Com candidaturas abertas até 5 de junho, o programa SPIN: Explore decorre entre 9 e 25 de junho e está aberto a investigadores de todas as áreas científicas e de qualquer fase de carreira. Não é exigida experiência em negócios ou empreendedorismo.

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There is a shape in physics that is remarkably hard to destroy. You can shake it, heat it, push it and disturb it in every way imaginable, but unless you physically tear the fabric it resides in, it will survive perfectly intact. This is not wishful thinking. It is a mathematical certainty. That shape is called a skyrmion.

The easiest way to picture a skyrmion is to imagine a dartboard covered in tiny arrows. At the very centre, every arrow points straight down into the board. At the outer edge, every arrow points straight up. In between, they rotate smoothly through every possible direction, completing a full rotation and closing back on themselves. This pattern has a score called the skyrmion number, and that score is locked at exactly ±1 (the sign simply defines which way the twist runs). Noise cannot nudge it. Heat cannot drift it. A stray disturbance cannot flip it. The only way to change it is to violently rip the whole pattern apart.

Scientists first found skyrmions hiding inside certain magnetic materials and immediately recognized them as dream candidates for carrying information (a skyrmion present means 1, a skyrmion absent means 0, and nothing in the environment can accidentally corrupt it). But magnetic materials are slow and confined to a chip. The next natural question was bold: what if you could take this indestructible shape and put it inside light itself, travelling freely through open space?

A team of researchers from Tianjin University in China, together with collaborators at Nanyang Technological University in Singapore and Oklahoma State University in the US, has now done exactly that – and gone one step further. As described in Optica, the researchers created not just one skyrmion in light, but two completely different kinds, and found a way to switch between them at will using nothing more than the rotation of a single thin optical half-wave plate.

The two types are an electric skyrmion, where the topological twist lives in the electric field of the light wave, and a magnetic skyrmion, where the same twist lives in the magnetic field. And they are as distinct from each other as a left-handed knot is from a right-handed one.

To generate these skyrmions, project leader Jiaguang Han and colleagues built a flat chip roughly the size of a small stamp, its surface packed with thousands of tiny C-shaped gold antennas, each one far smaller than a bacterium. When a structured laser beam hits this chip, the antennas absorb the incoming near-infrared light and re-radiate it as terahertz waves.

The key is how the antennas are arranged on the chip: one set is laid out in concentric rings pointing outward, while another set spirals around the centre like the spokes of a wheel. Each arrangement, when activated by the right kind of laser beam, generates a different skyrmion-carrying light pulse. Switching the laser from one beam shape to the other is done by rotating a single optical plate by just 45°, which flips the chip from producing one skyrmion type to the other, instantly and cleanly.

“The core innovation lies in the nonlinear metasurface that converts shaped near-infrared femtosecond laser pulses into tailored terahertz toroidal light pulses,” explains first author Li Niu in a press statement.

The team confirmed this process by mapping the full three-dimensional structure of each light pulse at multiple positions in space and time. The skyrmion numbers they measured came out at –0.990 and +0.992 for electric skyrmions, and –0.991 and +0.994 for magnetic skyrmions, within 1% of the mathematically perfect value of ±1. The tiny deviation from a perfect score of ±1 is simply down to the limits of any real measurement – sampling a fleeting pulse of light in three dimensions will always leave a small rounding error. However, the topology itself remains exactly intact.

The importance of this result reaches far beyond the elegance of the experiment. The next wave of wireless communication technology – already being designed to operate at terahertz frequencies, which can carry vastly more data than current mobile networks – has a serious enemy: the real world. Humidity, atmospheric turbulence, buildings and even rain can scramble a terahertz signal in ways that are very hard to protect against.

Conventional optical signals encode information in the brightness or precise timing of a wave, but both of those are fragile; noise corrupts them the same way that a smudge ruins ink on paper. A skyrmion signal is fundamentally different. The information is encoded in the topological shape of the light pulse, and that shape cannot be accidentally altered by the environment. It is protected not by better engineering or thicker shielding, but by mathematics itself.

On top of that, having two switchable skyrmion states, electric and magnetic, effectively enables two distinct channels of information to travel along the same beam, doubling the capacity without using any extra bandwidth.

What this team has built is a proof of concept for a new kind of communication: one where the message is written in a shape that the universe, by its own rules, refuses to erase.

The post Switchable skyrmions light up terahertz communications appeared first on Physics World.

Researchers in China have developed a new way of probing the magnetic domains within altermagnetic materials and used it to study a prominent altermagnet candidate, alpha-phase iron oxide. According to their measurements, this material shares certain properties with ferromagnets despite having a near-zero net magnetization – a fact the researchers say supports its classification as an altermagnet.

In most magnetically ordered materials, the spins of atoms (that is, their magnetic moments) can either line up parallel with each other or antiparallel, alternating up and down. These arrangements are driven by spin-exchange interactions between the atoms, and they lead to ferromagnetism and antiferromagnetism, respectively.

Altermagnets, which were identified as a distinct class of magnets in 2022, behave differently. While their neighbouring spins are antiparallel, like an antiferromagnet, the atoms hosting these antiparallel spins are related to each other by rotational or mirror symmetries rather than the spatial inversion and half-lattice translation symmetries found in conventional antiferromagnets, explain physicists Luyi Yang and Wanjun Jiang of Tsinghua University, Beijing, who led this study. This unique property leads to a zero net magnetization in altermagnets while still allowing for the spin-split electronic band structures typically found in ferromagnets.

An altermagnet candidate

Alpha-phase iron oxide (α-Fe₂O₃) is a naturally occurring mineral commonly known as haematite. It was long believed to be an antiferromagnet, but recent theoretical research has suggested that it should be relabelled as an altermagnet.

To shed more light on the nature of α-Fe₂O₃, the team turned to a phenomenon known as the giant magneto-optical Kerr effect (giant MOKE). Named after the Scottish physicist John Kerr, who discovered it in 1877, it occurs when linearly polarized light reflects off the surface of a magnet. Interactions between the light and the material’s magnetic domains cause the polarization vector of the light to rotate, and the direction of rotation can be reversed by reversing the direction of magnetization. The effect therefore provides a “window” into materials’ magnetization states, enabling scientists to monitor and characterize them.

The Tsinghua University researchers say they found evidence of a connection between the material’s MOKE responses and its Néel vector, which is a parameter that defines its so-called staggered magnetic order. In altermagnets, the orientation of this Néel vector determines the material’s magnetic space group, which in turn dictates whether magneto-optical responses are allowed or not, they explain.

“By using magnetic fields to switch the Néel vector through a tiny canted magnetization in α-Fe₂O₃, we selectively measured the symmetry-permitted MOKE signals and confirmed the absence of symmetry-forbidden components on different surface orientations of α-Fe₂O₃ single crystals,” they say.

The researchers also observed that at large applied magnetic fields, the MOKE signals remain constant. This finding further rules out contributions from canted magnetization, which should increase with the field. These experiments therefore strengthen the idea that the MOKE signal they measured is truly driven by the Néel vector and the corresponding symmetry of α-Fe₂O₃.

Broadening methods for imaging altermagnetic domains

To date, most experimental studies on altermagnets have focused on spin transport. Yang, Jiang and colleagues say that they turned to MOKE-based measurements because they would like to study insulating altermagnets, for which electrical transport measurements are inaccessible. “We aimed to uncover the symmetry requirements for magneto-optical responses and broaden the methods for imaging altermagnetic domains,” they explain.

The main challenge they encountered was proving that the MOKE they observed predominantly originates from the Néel vector, rather than from the canted weak magnetization. The researchers say they addressed this through symmetry analysis, first-principles calculations and performing the experiment in different configurations to show that the Kerr signal remains nearly constant even as the canted magnetization keeps increasing at large applied magnetic fields. “By examining such effects on single crystals with different surface orientations, we confirmed that different Néel vector orientations produce distinct MOKE responses, which are consistent with the symmetry of magnetic space group predicted by theory,” they tell Physics World.

The researchers say their work shows that MOKE responses are not limited to ferromagnets, as is conventionally understood. Provided the symmetry requirements are satisfied, altermagnets can also exhibit giant MOKE. “We have shown that standard MOKE imaging microscopy can be used to visualize altermagnetic domains and domain walls in α-Fe₂O₃,” they say. “This could accelerate the development of altermagnetic spintronics based on these structures, with potential applications in advanced memory and logic devices.”

The researchers now plan to extend their approach to other altermagnetic insulators and metals and to use the magneto-optical response to study the (presumably) ultrafast dynamics of domain walls. Their present study is detailed in Chinese Physics Letters.

The post Altermagnetic insulator shows giant magneto-optical Kerr effect appeared first on Physics World.

Physicists at ETH Zurich in Switzerland have produced magnetic fields as high as 40 T in a superconducting coil that has a bore diameter of just 3.1 mm. Until now, creating such intense fields required large and expensive facilities and tens of megawatts of power. The new miniaturized structure requires a few thousand times less power than larger magnets and it could help bring ultrastrong benchtop magnets closer to reality.

“All previous 40 T class magnets have been metres in size, weigh more than six tons, and require about 20 MW of power to operate,” says Alexander Barnes, who led the research effort. “Our miniature magnet can also generate a 40 T magnetic field, but it is small enough to fit in the palm of your hand and requires a few watts or less to operate.”

Such a device could be extremely useful for scientists who use strong magnets in their research, he adds. “Rather than having to travel to the few locations in the world that have the resources and space to house a strong magnetic field, with this technology scientists in the future could have access to these magnets in their own laboratory.”

Making the magnet tiny

Barnes and his colleagues, who are nuclear magnetic resonance (NMR) spectroscopists, came up with the idea for their new magnet by asking themselves a simple question: “what do we need to put inside it in our experiments?” The answer was: only the sample and an NMR detection coil.

“So, instead of making magnets expensive and big enough to house all different kinds of equipment, we decided to make the magnet tiny – and just big enough to be able to fit inside it what we need to fit inside it,” says Barnes. In this way, any bulky components can be placed outside the magnet and only the essential elements within the high-field region inside it.

“Think about the right-hand rule and the Biot-Savart law we all learn in first year physics,” he explains. “This law tells us the more electrons moving in a circle, the higher the magnetic field. And the more electrons moving in a circle in a smaller volume close to the sample also means a higher magnetic field. This is all we did – we tried to maximize the electrons moving in a circle near our sample.”

High-temperature superconducting tapes

Strong magnets are needed in a host of research and technology areas, from magnetic resonance imaging (MRI) and particle accelerators to NMR spectroscopy. Magnetic fields greater than 40 T can be produced using high-temperature superconducting (HTS) tapes. These structures can also be wound together to increase their already very high critical current even further, something that allows the resulting coils to reach higher magnetic fields. A famous example, Barnes reminds us, is the world-record 45.5 T steady-state magnet, which uses a HTS coil as an insert within a resistive background magnet. The problem, however, is that these high-field hybrid magnets are huge and require a lot of power.

Barnes’ team says it might now have overcome this issue with its two compact HTS magnets wound with a conducting tape coated with the superconducting ceramic REBCO. The first magnet, composed of two pancake coils, produces a magnetic field of 38 T and the second, composed of four (quad) pancake coils, a field of 42 T. The researchers say they used a specialized winding technique combined with soldering to make sure there was a jointless connection between the pancake coils at a winding diameter of 3.5 mm.

The strong magnetic fields of the coils stem from the high current-carrying ability of REBCO and the extremely small magnet bore diameter of 3.1 mm. “These magnets reach current densities of 2257 and 1880 Amm⁻² at peak currents of 1246 and 1038 A, respectively,” says Barnes, “and despite the much higher current density, they consume a few thousand times less power and require a coil volume over 1000 times smaller than that of the 45.5 T hybrid magnet.”

“Amazing” materials

He says he imagines a “bright future” where there are hundreds and thousands of benchtop magnets capable of 50 T and more, all over the world in academia and industry.  These magnets can be used for NMR and electron paramagnetic resonance (EPR) spectroscopy, but also quantum computers and other applications. For instance, the ETH Zurich team is working on a project that uses these magnets to build miniature gyrotrons, which are microwave generators. “We have plans to use such devices for spectroscopy, but also for nuclear fusion heating and even vaporizing holes deep in the Earth to extract geothermal energy,” Barnes tells Physics World.

It will not all be plain sailing, however, say the researchers. One of the main challenges in this work, which is detailed in Science Advances, is to avoid damaging the REBCO-coated tapes. These tapes are “amazing” materials, says Barnes. They are a single crystal of rare-earth barium copper oxide and are more than 100 m long, but the problem is that they are subject to mechanical strain. If this strain exceeds a certain, critical threshold, then the superconducting layer can crack, leading to reduced current-carrying capacity as the structure’s resistance increases.

The researchers say they are now busy working on increasing the magnetic fields – they are targeting 50 T soon – and performing NMR inside their existing coils. “ResonX, the commercial partner on this study, is also actively commercializing these magnets,” reveals Barnes.

The post Miniature magnets break field strength record appeared first on Physics World.