For the first time in nearly a century, physicists have identified a brand new type of magnetic material.

Traditionally, magnetic materials have been classified into two main types: ferromagnets and antiferromagnets. Ferromagnets are what most people think of when they hear "magnets"—these materials have a magnetic field that can hold up photos on a refrigerator or cause a compass to point north. Antiferromagnets, on the other hand, have no external magnetic field but exhibit other magnetic properties.

Now, physicists have expanded this classic pair into a trio with the discovery of a new class of magnetic materials called altermagnets. This breakthrough, which could pave the way for faster, more efficient computer hard drives, was reported recently by researchers. Despite the simplicity of the idea behind altermagnets, theoretical physicist Igor Mazin of George Mason University in Fairfax, Va., notes that “somehow … nobody thought about this possibility” until recently. The long-overlooked existence of a third magnetic category is “very surprising,” he adds.

The study of magnetic materials is an ancient science. Ferromagnets have been known for thousands of years, with magnetized lodestone fascinating the ancient Greeks and the Chinese using it to create the first compasses in the fourth century B.C. Antiferromagnets were discovered in the 1930s.

A few years ago, theoretical predictions suggested that altermagnets could exist. When scientists began searching for them, they quickly found that these magnetic materials were real and abundant.
Altermagnets: A Unique Class

At the microscopic level, a material’s magnetism comes from its atoms, which possess spin—a quantum mechanical property from the electrons. This spin makes each atom act like a tiny magnet. Spins can point in different directions, usually described as spin up and spin down. Any material with spins arranged in an orderly manner, without an externally imposed magnetic field, is considered magnetic by physicists.

In ferromagnets, the atoms' spins align so their magnetic fields combine, creating a surrounding magnetic field. Antiferromagnets have atoms with alternating spins that cancel each other out, resulting in no net magnetic field.

Altermagnets have a unique twist: while the atoms’ spins alternate like in antiferromagnets, the atoms themselves are also rotated. Imagine antiferromagnets as a checkerboard with alternating black and white squares representing spin up and spin down. Altermagnets, in contrast, resemble an M.C. Escher drawing with tessellating shapes that not only alternate in color but are also rotated relative to each other.

If you flip the spins in an altermagnet and rotate the material—say, by 90 degrees—it will look identical to its original state. This special type of symmetry distinguishes altermagnets from other magnetic materials. This unique symmetry, which places altermagnets in their own class, was highlighted by Jairo Sinova of Johannes Gutenberg University Mainz and colleagues in a paper published in Physical Review X in September 2022. This was one of several theoretical papers since 2019 that have helped establish altermagnets in the scientific community.