Researchers discover new kind of magnetism

More than 200 materials could be “altermagnets,” predicted just a few years ago

 

For thousands of years, people have been drawn to the apparent magic of magnets. Ancient Greek philosophers believed dark rocks called lodestones had souls because of their ability to move iron flakes.

Physicists now know that magnetic materials glean their power from the behavior of the atoms inside them. But magnetism still holds secrets. Researchers have recently found signs of a wholly new class of magnetism, one with characteristics of each of the two conventional kinds, ferromagnetism and antiferromagnetism.

More than 200 materials should exhibit the newfound phenomenon, according to theoretical predictions, and physicists are closing in on direct experimental evidence for it, which could lead to more efficient electronic devices. Already they have found a handful of materials that seem to exhibit this “fundamentally new type of magnetism,” says Paul McClarty, a physicist at the Léon Brillouin Laboratory. “It’s expanding our understanding of the ways that matter can work.”

Inside solid materials, atoms are surrounded by electrons that all have a property called spin, which endows each atom with its own tiny magnetic field. The total spin for each atom is represented by an arrow that can point in different directions. In ferromagnets, all the spins inside the material are aligned, resulting in a net magnetic field. In addition to sticking photos to the fridge, ferromagnets are useful because their spins can easily be flipped around by applying another magnetic field, creating distinct states that can be used as computer memory. This technique birthed the emerging technology of spintronics, in which information is encoded via electron spin rather than charge.

In the 1930s, scientists realized it’s much more common for the spins of neighboring atoms to point in opposite directions so their net magnetization cancels out. Because the staggered arrangement is much more stable than the uniform one, these antiferromagnets are nearly impossible to magnetize with applied magnetic or electric fields. When French physicist Louis Néel won a Nobel Prize in 1970 for his pioneering work on antiferromagnetism, he described the phenomenon as “interesting but useless.” Nevertheless, the concept has proved handy: During World War II, electric coils were used to make ship hulls behave like antiferromagnets and evade magnet-seeking mines.

More recently, scientists have begun to devise strategies for building spintronic devices out of antiferromagnets. Although their rigid spins are harder to manipulate, they can in principle flip 1000 times faster than those in ferromagnets, allowing for more energy efficient information storage and processing.

A few years ago, Libor Šmejkal, a physicist at the Johannes Gutenberg University of Mainz, was hunting for a possible antiferromagnetic spintronic material. He stumbled across a compound called ruthenium dioxide that seemed promising—but odd. His calculations suggested it should have no net magnetization, like a normal antiferromagnet. But he also predicted that when subjected to an electric current, the material would behave like a ferromagnet: Magnetic forces in the material would deflect the electrons in the current, leading to a strong voltage in the perpendicular direction. In 2020, a team in China experimentally confirmed ruthenium dioxide’s paradoxical properties.

The following year, Šmejkal and colleagues laid out a proposal explaining how materials like ruthenium dioxide could be part ferromagnet and part antiferromagnet. They called them altermagnets. In most materials, electron spin arrows align with the orientation of their host atoms within the crystal lattice. But in some materials, spin arrows can rotate independently of the atoms, and Šmejkal and colleagues considered one in which every other atom was rotated by 90° and its spin flipped by 180°.

Magnetism’s new twist

The properties of most magnetic materials depend on whether each atom’s magnetic field—denoted by its spin—is pointing up (pink) or down (blue). In altermagnets, the atoms and their spins rotate independently, giving them properties of both ferromagnets and antiferromagnets.

Ferromagnetic Electron spins Atom Antiferromagnetic Altermagnetic Rotated atom A. Mastin/Science

Altermagnets would combine the most prized features of ferromagnets and antiferromagnets. With zero net magnetization, they are graced with the stability and fast spin-flipping speeds of an antiferromagnet. But the spins in an altermagnet, like those in a ferromagnet, can be readily ushered into distinct up and down states, allowing for easier memory writing. “You can have your cake and eat it, too,” says Jairo Sinova, another physicist in the Mainz group. Whereas ferromagnetic spins are typically flipped with magnetic fields, spins in an altermagnet could be manipulated by applying currents in different directions.

Theorists were quick to accept Šmejkal’s description because of its mathematical elegance, but many are surprised the phenomenon went unnoticed for so long. “It’s one of those theoretical constructs which are unquestionable,” says Igor Mazin, a physicist at George Mason University. “Yet it has never been discussed before.”

More than 200 materials are predicted to be altermagnetic—more than double the number of known ferromagnetic materials. Researchers are now beginning to look for the property by shining laser light on a material to coax it to eject electrons. By measuring the properties of those electrons, scientists can look for a hallmark of altermagnetism: energy levels that fall within two distinct bands, reflecting both spin-up and spin-down electrons. (Antiferromagnets also have spin-up and spin-down electrons, but they sit at the same energy levels.)

Last month, a team in South Korea found the predicted split in electron energies in the material manganese telluride. Two additional recent studies identify similar signals in manganese telluride and ruthenium dioxide, and also attempt to tie the energy bands to specific spin polarities. “Crystal-clear proof is really hard to achieve experimentally,” says Suyoung Lee, a Ph.D. student at Seoul National University who led one of the latest studies. “But I would say that we now have sufficient experimental evidence … that altermagnetism is really a thing.”

McClarty says the new experiments are “consistent with altermagnetism,” but only reveal the spin behavior through a slice of the material’s magnetic landscape. Until experimentalists capture the behavior across an entire 3D structure, “I wouldn’t hang up my coat,” he says. Also, before altermagnets can be exploited in electronic devices, scientists must learn to synthesize materials that have a consistent altermagnetic orientation rather than a patchwork of shifting configurations.

Mazin says confirmation of materials’ altermagnetic makeup is all but certain. “There’s no way in nature that they wouldn’t be,” he says. He sees the verification effort akin to “an experiment that proves two times two is four.”

But for Lee, the hunt promises other payoffs: an opportunity to explore emergent complex phenomena that may lead to practical applications. “I think this is the starting point for a whole new field of altermagnetism,” she says. “I am happy to be part of it.”

Zack Savitsky - author