Magnetic vortices come full circle

The first experimental observation of three-dimensional magnetic ’vortex rings’ provides fundamental insight into intricate nanoscale structures inside bulk magnets and offers a fresh perspective for magnetic devices.

One of the main puzzles was why these structures are so unexpectedly stable - like smoke rings, they are only supposed to exist as moving objects

Claire Donnelly

Magnets often harbour hidden beauty. Take a simple fridge magnet: somewhat counterintuitively, it is ’sticky’ on one side but not the other. The secret lies in the way the magnetisation is arranged in a well-defined pattern within the material. More intricate magnetisation textures are at the heart of many modern technologies, such as hard drives.

Now, an international team of scientists from the University of Cambridge, the Paul Scherrer Institute, ETH Zurich, the Donetsk Institute for Physics and Engineering in Ukraine and the Institute for Numerical Mathematics RAS in Moscow have discovered unexpected magnetic structures inside a tiny pillar made of the magnetic material GdCo2.

The researchers observed sub-micrometre loop-shaped configurations, which they identified as magnetic vortex rings. Far beyond their aesthetic appeal, these textures might point the way to further complex three-dimensional structures arising in the bulk of magnets and could one day form the basis for new technological applications. Their results are reported .

Determining the magnetisation arrangement within a magnet is highly challenging, in particular for structures at the microand nanoscale, for which studies have been typically limited to looking at a shallow layer just below the surface. That changed in 2017 when researchers at PSI and ETH Zurich introduced a new X-ray method for the nanotomography of bulk magnets, which they demonstrated in experiments at the Swiss Light Source. That advance opened up a window into the inner life of magnets, providing a tool for determining three-dimensional magnetic configurations at the nanoscale within micrometre-sized samples.

Using these capabilities, the researchers ventured into new territory. The stunning loop shapes they observed appear in the same GdCo2 micropillar samples in which they had before detected complex magnetic configurations consisting of vortices - the sort of structures seen when water spirals down from a sink - and their topological counterparts, antivortices.

That was a first, but the presence of these textures has not been surprising in itself. Unexpectedly, however, the scientists also found loops that consist of pairs of vortices and antivortices. That observation proved to be puzzling. With the implementation of novel sophisticated data-analysis techniques they eventually established that these structures are so-called vortex rings - in essence, doughnut-shaped vortices.

Vortex rings are familiar to everyone who has seen smoke rings being blown, or who has watched dolphins producing loop-shaped air bubbles, for their own amusement as much as to that of their audience. The newly discovered magnetic vortex rings are captivating in their own right. Not only does their observation verify predictions made some two decades ago, settling the question whether such structures can exist. They also offered surprises. In particular, magnetic vortex rings have been predicted to be a transient phenomenon, but in the experiments now reported, these structures turned out to be remarkably stable.

"One of the main puzzles was why these structures are so unexpectedly stable - like smoke rings, they are only supposed to exist as moving objects," said Dr Claire Donnelly from Cambridge’s Cavendish Laboratory, and the paper’s first author. "Through a combination of analytical calculations and considerations of the data, we determined the root of their stability to be the magnetostatic interaction."

The stability of magnetic vortex rings could have important practical implications. For one, they could potentially move through magnetic materials, as smoke rings move stably though air, or air-bubble rings through water.

Learning how to control the rings within the volume of the magnet can open interesting prospects for energy-efficient 3D data storage and processing. There is interest in the physics of these new structures, too, as magnetic vortex rings can take forms not possible for their smoke and air counterparts. The team has already observed some unique configurations, and going forward, their further exploration promises to bring to light yet more magnetic beauty.

Claire Donnelly et al. ’Experimental observation of vortex rings in a bulk magnet.’ Nature Physics (2020). DOI: 10.1038/s41567-020-01057-3