The existence of a new kind of magnetism has been confirmed

by ARKANSAS DIGITAL NEWS

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Illustration of altermagnetism in a chemical compound

Altermagnetism works differently from standard magnetism

Libor Šmejkal and Anna Birk Hellenes

A new kind of magnetism has been measured for the first time. Altermagnets, which contain a blend of properties from different classes of existing magnets, could be used to make high capacity and fast memory devices or new kinds of magnetic computers.

Until the 20th century, there was thought to be only one kind of permanent magnet, a ferromagnet, the effects of which can be seen in objects with relatively strong external magnetic fields like fridge magnets or compass needles.

These fields are caused by the magnetic spins of the magnets’ electrons lining up in one direction.

But, in the 1930s, French physicist Louis Néel discovered another kind of magnetism, called antiferromagnetism, where the electrons’ spins are alternately up and down. Although antiferromagnets lack the external fields of ferromagnets, they do show interesting internal magnetic properties because of the alternating spins.

Then in 2019, researchers measured a perplexing electric current in the crystal structure of certain antiferromagnets, called the anomalous Hall effect, which couldn’t be explained by the conventional theory of alternating spins. The current was moving without any external magnetic field.

It seemed, when looking at a crystal in terms of sheets of spins, that a third kind of permanent magnetism might be responsible, which has been called altermagnetism. Altermagnets would look like antiferromagnets, but the sheets of spins would look the same when rotated from any angle. This would explain the Hall effect, but no one had seen the electronic signature of this structure itself, so scientists were unsure whether it was definitely a new kind of magnetism.

Now, Juraj Krempasky at the Paul Scherrer Institute in Villigen, Switzerland, and his colleagues have confirmed the existence of an altermagnet by measuring the electron structure in a crystal, magnesium telluride, that was previously thought to be antiferromagnetic.

To do this, they gauged how light bounced off magnesium telluride to find the energies and speeds of the electrons inside the crystal. After mapping out these electrons, they were found to almost exactly match the predictions given by simulations for an altermagnetic material.

The electrons seemed to be split into two groups, which allows them more movement inside the crystal and is the source of the unusual altermagnetic properties. “This gave direct evidence that we can talk about altermagnets and that they behave exactly as predicted by theory,” says Krempasky.

This electron grouping seems to come from the atoms of tellurium, which is non-magnetic, in the crystal structure, which separate the magnetic charges of the magnesium into their own planes and allow the unusual rotational symmetry.

“It’s really nice verification that these materials do exist,” says Richard Evans at the University of York, UK. As well as the electrons in altermagnets being freer to move than those in antiferromagnets, this new type of magnet also doesn’t have external magnetic fields like in ferromagnets, says Evans, so you can use them to make magnetic devices that don’t interfere with each other.

The property could boost the storage on computer hard drives, because commercial devices contain ferromagnetic material that is so tightly packed that the material’s external magnetic fields start to see interference – altermagnets could be packed more densely.

The magnets could even lead to spintronic computers that use magnetic spin instead of current to perform their measurements and calculations, says Joseph Barker at the University of Leeds, UK, combining memory and computer chips into one device. “It maybe gives more hope to the idea that we could make spintronic devices become a reality,” says Barker.

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