Physicists See Trippy ‘Vortex Rings’ In Magnetic Material For The First Time

Anywhere you have fluid you can also find vortex rings.

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 vortices that form when fluid returns on 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 it could help scientists identify even more complex magnetic structures that could be exploited to develop new technologies .

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

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

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

In these same tiny GdCo₂ pillars, the researchers also found closed magnetic loops, also present in the vortex-antivortex pairs. It was only after a computer analysis of these structures in the context of magnetic vorticity that the team discovered that they were ring-shaped annular vortices, interspersed with singularities of magnetization – a point where the magnetization disappears – which reflect the polarization reversal of the vortex and the anti-vortex.

(Donnelly et al., Nature Physics, 2020)

Above: a vortex-antivortex pair. Orange and green boxes indicate regions where polarization is reversed.

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

Instead, the vortices always remained in a static configuration, only disappearing after the GoCo₂ has been annealed – heated and exposed to a strong magnetic field, a process used to redirect magnetization.

“One of the main puzzles was why these structures are so surprisingly stable – like rings of smoke, they are only meant to 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 was the magnetostatic interaction.”

In other words, the vortices interact with the magnetizing structures surrounding them, which secure the rings in place, resulting in stabilization. Studying their formation and stability could help physicists learn 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 understand magnetization better. 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 more detail, such as solitons (magnetic waves).

“The computation and visualization of the magnetic vortex and pre-images have proven to be essential tools in the characterization of the observed three-dimensional structures,” the researchers wrote in their paper.

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

The research was published in Physics of nature.