A collaborative research team from Tsinghua University, the University of Science and Technology of China, and the Hong Kong University of Science and Technology has achieved a major breakthrough in characterizing altermagnets, a newly discovered class of magnetic materials with unique spin-splitting band structures. Their study, published in National Science Review, demonstrates the first successful real-space imaging of altermagnetic domains in RuO2, addressing a long-standing challenge in this emerging field.
Altermagnets represent a paradigm shift in magnetism, combining properties of both ferromagnets and antiferromagnets while exhibiting distinctive spin-splitting in momentum space. However, the coexistence of 0° and 180° domains (where Néel vectors are antiparallel) cancels macroscopic magnetic signals, making it extremely difficult to characterize these materials using conventional methods. This limitation has sparked scientific debate about whether altermagnetism truly exists in candidate materials like RuO2.
The research team, led by Professor Cheng Song from Tsinghua University and Professor Dazhi Hou from University of Science and Technology of China, developed an innovative approach that bypasses traditional magnetic signal measurements. Their method combines lock-in thermography, a technique with ultra-high temperature resolution (<0.1 mK), with the spin-dependent Peltier effect (SDPE). In SDPE, when a pure spin current is injected into an altermagnetic material, the different Peltier coefficients for spin-up and spin-down electrons generate a net heat flow at the interface. Crucially, the direction of this heat flow reverses when the Néel vector rotates by 180°, providing a thermal fingerprint that directly reflects the local magnetic domain orientation.
Using this technique, the team successfully visualized magnetic domains in epitaxial RuO2(110) films with micrometer-scale resolution. Their thermal imaging revealed that 0° and 180° domains exhibit preferential arrangements over tens of micrometers rather than random distribution, providing the first direct experimental evidence of domain structures in RuO2.
Perhaps more significantly, the researchers demonstrated that magnetic field cooling can deterministically control domain alignment. When cooling samples from above the Néel temperature (~370 K) to room temperature under an applied magnetic field, the field direction determines the final Néel vector orientation. This discovery solves the long-standing challenge of domain manipulation in altermagnets.
The team validated this controlled switching through measurements of the spin-dependent Seebeck effect, the reciprocal process of SDPE. After field annealing under different magnetic field directions, the sign of the hysteresis loop shift reversed, confirming the successful reorientation of Néel vectors.
"This work establishes a robust thermal fingerprint of altermagnetism," said Professor Cheng Song. "By directly mapping magnetic domains through their thermal signatures, we provide unambiguous evidence for altermagnetic order in RuO2 and open new avenues for characterizing other altermagnetic materials."
The findings carry significant scientific implications. They establish a practical method for real-space imaging of altermagnetic domains based on spin-dependent thermoelectric effects. Meanwhile, the direct observation of preferential domain arrangements in RuO2 provides experimental confirmation of altermagnetism in this material. In additions, the demonstration of spin-to-heat conversion and its reciprocal process extends the frontier of spin caloritronics into this emerging class of magnetic materials.
The research team believes this approach can be extended to other altermagnetic candidates such as Mn5Si3, V2Se2O, and V2Te2O, paving the way for deeper understanding and practical applications of altermagnets in next-generation spintronic devices.
