Wherever you have liquid, you can also find vortex rings.
Now scientists have discovered vortex rings somewhere fascinating – inside a small pillar made of magnetic material, the gadolinium-cobalt intermetallic compound GdCo2.
If you’ve seen smoke rings or rings with bubbles under water, you’ve seen vortex rings: donut-shaped vortices that form when liquid flows back onto itself after being pushed through a hole.
The new discovery is the first time vortex rings have been identified in magnetic material, confirming a decade-long prediction – and it could help scientists identify even more complex magnetic structures that could be used to develop new technologies.
Vortices with magnetic rings were predicted more than 20 years ago in 1
In fact, it wasn’t until 2017 that the technology was developed to depict magnetization in a material outside the surface layer. Researchers from the Paul Scherer Institute and ETH Zurich have developed an X-ray nanotomography technique to plot the three-dimensional structure of magnetization in GdCo2 bulk magnet.
During these experiments, researchers led by physicist Claire Donnelly of ETH Zurich identified vortices as those that occur when you remove the plug from a sink full of water. These vortices were paired with their topological counterparts, anti-vortices.
In the same little GdCo2 pillars, the researchers also found closed magnetic circuits, also present in vortex-antiortex pairs. It was only after a computational analysis of these structures in the context of magnetic vortices that the team realized that they were donut-shaped vortices intersected by magnetization features – the point at which magnetization disappears – that reflected the reversal of vortex and antiviral polarization.
Above: whirlwind-anti-vortex pair. The orange and green fields indicate the regions where the polarization is reversed.
But, surprisingly, they do not behave exactly as predicted. Fluid annular vortices are always in motion and do not last very long, so magnetic annular vortices were expected to behave in the same way, rolling through the magnetic material before dissipating.
Instead, the vortices remained stationary in a static configuration, disappearing only after GdCo2 was annealed – heated and exposed to a strong magnetic field, a process used to reorient 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 of the University of Cambridge.
“Through a combination of analytical calculations and data considerations, we determined the root of their stability to be the magnetostatic interaction.”
In other words, the vortices interact with the magnetizing structures around them, which secure the rings in place, leading to stabilization. Studying how they form and remain stable can 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 can also help us better understand magnetization. The role of features 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 calculation and visualization of magnetic vortices and preliminary images have proven to be key tools for characterizing the observed three-dimensional structures,” the researchers wrote in their report.
“The observation of stable magnetic vortex rings opens up opportunities for further studies of complex three-dimensional solitons in bulk magnets, allowing the development of applications based on three-dimensional magnetic structures.”
The study was published in Physics of nature.