Study: Quantum sensor may be able to identify new type of magnetism

BUFFALO, N.Y. — For nearly a century, there were two known kinds of magnets.

Ferromagnets are the classic magnets that attract metal and keep pictures stuck to the refrigerator. Antiferromagnets hide their magnetism at the atomic scale but are increasingly prized for their technological potential. 

A third category discovered within the last decade may combine the best qualities of both. Dubbed altermagnets, they could someday help create faster, more energy-efficient electronics. 

Now, University at Buffalo physicists are proposing a quantum sensing system to make identifying altermagnets much simpler.

Described in a study published in Physical Review Letters, the theoretical technique would measure how a suspected altermagnet disturbs a tiny magnetic defect in a nearby diamond. The way the defect’s magnetic signal relaxes could provide evidence 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.” 

Co-authors on the study include Marino’s former colleagues Libor Šmejkal and Jairo Sinova, physicists at Johannes Gutenberg University of Mainz who first proposed 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.”

In 2019, the Mainz team came across an effect they could not explain using either ferromagnets or antiferromagnets. Their calculations suggested that a compound called ruthenium dioxide should have no net magnetization, like an antiferromagnet, but would behave like a ferromagnet when subjected to an electric current.

With that, the concept of altermagnetism was born.

In ordinary magnets, atoms and their electron spins are usually organized in simple patterns. In ferromagnets, neighboring electron spins all align in the same direction, producing an external magnetic field. Those aligned spins can also be flipped relatively easily, creating distinct states that are often used for computer memory.

In antiferromagnets, neighboring spins point in opposite directions, canceling out any overall magnetism. This staggered arrangement is harder to manipulate but can switch much faster, potentially enabling more energy-efficient information storage and processing.

Altermagnets are more complex. Although their magnetism cancels out overall like in antiferromagnets, their atomic structure causes electrons to behave in ways usually 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.

The Mainz team and other researchers have experimentally observed signatures of altermagnetism in several materials. But theoretical predictions suggest more than 200 materials may be altermagnetic — more than double the number of known ferromagnetic materials.

That’s why Marino’s team developed their quantum sensing system. It involves placing a suspected altermagnet next to a diamond containing a tiny magnetic defect created by a nitrogen atom and a missing neighboring carbon atom. Such defects are extraordinarily sensitive to nearby magnetic behavior.

Researchers would rotate the defect’s magnetic spin in several directions and measure how quickly it relaxes. If the defect relaxes faster in some directions than others, it could provide evidence of the unusually complex spin pattern predicted for altermagnets.

Crucially, the quantum sensing system would be less invasive than many existing methods for probing altermagnetism. 

“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.

Marino stresses that the sensing system currently exists only in theory, developed using advanced models that simulate quantum dynamics. Experiments will still be needed to confirm whether it can reliably detect altermagnetism.

“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.”

Other 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.