Researchers led by Rice University, the University of Minnesota, and the Paul Scherrer Institute - with additional collaborators at Kyung Hee University, the Gwangju Institute of Science and Technology (GIST), Lawrence Berkeley National Laboratory, Brookhaven National Laboratory, Myongji University, the University of West Bohemia, and the University of Illinois Urbana-Champaign - have found spin texture consistent with altermagnetism in ultrathin, epitaxially strained films of ruthenium dioxide (RuO2), a material whose magnetic status has been debated for years.
Altermagnetism is a recently proposed third class of collinear magnetic order, alongside ferromagnetism and antiferromagnetism, in which compensated magnetic sublattices are related by rotation rather than by translation. That symmetry produces momentum-dependent spin splitting in a material's electronic structure even though it carries no net magnetization - a combination that could be useful for miniaturizing and improving RAM architecture in computers. RuO2 was one of the first materials proposed as an altermagnetic candidate, but a long line of studies on its bulk and strain-relaxed thick-film forms - using x-ray diffraction, neutron diffraction, muon spin rotation, infrared spectroscopy, quantum oscillations, torque magnetometry and other probes - had converged on the conclusion that RuO2 shows no magnetism at all, leaving the field in a persistent state of debate.
This study instead targeted the largely unexplored ultrathin, epitaxially strained regime. The team grew a 2-nanometer-thick RuO2 layer, with a 2-nanometer TiO2 buffer layer, on a TiO2 substrate using hybrid molecular beam epitaxy, reaching a fully strain regime - roughly -4.7% compressive strain along one in-plane axis and +2.3% tensile strain along the perpendicular axis - not previously accessed in spin-resolved photoemission experiments on RuO2. Using spin-resolved angle-resolved photoemission spectroscopy (spin-ARPES), supported by ab initio calculations, the team mapped the film's spin texture - how the material's electron spins are arranged in momentum space, which in turn reveals what type of magnetism, if any, is present.
The measurements revealed a coexistence of mirror-odd and mirror-even spin texture components. The mirror-odd component is consistent with ordinary Rashba-type spin-orbit splitting arising from the broken inversion symmetry at the film's interface, and doesn't by itself require magnetism. The mirror-even component, however, cannot be explained by that kind of non-magnetic effect. After a comprehensive symmetry analysis to rule out non-magnetic and measurement-related origins, the team concluded that the ultrathin, strained RuO2 film has intrinsically broken time-reversal symmetry, consistent with either weak ferromagnetism or a d-wave altermagnetic phase with magnetic moments lying in the plane of the film. Critically, the effect depended on strain: unstrained, relaxed RuO2 - closer to its natural bulk form - showed no sign of altermagnetism in the same kind of measurement.
Ming Yi, associate professor of physics and astronomy at Rice University, said: "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. Our research shows that its ultrathin form, on the other hand, may be the key to making it magnetic."
Yichen Zhang, the paper's first author and a recent Rice graduate, said: "After analyzing our measurements, including informing our interpretation with theoretical calculations, we found that, under our experimental conditions, ruthenium dioxide shows spin textures consistent with unconventional magnetism. This suggests that bulk and ultrathin ruthenium dioxide, under the right conditions, may have distinctly different magnetic properties." Zhang added: "The strain-dependent nature suggests that we may be able to use lattice strain as a tuning knob to induce or control altermagnetism. This could be extremely useful when thinking about next-generation spintronics and RAM architectures."
Yi noted the broader difficulty of the underlying science: "This work shows just how complex these questions can be. The high-quality material preparation and careful measurement protocol were critical to our observation of the correct electron spin properties. The results required careful analysis of spin-resolved angle-resolved photoemission spectroscopy. Through this, we were able to determine not only the magnetic state symmetries but also a potential way to manipulate it in next-generation quantum materials."