Demonstration of Altermagnetism in RuO₂ Thin Films—A New Magnetic Material for the AI Era

Ruthenium dioxide (RuO₂) has been attracting attention as a promising candidate for exhibiting altermagnetism—a proposed third fundamental class of magnetism.
Conventional ferromagnetic materials used in memory devices allow easy data writing via external magnetic fields but are prone to recording errors caused by stray magnetic fields, which limit further increases in data density.
Antiferromagnetic materials, on the other hand, are robust against such external disturbances. However, because their atomic-level spins cancel each other out, it is difficult to electrically read information from them.
This has driven demand for magnetic materials that combine resistance to external disturbances with compatibility with electrical readout, and ideally also allow rewritability.
However, experimental results concerning altermagnetism in RuO₂ have been inconsistent worldwide, hindering a clear understanding of its fundamental nature. Moreover, the lack of high-quality thin-film samples with uniform crystallographic orientation has prevented conclusive experimental verification.

The joint research team successfully fabricated single-orientation (single-variant) RuO₂ thin films with aligned crystallographic axes on sapphire substrates. They clarified the mechanism by which crystallographic orientation is determined through optimal substrate selection and fine-tuning of growth conditions.
Using X-ray magnetic linear dichroism, the team identified the spin arrangement and magnetic ordering in which the net magnetization (N–S poles) cancels out. Furthermore, they observed spin-split magnetoresistance—a phenomenon in which electrical resistance varies depending on spin orientation—thereby electrically verifying the spin-splitting electronic structure.
The results of the X-ray magnetic linear dichorism were consistent with first-principles calculations on the magneto-crystalline anisotropy, demonstrating that the RuO₂ thin films exhibit altermagnetism (see Figure).
This finding strongly supports the potential of RuO₂ thin films as promising materials for next-generation high-speed, high-density memory devices.

Building on these results, the research team aims to develop next-generation high-speed, high-density magnetic memory devices utilizing RuO₂ thin films.
Such devices are expected to contribute to more energy-efficient information processing by leveraging the inherently high-speed and high-density characteristics of altermagnetism.
Furthermore, the synchrotron-based magnetic analysis technique established in this study can be applied to the exploration of other altermagnetic materials and the development of spintronic devices.

• This project was carried out by a research team led by Zhenchao Wen (Senior Researcher, Spintronics Group (SG), Research Center for Magnetic and Spintronic Materials (CMSM), NIMS), Cong He (Postdoctoral Researcher, SG, CMSM, NIMS at the time of the research), Hiroaki Sukegawa (Group Leader, SG, CMSM, NIMS), Seiji Mitani (Managing Researcher, SG, CMSM, NIMS), Tadakatsu Ohkubo (Deputy Director, CMSM, NIMS), Jun Okabayashi (Associate Professor, School of Science, The University of Tokyo), Yoshio Miura (Professor, Kyoto Institute of Technology) and Takeshi Seki (Professor, Tohoku University).
• This work was supported by the JSPS Grants-in-Aid for Scientific Research (grant numbers: 22H04966, 24H00408); the MEXT Initiative to Establish Next-Generation Novel Integrated Circuits Centers (X-NICS) (grant number: JPJ011438); the GIMRT Program of the Institute for Materials Research, Tohoku University; and the Cooperative Research Projects of the Research Institute of Electrical Communication, Tohoku University.
• This research was published online in Nature Communications, an international scientific journal, on September 24, 2025.