A collaborative research team from the National Institute for Materials Science (NIMS), the University of Tokyo, the Kyoto Institute of Technology, and Tohoku University has made significant strides in the field of magnetism by demonstrating that thin films of ruthenium dioxide (RuO2) exhibit a newly identified form of magnetic behavior known as altermagnetism. This discovery adds a third fundamental class of magnetism to the established categories of ferromagnetism and antiferromagnetism, which are essential for current information technology, particularly in memory devices.
Magnetic materials are crucial in modern computing, enabling the storage and retrieval of data. Traditional ferromagnetic materials are effective for data writing using external magnetic fields. However, they are susceptible to interference from stray fields, which can lead to data errors as device density increases. On the other hand, antiferromagnetic materials resist such disturbances but present challenges for electrically reading stored information due to their canceling spin structures. Altermagnets, like RuO2, could resolve these issues by lacking net magnetization while allowing for electrical readout of spin-dependent properties. This unique combination positions them as promising candidates for high-speed and high-density memory applications.
Despite initial interest, previous experimental results on RuO2’s altermagnetism varied, primarily due to difficulties in producing high-quality samples. To overcome this challenge, the research team successfully fabricated RuO2 thin films with a single crystallographic orientation on sapphire substrates. They meticulously selected the substrate and optimized growth conditions, which allowed for precise control over the atomic lattice alignment during film formation. This careful alignment was essential for revealing consistent and interpretable magnetic behavior.
Using advanced techniques such as X-ray magnetic linear dichroism, the researchers were able to directly identify the spin arrangement within the thin films. Their findings confirmed that the magnetic poles effectively cancel each other out, supporting the altermagnetic behavior. Additionally, they observed spin-split magnetoresistance, indicating that the electrical resistance of the films varies based on spin orientation, further corroborating the presence of altermagnetism.
The team highlighted the importance of crystallographic orientation in their findings, drawing an analogy to laying tiles on a floor. When tiles are placed at random angles, it becomes challenging to discern patterns. Conversely, aligning them in a single direction clarifies the overall structure. Similarly, aligning the crystal axes of RuO2 made its underlying magnetic properties more observable.
“These results show that controlling crystallographic orientation is key to revealing and utilizing altermagnetism in RuO2 thin films,” stated a member of the research team. “This approach allows us to connect theoretical predictions with experimental observations.”
The experimental results aligned well with first-principles calculations, increasing confidence in the findings. Together, these results establish RuO2 thin films as a practical platform for studying altermagnetism and assessing its potential for future device applications.
Looking ahead, the research team is eager to explore the development of memory devices utilizing RuO2 thin films, aiming for efficient and high-speed information processing. The synchrotron-based magnetic analysis techniques developed during this research can also be applied to other candidate altermagnetic materials, paving the way for broader advancements in the field of spintronics.
The study detailing these findings was published in the journal Nature Communications on September 24, 2025.
