—Toward the Development of High-Speed, High-Density Memory for AI and Data Centers—
Background and Challenges
Ruthenium dioxide (RuO₂) has long been regarded as a promising candidate for exhibiting “altermagnetism,” the so-called third type of magnetism. Conventional ferromagnets can be easily written with external magnetic fields, but stray fields cause recording errors, posing a fundamental obstacle to high-density memory. Antiferromagnets are resistant to external disturbances such as stray fields; however, because atomic spins (N–S poles) cancel each other out, electrical readout is extremely challenging.
This created the demand for a new class of magnetic material that combines the best of both worlds—robustness against disturbances while still enabling electrical readout and, potentially, future rewriting. Yet, worldwide experimental results on RuO₂’s altermagnetism have been inconsistent, and the lack of high-quality thin films with uniform crystal orientation prevented definitive demonstration.
Key Achievements
A collaborative research team from NIMS, the University of Tokyo, Kyoto Institute of Technology, and Tohoku University succeeded in fabricating single-variant RuO₂ thin films with aligned crystal orientation on sapphire substrates. By optimizing substrate choice and growth conditions, the team clarified the mechanism that determines orientation.
Using X-ray magnetic linear dichroism (XMLD) measurements at the Photon Factory synchrotron facility of KEK, the researchers identified both the magnetic order—where total magnetization (N–S poles) cancels out—and the spin orientation. They further observed spin-split magnetoresistance, a phenomenon in which electrical resistance changes depending on spin orientation, thereby confirming electronic differences in spin states by electrical means.
Read more on the KEK website
Image: Conceptual illustration of altermagnetism in single-variant RuO₂ thin films, showing XMLD signals and spin orientations