found vortex rings somewhere fascinating – inside a tiny pillar made of a magnetic material, the gadolinium-cobalt intermetallic compound GdCo.₂.

If you’ve seen smoke rings or bubble rings underwater, you’ve seen vortex rings – donut-shaped swirls that form when fluid ebbs back into itself after being forced through a hole.

The new discovery is the first time that vortex rings have been identified in a magnetic material, confirming a decades-old prediction and could help scientists identify even more complex magnetic structures that could be exploited to develop new technologies.

The magnetic vortices of the rings were predicted over 20 years ago in 1998, when physicist Nigel Cooper of the University of Cambridge showed that magnetic vortices are analogous to the vortex rings seen in fluid dynamics. In reality, finding them, however, was much more difficult to do.

In fact, it wasn’t until 2017 when the technology was developed for magnetizing images within a material beyond the surface layer. Researchers from the Paul Scherrer Institute and ETH Zurich have developed an X-ray nanotomography technique to visualize the three-dimensional magnetization structure within a GdCo₂ loose magnet.

During those experiments, researchers, led by physics Claire Donnelly of ETH Zurich, identified eddies like those that appear when you unplug from a sink full of water. These vortices were associated with their topological counterparts, the antivortices.

In those same tiny GdCo₂ pillars, the researchers also found closed magnetic rings, also present in the vortex-vortex pairs. It was only after computationally analyzing these structures in the context of magnetic vorticity that the team discovered that they were donut-shaped ring vortices, intersected by magnetization singularities – a point where the magnetization vanishes – reflecting the inversion. polarization of the vortex and the anti-vortex.

(Donnelly et al., Nature Physics, 2020)

Above: a vortex-antivortex pair. The orange and green boxes indicate the regions where the polarization reverses.

But surprisingly, they don’t behave exactly as expected. The swirls of fluid rings are always in motion and don’t last very long, so the swirls of magnetic rings were expected to behave the same way, rolling through the magnetic material before dissipating.

Instead, the vortices remained stationary in a static configuration, disappearing only after the GdCo₂ it was annealed, heated and exposed to a strong magnetic field, a process used to reorient the magnetization.

“One of the main puzzles was why these structures are so unexpectedly stable – like smoke rings, they should only exist as moving objects,” said Donnelly, now at Cambridge University.

‘Through a combination of analytical calculations and data considerations, we determined that the root of their stability is the magnetostatic interaction.’

In other words, the vortices interact with the magnetization structures surrounding them, which fix the rings in place, resulting in a stabilization. Studying how they form and remain stable could help physicists learn how to control magnetic vortex rings, which in turn could help develop better technologies, such as data storage and neuromorphic engineering.

But vortex rings could also help us better understand magnetization. The role of singularities in magnetization processes, for example, is poorly understood. And the observation of vortex rings suggests that other complex structures could be studied in greater detail, such as solitons (magnetic waves).

“Computation and visualization of magnetic vorticity and pre-images have proved to be essential tools in characterizing the observed three-dimensional structures,” the researchers wrote in their paper.

“The observation of stable magnetic vortex rings opens up possibilities for further studies of complex three-dimensional solitons in bulk magnets, enabling the development of applications based on three-dimensional magnetic structures.”

The research was published in Physics of nature.