Ultrathin Films of a 'Nonmagnetic' Material Reveal a New Kind of Magnetism

A recently proposed form of magnetism called altermagnetism could eventually help make computer memory smaller, faster and more efficient. Now, researchers led by Rice University physicist Ming Yi, working with Bharat Jalan of the University of Minnesota and Milan Radovic of the Paul Scherrer Institute, report evidence that ruthenium dioxide — a material long considered nonmagnetic in its natural, bulk form — may display this unusual magnetism when prepared as an ultrathin film only a few atomic layers thick.

"Ruthenium dioxide was one of the first materials to be proposed as an altermagnetic candidate, but studies on its bulk form didn't return evidence of magnetism," Yi said. "Our research shows that its ultrathin form, on the other hand, may be the key in making it magnetic." The team, reporting in Science Advances, studied the material's spin texture — how the magnetic moments of its electrons are arranged — using a technique called spin-resolved angle-resolved photoemission spectroscopy.

The unusual spin behavior appeared only under specific conditions: the ultrathin ruthenium dioxide had to be under lattice strain, meaning pressure on its atomic structure. Without that strain, as in the material's natural bulk form, the electron spins showed no sign of altermagnetism. "The strain-dependent nature suggests that we may be able to use lattice strain as a tuning knob to induce or control altermagnetism," said Yichen Zhang, the study's first author and a recent Rice graduate. "This could be extremely useful when thinking about next-generation spintronics and RAM architectures."

Physicists had long debated whether bulk ruthenium dioxide was magnetic and eventually agreed that it was not. The new findings show that changing the material's thickness and placing it under strain can produce different behavior. The work was funded by the U.S. Department of Energy, the Gordon and Betty Moore Foundation's EPiQS Initiative, and the Robert A. Welch Foundation.