̽��ֱ�� of Cambridge - Claire Donnelly

Magnetic vortices come full circle

sc604 — Mon, 30 Nov 2020 16:03:24 +0000

Magnets often harbour hidden beauty. Take a simple fridge magnet: somewhat counterintuitively, it is ‘sticky’ on one side but not the other. ̽��ֱ��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 ̽��ֱ�� of Cambridge, the Paul Scherrer Institute (PSI), 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.

̽��ֱ��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 in the journal Nature Physics.

Determining the magnetisation arrangement within a magnet is highly 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 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. ̽��ֱ��stunning loop shapes they observed appear in the same GdCo₂micropillar 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. ̽��ֱ��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.”

̽��ֱ��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. ̽��ֱ��team has already observed some unique configurations, and going forward, their further exploration promises to bring to light yet more magnetic beauty.

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

Adapted from a PSI press release.

̽��ֱ��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

Reconstructed vortex rings inside a magnetic micropillar

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Watching magnetic nano ‘tornadoes’ in 3D

sc604 — Mon, 24 Feb 2020 15:01:20 +0000

̽��ֱ��team, from the Universities of Cambridge and Glasgow in the UK and ETH Zurich and the Paul Scherrer Institute in Switzerland, used their technique to observe how the magnetisation behaves, the first time this has been done in three dimensions. ̽��ֱ��technique, called time-resolved magnetic laminography, could be used to understand and control the behaviour of new types of magnets for next-generation data storage and processing. ̽��ֱ��results are reported in the journal Nature Nanotechnology.

Magnets are widely used in applications from data storage to energy production and sensors. In order to understand why magnets behave the way they do, it is important to understand the structure of their magnetisation, and how that structure reacts to changing currents or magnetic fields.

“Until now, it hasn’t been possible to actually measure how magnets respond to changing magnetic fields in three dimensions,” said Dr Claire Donnelly from Cambridge’s Cavendish Laboratory, and the study’s first author. “We’ve only really been able to observe these behaviours in thin films, which are essentially two dimensional, and which therefore don’t give us a complete picture.”

Moving from two dimensions to three is highly complex, however. Modelling and visualising magnetic behaviour is relatively straightforward in two dimensions, but in three dimensions, the magnetisation can point in any direction and form patterns, which is what makes magnets so powerful.

“Not only is it important to know what patterns and structures this magnetisation forms, but it’s essential to understand how it reacts to external stimuli,” said Donnelly. “These responses are interesting from a fundamental point of view, but they are crucial when it comes to magnetic devices used in technology and applications.”

One of the main challenges in investigating these responses is tied to the very reason magnetic materials are so relevant for so many applications: changes in the magnetisation typically are extremely small, and happen extremely fast. Magnetic configurations – so-called domain structures – exhibit features on the order of tens to hundreds of nanometres, thousands of times smaller than the width of a human hair, and typically react to magnetic fields and currents in billionths of a second.

Now, Donnelly and her collaborators from the Paul Scherrer Institute, the ̽��ֱ�� of Glasgow and ETH Zurich have developed a technique to look inside a magnet, visualise its nanostructure, and how it responds to a changing magnetic field in three dimensions, and at the size and timescales required.

̽��ֱ��technique they developed, time-resolved magnetic laminography, uses ultra-bright X-rays from a synchrotron source to probe the magnetic state from different directions at the nanoscale, and how it changes in response to a quickly alternating magnetic field. ̽��ֱ��resulting seven-dimensional dataset (three dimensions for the position, three for the direction and one for the time) is then obtained using a specially developed reconstruction algorithm, providing a map of the magnetisation dynamics with 70 picosecond temporal resolution, and 50 nanometre spatial resolution.

What the researchers saw with their technique was like a nanoscale storm: patterns of waves and tornadoes moving side to side as the magnetic field changed. ̽��ֱ��movement of these tornadoes, or vortices, had previously only been observed in two dimensions.

̽��ֱ��researchers tested their technique using conventional magnets, but they say it could also be useful in the development of new types of magnets which exhibit new types of magnetism. These new magnets, such as 3D-printed nanomagnets, could be useful for new types of high-density, high-efficiency data storage and processing.

“We can now investigate the dynamics of new types of systems that could open up new applications we haven’t even thought of,” said Donnelly. “This new tool will help us to understand, and control, their behaviour.”

̽��ֱ��research was funded in part by the Leverhulme Trust, the Isaac Newton Trust and the European Union.

Reference:
Claire Donnelly et al. ‘Time-resolved imaging of three-dimensional nanoscale magnetization dynamics.’ Nature Nanotechnology (2020). DOI: 10.1038/s41565-020-0649-x

Scientists have developed a three-dimensional imaging technique to observe complex behaviours in magnets, including fast-moving waves and ‘tornadoes’ thousands of times thinner than a human hair.

We can now investigate the dynamics of new types of systems that could open up new applications we haven’t even thought of

Claire Donnelly

Reconstruction of 3D magnetic structure

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