The first experimental observation of three-dimensional magnetic “vortex rings”

Reconstructed vortex ring inside a magnetic micropile. Credit: Claire Donnelly

The first experimental observation of three-dimensional magnetic ‘vortex rings’ provides substantial insight into complex nanoscale structures within bulk magnets and offers new perspectives for magnetic devices.

Magnets often carry hidden beauty. Get a simple fridge magnet: Somewhat counterintuitively, it ‘sticks’ on one side but not the other. The secret lies in how the magnetization is arranged in a well-defined pattern within the material. The most complex magnetization texts are at the heart of many modern technologies, such as hard drives. Now, an international team of scientists at the Paul Scherrer Institute PSI, ETH Zurich, the University of Cambridge, the Donetsk Institute for Physics and Engineering and the RAS Institute for Numerical Mathematics in Moscow report the discovery of unexpected magnetic structures inside a small pole made of the material magnetic cobalt gadolinium.

As they write in a paper published on November 30, 2020, in the diary Physics of nature,[1] the researchers observed loop-shaped configurations under the micrometer, which they identified as magnetic vortex rings. Far beyond their aesthetic appeal, these structures could pave the way for further complex three-dimensional structures emerging from most magnets and could one day form the basis for new technological applications.

Fascinating views

Determining the magnetization regulation within a magnet is extremely challenging, especially for micro- and nanoscale structures, for which studies have usually been limited to viewing a shallow layer below the surface. That changed in 2017 when researchers at PSI and ETH Zurich introduced a new X-ray method for nanomatography of bulk magnets, which they demonstrated in experiments at the Swiss Light Source SLS.[2] This breakthrough opened a unique window into the inner life of magnets, providing a means of defining three-dimensional magnetic configurations in nanoscale within micrometer-sized samples.

Vortex Ring Inside the Magnetic Micropil

Reconstructed vortex ring inside a magnetic micropile. Credit: Claire Donnelly

Using these skills, the original team members, along with international collaborators, were now introduced to new territory. The stunning loop shapes they observed appear in the same micropillar cadolin gadolinium specimens in which they had previously discovered complex magnetic configurations consisting of vortices – the type of structures seen when water flows from a sink – and their topological counterparts, antivortices . This was the first, but the presence of these texts has not been surprising in itself. However, suddenly, scientists also found loops consisting of pairs of vortices and antivortices. This observation initially proved to be surprising. By applying new sophisticated data analysis techniques, they eventually proved that these structures are so-called vortex rings – essentially donut-shaped vortices.

A new twist on an old story

Whirlpool rings are known to anyone who has seen smoke rings swell, or who have watched dolphins produce loop-shaped air bubbles, for their entertainment as much as that of their audience. The newly discovered magnetic vortex rings are fascinating in themselves. Not only does their observation verify predictions made about two decades ago, resolving the question of whether such structures may exist. They also offered surprises. In particular, magnetic vortex rings are predicted to be a transient phenomenon, but in the experiments reported now, these structures turned out to be extremely stable.

The stability of magnetic vortex rings must have important practical implications. For one, they can potentially move through magnetic materials, as the smoke rings move steadily through the air, or the air bubble rings through the water. Learning how to control the rings within the volume of the magnet can open up interesting prospects for storing and processing 3D data with energy efficiency. There is also interest in the physics of these new structures, as magnetic vortex rings can take shapes impossible for their smoke and bubble counterparts. The team has already observed some unique configurations, and going forward, their further exploration promises to bring to light even more magnetic beauty.

References:

  1. “Experimental observation of vortex rings in a large magnet” by Claire Donnelly, Konstantin L. Metlov, Valerio Scagnoli, Manuel Guizar-Sicairos, Mirko Holler, Nicholas S. Bingham, Jörg Raabe, Laura J. Heyderman, Nigel R. Cooper and Sebastian Gliga, November 30, 2020, Physics of nature.
    DOI: 10.1038 / s41567-020-01057-3
  2. “Three-dimensional magnetic structures detected by X-ray vector nanotomography” by Claire Donnelly, Manuel Guizar-Sicairos, Valerio Scagnoli, Sebastian Gliga, Mirko Holler, Jörg Raabe and Laura J. Heyderman, July 20, 2017, Nature.
    DOI: 10.1038 / natyra23006

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