One is the familiar ferromagnet, the kind found in refrigerator magnets and countless everyday devices. The other is the antiferromagnet, whose magnetic properties are hidden at the atomic level but have attracted growing interest because of their potential use in advanced technologies.

More recently, researchers identified a third category known as altermagnets. First proposed within the last decade, these materials may combine some of the most useful characteristics of both ferromagnets and antiferromagnets, potentially opening the door to faster, more energy-efficient electronics.

Now, physicists at the University at Buffalo have proposed a new quantum sensing approach that could make altermagnets much easier to identify.

The proposed method, described in Physical Review Letters, would detect how a suspected altermagnet affects a tiny magnetic defect inside a nearby diamond. By monitoring how the defect's magnetic signal relaxes over time, researchers may be able to identify telltale signs of altermagnetism.

"This could be the first building block of a new generation of experiments that determine whether a material is an altermagnet," says corresponding author Jamir Marino, PhD, assistant professor in the UB Department of Physics, College of Arts and Sciences. "Altermagnets could completely revolutionize the way we transport information, but to confirm if this elegant theory is true, we need experiments that identify altermagnets and confirm they behave the way scientists predict."

The study's co-authors include Marino's former colleagues Libor Šmejkal and Jairo Sinova of Johannes Gutenberg University of Mainz, the researchers who originally proposed the concept of altermagnets.

"This sensing technique could become a very important tool for exploring candidate altermagnetic materials," Sinova says. "It offers advantages over conventional experimental techniques by detecting subtle directional magnetic patterns across different regions of a material without significantly disturbing it."

What Makes Altermagnets Different?

The idea of altermagnetism emerged in 2019 when researchers in Mainz encountered behavior that could not be explained by either ferromagnets or antiferromagnets.

Their calculations suggested that ruthenium dioxide should have no overall magnetization, much like an antiferromagnet. Yet when exposed to an electric current, it appeared to behave more like a ferromagnet.

That unexpected result led to the development of the altermagnet concept.

In conventional magnets, atoms and their electron spins typically arrange themselves in relatively simple patterns. In ferromagnets, neighboring electron spins point in the same direction, creating an external magnetic field. Because these spins can be switched relatively easily, ferromagnets are widely used for information storage.

Antiferromagnets work differently. Neighboring spins point in opposite directions, causing their magnetic effects to cancel each other out. Although this arrangement is more difficult to control, it can switch states much more rapidly, making antiferromagnets attractive for future information processing technologies.

Altermagnets occupy a middle ground. Like antiferromagnets, their overall magnetism cancels out. However, the arrangement of atoms inside the material causes electrons to behave in ways normally associated with ferromagnets.

"That arrangement allows altermagnets to combine the rapid switching behavior of antiferromagnets with some of the more easily controllable electronic properties of ferromagnets," Marino says.

Using Diamond Defects To Detect Hidden Magnetism

Researchers in Mainz and elsewhere have already reported experimental signatures of altermagnetism in several materials. Theoretical studies suggest the class could be much larger, with more than 200 materials potentially qualifying as altermagnets. That would be more than twice the number of known ferromagnetic materials.

To help identify these candidates, Marino's team developed its proposed quantum sensing technique.

The approach relies on a diamond containing a microscopic magnetic defect formed by a nitrogen atom and a missing neighboring carbon atom. These defects are exceptionally sensitive to nearby magnetic activity.

In the proposed experiments, researchers would rotate the defect's magnetic spin in different directions and measure how quickly it relaxes. If relaxation occurs more rapidly in certain directions than others, that behavior could reveal the complex spin arrangements predicted for altermagnets.

An important advantage of the technique is that it would be less disruptive than many existing methods used to study magnetic materials.

"You don't want your measurement to strongly perturb the material you're studying because it can become harder to tell whether you're seeing the material's natural behavior or behavior caused by the experiment," Marino says.

Toward Faster, More Efficient Electronics

Marino emphasizes that the sensing system currently exists only as a theoretical proposal. The team developed it using sophisticated models that simulate quantum dynamics, but experimental validation will still be required before researchers know whether it can reliably identify altermagnets.

"Efficiently identifying altermagnetic materials is a crucial step toward one day actually using them in electronics," Marino says. "Altermagnets would make transport of information radically more efficient. That could allow technology to scale down and be less power consuming."

Additional co-authors include Hossein Hosseinabadi, PhD, a former graduate student in Marino's lab who is now an independent distinguished postdoctoral scholar at the Max Planck Institute for the Physics of Complex Systems in Germany, and V.A.S.V. Bittencourt of the University of Strasbourg/Max Planck Institute for the Science of Light.

The research was supported by the German Research Foundation.