Reconstructed vortex rings inside a magnetic micropillar. Courtesy: Claire
Donnelly
Magnets
often harbor 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 magnetization textures are at the heart of many
modern technologies, such as hard disk 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 Institute
for Numerical Mathematics RAS in Moscow report the discovery of unexpected
magnetic structures inside a tiny pillar made of the magnetic material
gadolinium cobalt. As they write in a paper published today in the journal
Nature Physics, the researchers observed sub-micrometer 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 novel technological applications.
Mesmerizing
insights
Determining
the magnetisation arrangement within a magnet is extraordinarily challenging,
in particular for structures at the micro- and 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 novel X-ray method for the nanotomography of bulk magnets, which they
demonstrated in experiments at the Swiss Light Source SLS. That advance opened
up a unique window into the inner life of magnets, providing a tool for
determining three-dimensional magnetic configurations at the nanoscale within
micrometer-sized samples.
Utilizing these capabilities, members of the original team, together with international collaborators, now ventured into new territory. The stunning loop shapes they observed appear in the same gadolinium cobalt 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 initially. 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.
Reconstructed
vortex rings inside a magnetic micropillar. Courtesy: Claire Donnelly
A new
twist on an old story
Vortex
rings are familiar to everyone who has seen smoke rings being blown, or who
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.
The
stability of magnetic vortex rings should 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 3-D 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 bubble
counterparts. The team has already observed some unique configurations, and
going forward, their further exploration promises to bring to light yet more
magnetic beauty.