New type of magnetism discovered

Mankind first came into contact with magnetism thousands of years ago: The strange forces emanating from ferromagnetic magnetite were noticed by scholars in what is now China and Greece, among other places. Since then, physicists have extensively investigated the mechanisms behind magnetic materials - and yet a team led by solid-state physicist Libor Šmejkal from Johannes Gutenberg University Mainz made an astonishing discovery in 2020: It observed a strange behaviour of electrons that could only be explained by a new, previously unknown form of magnetism. Whether such "alter magnetism" really exists initially remained an open question. However, three research groups have independently proven in laboratory experiments that alter magnetism is real. And not only that: it could prove to be extremely useful in practice.

What seemed like a magical force in the past is now fully understood. Magnets are crystalline solids that consist of atoms arranged in regular lattices. These atoms have a kind of intrinsic angular momentum, a "spin", which can point in different directions. If the atomic spins in a crystal are all aligned in the same direction, it is a ferromagnet, such as magnetite, which was already observed in ancient times. In other materials, however, the spins are so wildly mixed up that they do not follow any recognisable structure - such materials are considered non-magnetic. In 1930, the physicist Louis Néel discovered a second form of magnetism in so-called antiferromagnets. Although they do not generate an external magnetic field, the spins inside them are organised: They alternately point in different directions. If the spin of one atom points north, the spins of the neighbouring atoms point south. This means that the magnetic moments of the particles cancel each other out. Antiferromagnets occur much more frequently in nature than ferromagnets.

Long overlooked form of magnetism

As it turns out, even experts have overlooked a third form of magnetism for decades. "Alter magnets virtually combine the properties of ferromagnets and antiferromagnets," says solid-state physicist Hans-Joachim Elmers from Johannes Gutenberg University in Mainz, who was involved in the detection of alter magnets. From the outside, these materials, like antiferromagnets, do not generate a magnetic field. However, they actually possess characteristics that are actually reserved for ferromagnets.

Smejkal and his team discovered this when they carried out what is known as a Hall experiment, which investigates how a magnetic field influences the flow of current through a conductor. It has been known since the 19th century that this creates a Lorentz force that deflects the current from its path. However, as the physicist Edwin Hall recognised, this deflection can also be observed if the conductor itself is magnetic - in which case no external magnetic field is required. This phenomenon is known as the "anomalous Hall effect".

Šmejkal and his colleagues observed the anomalous Hall effect in ruthenium oxide, an antiferromagnetic substance, in 2020. "This was very surprising, as it had previously been assumed that the Hall effect was compensated for by these opposing magnetic moments," said Šmejkal in a 2020 press release. Apparently, not all materials previously classified as antiferromagnets are the same. Among them lurk substances that apparently belong to a different form of magnetism, known as alter magnetism.

In the following years, researchers realised that numerous materials classified as antiferromagnets could in fact be age-magnetic. "Experts realised that while ageing magnets share some of their key properties with antiferromagnets, they have even more in common with ferromagnets," wrote physicist Igor Mazin from George Mason University in Fairfax in an article published by the American Physical Society. In addition to the anomalous Hall effect, old magnets exhibit other electronic properties that otherwise only occur in ferromagnets. However, as with antiferromagnets, their atomic spins are aligned in the opposite direction.

A closer look, however, reveals an additional order that antiferromagnets do not have: In alter magnets, all electrons moving in the same direction have an equally aligned spin. "This ordering phenomenon has nothing to do with the spatial order - i.e. the location of the electrons - but only with the directions of the electron velocities," says Elmers. This significant difference means that the materials have unexpected characteristics that are actually reserved for ferromagnets.

Experimental proof of alter magnetism

The three research groups have succeeded in confirming the theoretical predictions about ageing magnets in experiments - and thus proving the existence of a new form of magnetism. The research group led by Elmers irradiated a thin layer of ruthenium dioxide with X-ray light at the German Electron Synchrotron in Hamburg, which excited the electrons of the material to such an extent that they were knocked out of the layer and detected. This made it possible to determine both the speed and the spin direction of the particles. The results suggest that ruthenium dioxide is an alter magnet as theoretically predicted. The researchers published the results in "Science Advances" on 31 January 2024.

Another research team led by Tomas Jungwirth from the University of Nottingham and a group led by Chang Liu from the Southern University of Science and Technology in Shenzhen have investigated manganese telluride and manganese ditelluride. According to theoretical calculations, both materials should also be old magnets. Using angle-resolved photoelectron spectroscopy, both groups have succeeded in detecting clear age-magnetic properties in both materials. They published their results on 14 February 2024 in the scientific journal "Nature". "The two groups used different experimental approaches and methods of analysis and were able to shed light on the complex magnetic structures of these materials," writes physicist Carmine Autieri from the Polish Academy of Sciences in Warsaw, who was not involved in the work, in an accompanying article.

The newly discovered form of alter magnetism could enable a wide range of technological applications. In the field of spintronics in particular, where signals are transported by the spins of particles rather than the charge, alter magnets could be helpful. "Ferromagnets can induce stray magnetic fields that affect the performance of the material, but these fields do not occur with antiferromagnets," writes Autieri. With antiferromagnets, ferromagnetic properties can therefore be utilised without generating disruptive magnetic fields. In addition, old magnets could enable components with higher clock frequencies than before. "These newly emerging concepts will probably soon be an integral part of physics textbooks," says Autieri.

Spectrum of Science

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