2026-03-06 ミネソタ大学
Using state-of-the-art epitaxial synthesis and advanced laser-based optical probes, the researchers directly observed the transition to an altermagnetic state in films only two nanometers thick. Photo by Kalie Pluchel, University of Minnesota Twin Cities
＜関連情報＞
- https://cse.umn.edu/college/news/tuning-nonmagnetic-material-reveals-hidden-magnetic-state
- https://www.pnas.org/doi/10.1073/pnas.2526641123
超薄エピタキシャル歪RuO₂膜における異磁性極性金属相 Altermagnetic polar metallic phase in ultrathin epitaxially strained RuO2 films
Seung Gyo Jeong, In Hyeok Choi , Sreejith Nair, +13 , and Bharat Jalan
Proceedings of the National Academy of Sciences Published:March 6, 2026
DOI:https://doi.org/10.1073/pnas.2526641123
Significance
Altermagnetism, a recently proposed form of magnetic order, has been predicted in rutile RuO2 but its existence has remained controversial, as bulk crystals or thick films appear nonmagnetic. We demonstrate that epitaxial strain in ultrathin films stabilizes altermagnetism and enables direct observation of its magnetic transition using symmetry-sensitive magneto-optics, atomic-resolution probes, and theory. These results resolve long-standing debates about RuO2 and reveal that strain can transform a nominally nonmagnetic material into an altermagnet. More broadly, our work establishes strain engineering as a powerful route to create emergent quantum states, advancing design strategies for functional oxides with potential applications in spintronics and quantum technologies.
Abstract
Altermagnetism refers to a wide class of magnetic orders featuring magnetic sublattices with opposite spins related by rotational symmetries, resulting in nontrivial spin splitting and magnetic multipoles. However, the direct observation of the altermagnetic transition remains elusive. Here, by combining theoretical analysis, electrical transport, X-ray, and optical spectroscopies, we establish a phase diagram in hybrid molecular beam epitaxy-grown RuO2/TiO2 (110) films, mapping symmetries along with altermagnetic/electronic/structural phase transitions as functions of film thickness and temperature. This features an altermagnetic metallic polar phase in epitaxially strained 2 nm films, suggesting a potential link between polar metals and altermagnetic materials. Such a clear signature of a magnetic phase transition at ~500 K is observed exclusively in ultrathin strained films, unlike in bulk RuO2 single crystals. These results highlight the power of epitaxial heterostructure engineering to induce altermagnetism in systems initially nonmagnetic, opening avenues for realizing emergent quantum phases with multifunctional properties.
