̽��ֱ�� of Cambridge - magnetic field

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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Death of a dynamo – a hard drive from space

sc604 — Wed, 21 Jan 2015 18:00:01 +0000

̽��ֱ��dying moments of an asteroid’s magnetic field have been successfully captured by researchers, in a study that offers a tantalising glimpse of what may happen to the Earth’s magnetic core billions of years from now.

Using a detailed imaging technique, the research team were able to read the magnetic memory contained in ancient meteorites, formed in the early solar system over 4.5 billion years ago. ̽��ֱ��readings taken from these tiny ‘space magnets’ may give a sneak preview of the fate of the Earth’s magnetic core as it continues to freeze. ̽��ֱ��findings are published today (22 January) in the journal Nature.

Using an intense beam of x-rays to image the nanoscale magnetisation of the meteoritic metal, researchers led by the ̽��ֱ�� of Cambridge were able to capture the precise moment when the core of the meteorite’s parent asteroid froze, killing its magnetic field. These ‘nano-paleomagnetic’ measurements, the highest-resolution paleomagnetic measurements ever made, were performed at the BESSY II synchrotron in Berlin.

̽��ֱ��researchers found that the magnetic fields generated by asteroids were much longer-lived than previously thought, lasting for as long as several hundred million years after the asteroid formed, and were created by a similar mechanism to the one that generates the Earth’s own magnetic field. ̽��ֱ��results help to answer many of the questions surrounding the longevity and stability of magnetic activity on small bodies, such as asteroids and moons.

“Observing magnetic fields is one of the few ways we can peek inside a planet,” said Dr Richard Harrison of Cambridge’s Department of Earth Sciences, who led the research. “It’s long been assumed that metal-rich meteorites have poor magnetic memories, since they are primarily composed of iron, which has a terrible memory – you wouldn’t ever make a hard drive out of iron, for instance. It was thought that the magnetic signals carried by metal-rich meteorites would have been written and rewritten many times during their lifetime, so no-one has ever bothered to study their magnetic properties in any detail.”

̽��ֱ��particular meteorites used for this study are known as pallasites, which are primarily composed of iron and nickel, studded with gem-quality silicate crystals. Contained within these unassuming chunks of iron however, are tiny particles just 100 nanometres across – about one thousandth the width of a human hair – of a unique magnetic mineral called tetrataenite, which is magnetically much more stable than the rest of the meteorite, and holds within it a magnetic memory going back billions of years.

“We’re taking ancient magnetic field measurements in nanoscale materials to the highest ever resolution in order to piece together the magnetic history of asteroids – it’s like a cosmic archaeological mission,” said PhD student James Bryson, the paper’s lead author.

̽��ֱ��researchers’ magnetic measurements, supported by computer simulations, demonstrate that the magnetic fields of these asteroids were created by compositional, rather than thermal, convection – meaning that the field was long-lasting, intense and widespread. ̽��ֱ��results change our perspective on the way magnetic fields were generated during the early life of the solar system.

These meteorites came from asteroids formed in the first few million years after the formation of the Solar System. At that time, planetary bodies were heated by radioactive decay to temperatures hot enough to cause them to melt and segregate into a liquid metal core surrounded by a rocky mantle. As their cores cooled and began to freeze, the swirling motions of liquid metal, driven by the expulsion of sulphur from the growing inner core, generated a magnetic field, just as the Earth does today.

“It’s funny that we study other bodies in order to learn more about the Earth,” said Bryson. “Since asteroids are much smaller than the Earth, they cooled much more quickly, so these processes occur on shorter timescales, enabling us to study the whole process of core solidification.”

Scientists now think that the Earth’s core only began to freeze relatively recently in geological terms, maybe less than a billion years ago. How this freezing has affected the Earth’s magnetic field is not known. “In our meteorites we’ve been able to capture both the beginning and the end of core freezing, which will help us understand how these processes affected the Earth in the past and provide a possible glimpse of what might happen in the future,” said Harrison.

However, the Earth’s core is freezing rather slowly. ̽��ֱ��solid inner core is getting bigger, and eventually the liquid outer core will disappear, killing the Earth’s magnetic field, which protects us from the Sun’s radiation. “There’s no need to panic just yet, however,” said Harrison. “ ̽��ֱ��core won’t completely freeze for billions of years, and chances are, the Sun will get us first.”

̽��ֱ��research was funded by the European Research Council (ERC) and the Natural Environment Research Council (NERC).

Hidden magnetic messages contained within ancient meteorites are providing a unique window into the processes that shaped our solar system, and may give a sneak preview of the fate of the Earth’s core as it continues to freeze.

It’s like a cosmic archaeological mission

James Bryson

̽��ֱ��Esquel pallasite from the Natural History Museum collections, consists of gem-quality crystals of the silicate mineral olivine embedded in a matrix of iron-nickel alloy.

̽��ֱ��text in this work is licensed under a Creative Commons Licence. If you use this content on your site please link back to this page. For image rights, please see the credits associated with each individual image.

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Spin with a new twist

tdk25 — Fri, 10 Oct 2014 06:56:28 +0000

A new method of controlling the “spin” of an electron, one of the fastest-developing research topics in quantum-based technologies and widely seen as the potential foundation of numerous future advances, has been demonstrated by scientists.

In quantum physics, the “spin” of a particle refers to its intrinsic angular momentum. This can be controlled so that it is aligned with one of two directions, typically referred to as “up” or “down”.

Usually, researchers define the direction by applying a magnetic field to orientate the electron, called the “quantisation axis”. ̽��ֱ��process can, however, be distorted by the natural magnetic environment around the electron itself, which is usually seen as one of the biggest obstacles to controlling spin.

Uniquely, the new study, by a team at the ̽��ֱ�� of Cambridge and the Joint Quantum Institute (JQI) in the USA, instead turned this magnetic field into a natural advantage which allowed the electron to be held in place. ̽��ֱ��researchers did this by firing two precisely-tuned lasers at the particle, creating what they call a “dark state” from which it could be manipulated and measured.

̽��ֱ��implications for controlling quantum systems are significant because, since the 1990s, researchers have theorised that a particle’s spin could be used to store and manipulate information. Using the “up” or “down” as an alternative to the binary coding of 0s and 1s that characterises computers today, spin-based quantum computers would be able to compute difficult problems and vast amounts of data much more efficiently.

Any such development, however, depends on finding ways to bring electron spin under control in the first place. To date, researchers have had to find ways to do this in spite of the randomising effect that the magnetic field around an electron has on the orientation of its spin.

̽��ֱ��spin of an electron cannot be observed continuously without altering it, so it has to be measured before and after an attempt to manipulate its quantum state. This measurement reveals whether the spin is up or down, but the surrounding magnetic environment can also take effect at any time. If it does so, the quantisation axis of the electron is altered, and the whole picture is distorted. ̽��ֱ��effect is similar to trying to measure longitude and latitude in a world where the positions of the north and south poles are changing randomly all the time.

Dr Mete Atatüre, a researcher at St John’s College, Cambridge who led part of the new study, said: “In order to perform reliable measurements, we constantly have to fight against this fluctuating magnetic environment. In fact, most research is about trying to keep electrons detached or isolated from it. What is unique about this experiment is that we did the opposite and used this environment as a resource. We created a quantum state that wouldn’t be accessible if the magnetic field wasn’t there.”

̽��ֱ��electron was trapped inside a self-assembled “quantum dot”, a tiny structure made from a 10 nanometre-thick indium arsenide droplet, surrounded by gallium arsenide. While both materials are semiconductors, an electron can have a lower energy inside the “quantum dot” than in gallium arsenide. “This forms a natural and stable trap for single electrons within a semiconductor device, providing the desired conditions for defining a spin quantum bit” explained Carsten Schulte, a Cambridge graduate student who worked on the project.

An electron isolated in this fashion can then be targeted with lasers to manipulate its spin. If a laser strikes the quantum dot at certain wavelengths, the electron is optically excited and emits light, or fluoresces, which changes its spin. If, however, the magnetic environment interferes with this the change becomes uncontrollable.

To resolve this, the researchers fired two separate lasers at the quantum dot – one tuned to excite the “up” spin state, the other to excite the “down” state. These interfered with each other destructively, preventing any fluorescence at all and creating a so-called “dark state”.

“You would expect two lasers to raise the level of optical excitation even more, but in fact when this is done no light comes out of the quantum dot,” Jack Hansom, another graduate student in the research team, said. “Optical excitation ceases and the electron finds a unique quantum superposition state, which is neither up nor down.”

By changing the relative phase between the two lasers, they were able to redefine the specific dark state and force the electron into it. This showed that the electron could be manipulated in its own up/down coordinate system without the researchers ever knowing the orientation of up and down during the whole process.

“What is profound is that the electron is always in the same quantum superposition state, but the basis in which it is represented evolves with the nuclear field that remains unknown to us,” Atatüre added. “This research shows that the magnetic environment around the quantum dot does not need to remain a problem, but can be utilized for the definition and control of a quantum bit.”

̽��ֱ��full report appears in the October issue of Nature Physics.

Scientists have successfully demonstrated a new way to control the “spin” of an electron – the natural intrinsic angular momentum of electrons which could underpin faster computing in the future. ̽��ֱ��technique counterintuitively makes use of the ever-changing magnetic field of the electron’s environment - one of the main obstacles to traditional methods of spin control.

Most research is about trying to keep electrons isolated from the magnetic environment. What is unique about this experiment is that we did the opposite and used it as a resource

Mete Atature

Carsten Schulte

Spin manipulation in a noisy environment. In the quest for ever more precise manipulation of quantum systems, any uncontrolled interaction with the environment is usually considered detrimental.

Yes

Superconducting secrets solved after 30 years

sc604 — Sun, 15 Jun 2014 17:00:00 +0000

Harnessing the enormous technological potential of high-temperature superconductors – which could be used in lossless electrical grids, next-generation supercomputers and levitating trains – could be much more straightforward in future, as the origin of superconductivity in these materials has finally been identified.

Superconductors, materials which can carry electric current with zero resistance, could be used in a huge range of applications, but a lack of understanding about where their properties originate from has meant that the process of identifying new materials has been rather haphazard.

Researchers from the ̽��ֱ�� of Cambridge have found that ripples of electrons, known as charge density waves or charge order, create twisted ‘pockets’ of electrons in these materials, from which superconductivity emerges. ̽��ֱ��results are published in the June 15th issue of the journal Nature.

Low-temperature, or conventional, superconductors were first identified in the early 20th century, but they need to be cooled close to absolute zero (zero degrees on the Kelvin scale, or -273 degrees Celsius) before they start to display superconductivity. So-called high-temperature superconductors however, can display the same properties at temperatures up to 138 Kelvin (-135 degrees Celsius), making them much more suitable for practical applications.

Since they were first identified in the mid-1980s, the process of discovering new high-temperature superconductors could be best described as random. While researchers have identified the ingredients that make for a good low-temperature superconductor, high-temperature superconductors have been more reluctant to give up their secrets.

In a superconductor, as in any electronic device, current is carried via the charge on an electron. What is different about superconductors is that the electrons travel in tightly bound pairs. When travelling on their own, electrons tend to bump into each other, resulting in a loss of energy. But when paired up, the electrons move smoothly through a superconductor’s structure, which is why superconductors can carry current with no resistance. As long as the temperature is kept sufficiently low, the electron pairs will keep moving through the superconductor indefinitely.

Key to conventional superconductors are the interactions of electrons with the lattice structure of the material. These interactions generate a type of ‘glue’ which holds the electrons together. ̽��ֱ��strength of the glue is directly related to the strength of the superconductor, and when the superconductor is exposed to an increase in temperature or magnetic field strength, the glue is weakened, the electron pairs break apart and superconductivity is lost.

“One of the problems with high-temperature superconductors is that we don’t know how to find new ones, because we don’t actually know what the ingredients are that are responsible for creating high-temperature superconductivity in the first place,” said Dr Suchitra Sebastian of the Cavendish Laboratory, lead author of the paper. “We know there’s some sort of glue which causes the electrons to pair up, but we don’t know what that glue is.”

In order to decode what makes high-temperature superconductors tick, the researchers worked backwards: by determining what properties the materials have in their normal, non-superconducting state, they might be able to figure out what was causing superconductivity.

“We’re trying to understand what sorts of interactions were happening in the material before the electrons paired up, because one of those interactions must be responsible for creating the glue,” said Dr Sebastian. “Once the electrons are already paired up, it’s hard to know what made them pair up. But if we can break the pairs apart, then we can see what the electrons are doing and hopefully understand where the superconductivity came from.”

Superconductivity tends to override other properties. For example, if in its normal state a superconductor was a magnet, suppressing that magnetism has been found to result in superconductivity. “So by determining the normal state of a superconductor, it would make the process of identifying new ones much less random, as we’d know what sorts of materials to be looking for in the first place,” said Dr Sebastian.

Working with extremely strong magnetic fields, the researchers were able to kill the superconducting effect in cuprates - thin sheets of copper and oxygen separated by more complex types of atoms.

Previous attempts to determine the origins of superconductivity by determining the normal state have used temperature instead of magnetic field to break the electron pairs apart, which has led to inconclusive results.

As cuprates are such good superconductors, it took the strongest magnetic fields in the world – 100 Tesla, or roughly one million times stronger than the Earth’s magnetic field – in order to suppress their superconducting properties.

These experiments were finally able to solve the mystery surrounding the origin of pockets of electrons in the normal state that pair to create superconductivity. It was previously widely held that electron pockets were located in the region of strongest superconductivity. Instead, the present experiments using strong magnetic fields revealed a peculiar undulating twisted pocket geometry -similar to Jenga bricks where each layer goes in a different direction to the one above or beneath it.

These results pinpointed the pocket locations to be where superconductivity is weakest, and their origin to be ripples of electrons known as charge density waves, or charge order. It is this normal state that is overridden to yield superconductivity in the family of cuprate superconductors studied.

“By identifying other materials which have similar properties, hopefully it will help us find new superconductors at higher and higher temperatures, even perhaps materials which are superconductors at room temperature, which would open up a huge range of applications,” said Dr Sebastian.

A breakthrough has been made in identifying the origin of superconductivity in high-temperature superconductors, which has puzzled researchers for the past three decades.

By identifying other materials which have similar properties, hopefully it will help us find new superconductors at higher and higher temperatures

Suchitra Sebastian

Nicolle R Fuller

Map of superconducting copper oxide structure.

Yes