Newly discovered ‘altermagnets’ disrupt the magnetic status quo

Although the idea behind altermagnets is remarkably simple, says theoretical physicist Igor Mazin of George Mason University in Fairfax, Va., “somehow … no one thought of this possibility” until recently. That a third magnetic category could have gone unnoticed for so long, “is very surprising to me.”

This is because the study of magnetic materials is an ancient science. Ferromagnets have been known for thousands of years. Lodestone, a magnetized form of the mineral magnetite, fascinated the ancient Greeks. The Chinese forged the magnetized mineral into the first compasses in the fourth century BC (SN: 1/28/11). Antiferromagnets were discovered in the 1930s.

Then, a few years ago, theoretical predictions suggested that altermagnets might exist. And when scientists started looking for them, researchers quickly discovered that the magnetic materials were real and plentiful.

Altermagnets are their own class

At a microscopic level, materials derive their magnetism from their atoms. Atoms have spin, a quantum mechanical property imparted by the atoms’ electrons. This spin causes each atom to act like a tiny magnet. Spins can be directed in different directions, commonly called spin up and spin down. Any material with regularly arranged spins—in the absence of any externally imposed magnetic field—is considered a magnetic material by physicists.

In ferromagnets, the spins of the atoms align so that their magnetic fields combine to create a magnetic field that surrounds the material. Antiferromagnets do the opposite: the spins of the atoms point in alternating directions and their magnetic fields cancel, producing no net field.

In the altermagnet, the spins of the atoms alternate, but with an added twist. Not only are the spins of neighboring atoms opposite, but the atoms also spin. If you think of antiferromagnets as a chessboard, with alternating black and white squares standing for spin up and down, then altermagnets are like an MC Escher drawing, with little shapes—birds, horses, or other Escher motifs— that don’t just change color, but are also rotated relative to each other.

If you take an altermagnet, flip its spins around, and rotate the material—by 90 degrees, for example—it will look identical to its original state. This is a special type of symmetry, different from other magnetic materials. And this symmetry puts altermagnets in a class of their own, argued Jairo Sinova of Johannes Gutenberg University Mainz and colleagues in Physical Review X in September 2022 – one of many theoretical papers since 2019 that helped put altermagnets on the map.

Experiments are now beginning to confirm the altermagnetic identities of some materials.

Altermagnets become real

The scientists predicted that the electrons inside the altermagnet materials would have some unusual characteristics. To confirm the altermagnetic nature of a given material, scientists must determine that electron behavior. Particularly important is the drawing of how the energy of an electron in the material is related to its momentum. In a ferromagnet, electrons with a certain energy in that map are separated: The momentum depends on the spin. Spin up electrons will have a different momentum than spin down electrons of the same energy.

However, in antiferromagnets, the electrons spin up and spin down are the same. For a given energy, both spins will have the same momentum.

This is where the strange dual nature of altermagnets comes into play. The scientists predicted that the materials’ electrons would be separated by spin, but only for electrons moving in certain directions. That is, in some orientations the material will act as a ferromagnet, and in others as an antiferromagnet.

To confirm this effect, the scientists used a technique called angle-resolved photoemission spectroscopy, which measures the electrons emitted when a material is hit with light. With this method, the researchers observed spin separation in the manganese telluride material. The material has been studied since the 1960s and was previously thought to be well understood as an antiferromagnet. But the results were consistent with predicted altermagnetic behavior, the researchers reported on February 15 Nature.

Around the same time, two other teams also found evidence of spin splitting in manganese telluride, according to papers published on January 19 Physical review papers and March 15 Physical review B.

And more altermagnetic materials are emerging. A newspaper on February 2 Advances in science found distinctive signs of altermagnetism in ruthenium dioxide and a March 8 paper in Nature Communications described the altermagnetic behavior in thin films of a chromium and antimony compound.

“The bottom line is … it’s not just a rare system” that hosts an altermagnet, says physicist Libor Šmejkal of Johannes Gutenberg University in Mainz in Germany. And the results confirmed that altermagnets are not just theoretical. They are a new, third class of magnetic material.

Not only are altermagnets found in numerous materials, but there are more altermagnet candidates than ferromagnets. And the materials are not opaque or toxic, says experimental physicist Helena Reichlová of the Institute of Physics of the Czech Academy of Sciences in Prague. Researchers already know how to produce and work with these materials. “They are already here with us, they were just hiding from us.”

Young magnets find their place

The nature of altermagnets can make them particularly suitable for some technological applications. Currently, ferromagnets are used for magnetic computer hard drives, which encode 0’s and 1’s into small magnetic pieces. But the technology is limited by the magnetic fields of ferromagnets. “This magnetization in ferromagnets is the source of all these exciting effects that we, for example, use in hard drives,” says Šmejkal. “But at the same time, it is [an] enemy.”

Magnetic parts are difficult to pack tightly: Ferromagnets placed in close proximity can interfere with each other through their magnetic fields. And magnetic bits have a speed limit: they can switch from 0 to 1 only so fast. So scientists thought of replacing ferromagnets with antiferromagnets, which have no magnetic field. But there’s a problem with that plan. To read data, hard drives take advantage of the spin-splitting behavior of ferromagnets. In antiferromagnets, electrons are not separated by spin.

Altermagnets, which have no net magnetic field but share electrons with spin, can offer the best of both worlds. Altermagnetism “seems to correct some of the main limitations of ferromagnets,” says physicist Tomáš Jungwirth of the Institute of Physics of the Czech Academy of Sciences.

Additionally, while ferromagnets tend to be metals, altermagnets can be made from a variety of materials (SN: 1/11/23). Manganese telluride, for example, is a semiconductor. Because semiconductors are used to make computer chips, scientists hoped that a magnetic material that is also a semiconductor might allow the possibility of combining a memory and processor in one material (SN: 10/4/13).

With “the best of antiferromagnets, the best of ferromagnets, and some things that are unique to themselves,” says Sinova, altermagnets are shattering the limitations of the magnetic status quo. “These materials break all those barriers. They just plow right into them.”