A joint research team from the National Institute for Materials Science (NIMS), the University of Tokyo, Kyoto Institute of Technology, and Tohoku University has demonstrated that thin films of ruthenium dioxide (RuO?) exhibit altermagnetism, a third fundamental class of magnetic behavior, distinct from ferromagnetism and antiferromagnetism.

Magnetic materials play a central role in modern information technology, particularly in memory devices. Conventional ferromagnetic materials allow data to be written easily using external magnetic fields, but they are vulnerable to interference from stray fields, which can cause errors as device density increases. Antiferromagnetic materials are resistant to such disturbances, yet their canceling spin structure makes it difficult to electrically read stored information.

Altermagnets offer an alternative. They possess no net magnetization, like antiferromagnets, while still allowing electrical readout of spin-dependent properties. This combination has attracted interest for applications such as high-speed and high-density memory. However, experimental results on whether RuO? truly exhibits altermagnetism have varied, partly due to challenges in fabricating high-quality samples.

To address this issue, the research team fabricated RuO? thin films with a single crystallographic orientation on sapphire substrates. By carefully selecting the substrate and fine-tuning growth conditions, the researchers were able to control how the atomic lattice aligned during film formation. This control was essential for obtaining consistent and interpretable magnetic behavior.

Using X-ray magnetic linear dichroism, the team directly identified the spin arrangement in the films and confirmed that the magnetic poles cancel each other out. They also observed spin-split magnetoresistance, meaning that the electrical resistance depends on spin orientation. This provided electrical evidence of a spin-split electronic structure, supporting the presence of altermagnetism.

The relationship between crystallographic orientation and magnetic behavior can be compared to laying tiles on a floor. If the tiles are placed at random angles, patterns are difficult to recognize. When they are aligned in a single direction, the overall structure becomes clear. In a similar way, aligning the crystal axes of RuO? made its underlying magnetic properties observable.

"These results show that controlling crystallographic orientation is key to revealing and utilizing altermagnetism in RuO? thin films," said a member of the research team. "This approach allows us to connect theoretical predictions with experimental observations."

The experimental findings were consistent with first-principles calculations, strengthening confidence in the interpretation. Together, the results identify RuO? thin films as a practical platform for studying altermagnetism and evaluating its suitability for device applications.

Looking ahead, the team plans to explore memory devices that use RuO? thin films to achieve efficient, high-speed information processing. The synchrotron-based magnetic analysis techniques developed in this work can also be applied to other candidate altermagnetic materials, supporting broader research in spintronics.

The study was published online in Nature Communications on September 24, 2025.

Publication Details:

Title: Evidence for single variant in altermagnetic RuO2(101) thin films

Authors: Cong He, Zhenchao Wen, Jun Okabayashi, Yoshio Miura, Tianyi Ma, Tadakatsu Ohkubo, Takeshi Seki, Hiroaki Sukegawa, and Seiji Mitani

Journal: Nature Communications

DOI: 10.1038/s41467-025-63344-y