An international research team from NIMS, The University of Tokyo, Kyoto Institute of Technology, and Tohoku University has shown that ultra-thin films of ruthenium dioxide (RuO2) display altermagnetism. This property defines what scientists now recognize as a third fundamental category of magnetic materials. Altermagnets are drawing growing interest because they could overcome key limitations of today's magnetic memory technologies and enable faster, more compact data storage.

The researchers also found that the performance of RuO2 thin films can be improved by carefully controlling how their crystal structure is oriented during fabrication. Their findings were published in Nature Communications.

Why Scientists Are Searching for New Magnetic Materials

Ruthenium dioxide (RuO2) has long been considered a promising candidate for altermagnetism, a recently proposed form of magnetism that differs from conventional types. Standard ferromagnetic materials used in memory devices allow data to be written easily using external magnetic fields. However, they are vulnerable to interference from stray magnetic fields, which can cause errors and limit how densely information can be stored.

Antiferromagnetic materials offer much better resistance to external magnetic disturbances. The challenge is that their internal magnetic spins cancel each other out, making it difficult to read stored information using electrical signals. As a result, scientists have been searching for materials that combine magnetic stability with electrical readability and, ideally, the ability to be rewritten. While altermagnets promise this balance, experimental results for RuO2 have varied widely around the world. Progress has also been slowed by the difficulty of producing high-quality thin films with a consistent crystallographic orientation.

How the Team Verified Altermagnetism

The research team overcame these obstacles by successfully creating RuO2 thin films with a single crystallographic orientation on sapphire substrates. By carefully choosing the substrate and fine-tuning the growth conditions, they were able to control how the crystal structure formed.

Using X-ray magnetic linear dichroism, the researchers mapped the spin arrangement and magnetic order in the films, confirming that the overall magnetization (N-S poles) cancels out. They also detected spin-split magnetoresistance, meaning the electrical resistance changes depending on the spin direction. This effect provided electrical evidence of a spin-split electronic structure.

The experimental results matched first-principles calculations of magneto-crystalline anisotropy, confirming that the RuO2 thin films truly exhibit altermagnetism (see Figure). Together, these findings strongly support the potential of RuO2 thin films for next-generation high-speed, high-density magnetic memory devices.