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Strain Unlocks Unconventional Magnetism in Ruthenium Dioxide

Scientific Achievement

Using spin-resolved ARPES at the Advanced Light Source (ALS), researchers found evidence for altermagnetism in thin, strained ruthenium dioxide, previously thought non-magnetic in its bulk form.

Significance and Impact

This finding revives ruthenium dioxide as a candidate for spintronic devices that could be faster and more energy-efficient than today’s ferromagnet-based electronics.

Research Details

  • Synthesized ruthenium dioxide (RuO2) as an ultrathin film on a substrate to induce a strained, rigid crystal structure.
  • Used spin-resolved angle-resolved photoemission spectroscopy (spin-resolved ARPES) at ALS beamline 10.0.1 to map the sample’s spin polarization.
  • Experiments revealed that the electronic bands were spin-polarized, and that the polarization persisted across a specific mirror plane located at the zone center—a signature of altermagnetism.
Four-panel figure of spin-resolved photoemission data for a material with a (1-10) mirror plane: a Fermi surface map, an energy-momentum band cut, and two spin polarization spectra along [001] at opposite momentum points, showing mostly negative polarization in the mirror-odd region and positive polarization near the Fermi level.
Left: Gray-scale intensity plots from ALS beamline 10.0.1, which researchers used to navigate the momentum space for spin-polarized measurements. A grey bar indicates the mirror plane located at the Brillouin center. On opposite sides of the mirror plane, a purple hexagon and star indicate the positions in momentum space where the spin-polarized spectra were taken. Right: Spin polarization of photoelectrons measured at ALS beamline 10.0.1 at momentum points on opposite sides of the mirror plane (star and hexagon). The spin polarization is not symmetric across the plane, indicating time-reversal symmetry breaking and altermagnetic spin-polarization components in the material (Credit: Yichen Zhang/Rice University).

Research Summary

Altermagnetism, a recently discovered form of magnetism, has unique properties that could allow future electronic devices to use electron spin, not just charge, to transfer information. Altermagnetic materials could be used to make data storage and computing devices orders of magnitude faster and more energy-efficient than today’s ferromagnet-based technology. RuO2 was the first proposed altermagnetic material, but follow-up experiments looking into properties of bulk samples found no evidence of altermagnetism. However, researchers hypothesized that RuO2 in the form of thin, strained films—which are better aligned with the realities of microelectronics—might support altermagnetism.

Researchers grew ultrathin films (less than five nanometers) of RuO2 on a TiO2 substrate, producing a highly strained crystal structure. They then used spin-resolved angle-resolved photoemission spectroscopy (ARPES) at ALS beamline 10.0.1 to study how electron spin was distributed across the material’s electronic structure after it had been magnetized. The data revealed two hallmarks of altermagnetism: RuO2’s electronic bands were spin-polarized, and that polarization persisted across a specific mirror plane located at the Brillouin zone center, or the edge of a repeating unit of electron states. Observed together, these properties indicate that thin, strained RuO2 hosts unconventional magnetism, possibly altermagnetism.

This measurement was possible due to the spin sensitivity of the ARPES instrument (spin-ARPES) operating at the beamline: The beamline gave researchers information about the structure and the spin directions of electrons in the sample. The team used this data to conduct analysis and simulations that ruled out alternative explanations for their observations, such as the crystal’s polar structure or the photoemission process itself. Neither explained their results. The remaining explanation was that time-reversal symmetry was broken in the sample, revealing its unconventional magnetism.

The result shows that this magnetism in RuO2 depends on the material being thin and strained, the same parameters relevant to future spintronic devices. The findings also suggest that strain could be a tool for inducing altermagnetic behavior in other candidate materials.

Contacts: Yichen Zhang, Ming Yi, and Alexei Fedorov

Affiliations and Institutions: Lawrence Berkeley National Laboratory, ALS; Brookhaven National Laboratory, National Synchrotron Light Source II (NSLS-II); Gwangju Institute of Science and Technology; Kyung Hee University; Myongji University; Paul Scherrer Institute; Rice University; University of Illinois; University of Minnesota; University of West Bohemia.

Funding: This research was supported by the US Department of Energy (DOE), Office of Science (SC), Basic Energy Sciences (BES) program; National Science Foundation; Air Force Office of Scientific Research; Rice University’s Center for Research Computing; Swiss National Science Foundation; Programme Johannes Amos Comenius; National Research Foundation of Korea; Gordon and Betty Moore Foundation; Robert A. Welch Foundation. Operation of the ALS and the NSLS-II is supported by the DOE BES.

Y. Zhang, S.G. Jeong, L. Buiarelli, S. Lee, Y. Guo, J. Wen, H. Li, S. Nair, I.H. Choi, Z. Ren, Z. Yue, J. Hyun, T. Zhang, A. Fedorov, S.-K. Mo, H. Lim, A. Hunt, I. Waluyo, J. Kono, J. Minár, J.S. Lee, T. Low, T. Birol, R.M. Fernandes, M. Radovic, B. Jalan, and M. Yi, “Observation of mirror-odd and mirror-even spin texture in ultrathin epitaxially strained RuO2 films,” Sci. Adv. 12, eaec2917 (2026), doi:10.1126/sciadv.aec2917.