New review maps out the altermagnetism debate and charts a path forward

A comprehensive review published in Science China: Physics, Mechanics & Astronomy provides a critical assessment of the ongoing debate surrounding altermagnetism in ruthenium dioxide (RuO₂), one of the most extensively investigated candidate materials in the emerging field of altermagnetism. The work, led by researchers from China Three Gorges University, Southern University of Science and Technology, and other institutions, synthesizes a decade of experimental and theoretical efforts, offering a clear roadmap of the studies conducted on RuO₂, which is helpful for guiding future research.

Altermagnets represent a newly discovered class of magnetic materials that combine the zero net magnetization of conventional antiferromagnets with the spin split electronic bands typically found in ferromagnets. This unique combination, arising from specific crystal symmetries rather than spin-orbit coupling, has opened exciting avenues for next-generation spintronic devices. RuO₂ was one of the first materials predicted to host altermagnetism and has since become the focus of extensive experimental and theoretical investigation, while its magnetic nature continues to be actively debated.

The review systematically examines the crystal and magnetic structures of RuO₂, its electronic band properties, and a broad range of transport phenomena, including the anomalous Hall effect, spin-splitting torque, and inverse altermagnetic spin-splitting effects. It also discusses emerging research directions such as strain-induced superconductivity and magneto-optical responses. A major focus of the review is the ongoing discrepancy between results obtained from bulk crystals and those reported for thin-film systems, which lies at the center of the current debate over altermagnetism in RuO₂.

According to the authors, measurements on pristine bulk RuO₂, including neutron diffraction, muon spin rotation, and bulk ARPES, have detected no long-range magnetic order in high-quality RuO₂ single crystals. By contrast, thin films frequently exhibit transport signatures commonly associated with altermagnetism. The review argues, however, that these thin-film observations may also be affected by epitaxial strain, stoichiometric deviations, defects, or interface-related effects, making the interpretation of their magnetic properties considerably more complex.

 The ongoing controversies surrounding altermagnetism in RuO₂, the authors conclude, likely arise from four factors. First, most bulk-sensitive probes detect no magnetic order, while thin films are often affected by epitaxial strain, stoichiometric deviations, or interfacial symmetry breaking, and then exhibit magnetic signatures. Second, the magnetic response of RuO₂ is highly sensitive to strain, Ru vacancies, and disorder, which may stabilize strain- or defect-induced magnetic states rather than intrinsic altermagnetism. Third, theoretical studies indicate that RuO₂ resides near a magnetic quantum critical boundary, implying that any magnetic order is fragile and strongly sample-dependent. Fourth, transport signatures frequently attributed to altermagnetism can also arise from impurities, interfaces, or surface magnetism, complicating their interpretation.Two key future research directions are suggested by the authors. For RuO₂ thin films, there is a pressing need for the direct detection of magnetic order in thin film samples that exhibit transport signatures consistent with altermagnetism. Nanoscale X-ray magnetic circular dichroism combined with photoemission electron microscopy can visualize magnetic domains, which would be very helpful in resolving the current controversy. Meanwhile, experiments on tuning the crystal properties of RuO₂ are highly recommended, as theoretical studies suggest that RuO₂ is proximate to a quantum phase transition. Any means that can potentially affect electronic interactions in RuO₂ may serve as a control parameter. By altering the state of Fermi surface instability, it becomes possible to turn itinerant magnetism on and off in RuO₂.

Despite the continuing debate over the microscopic origin of the observed phenomena, the review highlights the remarkable spintronic functionality already demonstrated in RuO₂ thin films. These systems exhibit highly efficient spin-charge conversion, large spin Hall angles, and field-free switching of adjacent ferromagnetic layers. As a result, RuO₂ remains a highly promising platform for energy-efficient magnetic memory and spintronic logic devices, regardless of whether the underlying magnetism ultimately proves to be intrinsic or induced.

The review, titled “Exploration of altermagnetism in RuO₂,” is published in Science China: Physics, Mechanics & Astronomy (Vol. 69, No. 5, 2026). The work was supported by the National Key R&D Program of China, the National Natural Science Foundation of China, and other funding agencies.

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