Altermagnets represent a striking departure from traditional magnetic materials, inhabiting a unique middle ground between ferromagnets and antiferromagnets. Unlike ferromagnets, where magnetization manifests as a net macroscopic field, or antiferromagnets, composed of antiparallel aligned spins canceling out macroscopic magnetization, altermagnets display spin splitting in momentum space governed by their crystallographic symmetries. This complex spin structure leads to the coexistence of 0° and 180° domains, distinguished by Néel vectors pointing in antiparallel directions, thereby neutralizing net magnetic signals and frustrating conventional magnetic imaging techniques. The enigmatic nature of this magnetism had sparked controversy about the actual embodiment of altermagnetic order in materials such as RuO₂.

Overcoming these challenges, the research team devised a novel approach, ingeniously circumventing the limitations of direct magnetic detection. At the heart of their method is the application of lock-in thermography, a sensitive imaging technique capable of capturing minute temperature fluctuations—down to sub-millikelvin levels—coupled with the spin-dependent Peltier effect (SDPE). The SDPE emerges when a pure spin current is injected into an altermagnet: the distinct Peltier coefficients associated with spin-up and spin-down electrons yield a directional heat flow at the interface. Crucially, this heat flow reverses upon flipping the orientation of the Néel vector by 180°, creating a direct thermal fingerprint that faithfully maps the altermagnetic domain structure.

Implementing this powerful technique, the scientists imaged epitaxial RuO₂(110) thin films with micrometer spatial resolution, revealing striking patterns in domain distribution. Instead of a random landscape of domains, the thermal maps depicted large-scale preferential arrangements of 0° and 180° domains spanning tens of micrometers. This highly organized domain configuration provides the first unequivocal experimental evidence validating the altermagnetic state within RuO₂, conclusively settling the debate about its magnetic character after years of theoretical and indirect experimental examinations.

The breakthrough also extended to active control of these domain structures. By cooling samples through the Néel temperature (~370 K) under external magnetic fields—a process known as magnetic field cooling—the team achieved deterministic alignment of the Néel vectors. The magnetic field direction during cooling robustly set the final antiferromagnetic domain orientation, demonstrating that even in the absence of net magnetization, the domains in altermagnets can be manipulated with high fidelity. This capability solves a critical puzzle in altermagnetic research: the controllable switching and stabilization of magnetic order in materials lacking conventional magnetic signatures.

To further confirm their results and complete the reciprocity of the effect, the researchers measured the spin-dependent Seebeck effect, the inverse phenomenon of SDPE. By analyzing hysteresis loop shifts in samples annealed under different field orientations, they observed sign reversals that perfectly corresponded to controlled Néel vector switching. This reciprocal verification solidified the robustness of their experimental methodology and the deep spin-heat coupling intrinsic to altermagnets.

Professor Cheng Song, leading the effort from Tsinghua University, emphasized the significance of these findings: “Our experiments establish a definitive thermal fingerprint of altermagnetism, enabling direct imaging of magnetic domains through heat flow patterns. This breakthrough provides unambiguous proof of altermagnetic order in RuO₂ and opens an entirely new pathway for exploring and harnessing this novel magnetic phase.”

The implications of this work resonate beyond fundamental physics. By pioneering a practical imaging tool for altermagnetic domains, the team opens the door to exploring a broad family of altermagnetic compounds, including Mn₅Si₃, V₂Se₂O, and V₂Te₂O. Each presents unique spin textures promising transformative advances in spin caloritronics—the burgeoning field that leverages spin-dependent heat and charge transport processes for novel device functionalities.

Moreover, the ability to thermally detect and control antiferromagnetic domains with micrometer precision paves the way for innovative spintronic devices that combine ultrafast operation with negligible stray fields. This technology could revolutionize memory, logic, and sensing applications by exploiting the intrinsic robustness and symmetry-driven spin textures of altermagnets, bypassing the limitations of conventional ferromagnetic systems.

The research also establishes an exciting scientific narrative by merging spin-caloritronic effects with new magnetic orders, enriching our understanding of how spin currents and thermal gradients intertwine in complex quantum materials. By capturing the spin-dependent heat transfer associated with subtle spin-momentum locking, the study pushes the frontier of both magnetism and energy conversion technologies.

Looking forward, the research collective is enthusiastic about expanding their approach to a wider array of candidate materials, unlocking fresh opportunities for investigating altermagnetic phenomena across diverse chemical compositions and crystallographic structures. Their technique offers the scientific community a potent and versatile tool to probe hidden antiferromagnetic orders that evade traditional magnetometry, heralding a new era of magnetic materials research driven by precision thermal imaging.

In conclusion, the successful imaging and controlled manipulation of altermagnetic domains in RuO₂ constitute a major leap in magnetism research, proving that even materials without net magnetization can manifest rich and controllable spin textures. Combining cutting-edge thermography with the spin-dependent Peltier effect provides a high-resolution and direct means to visualize and tune these exotic magnetic states, challenging existing paradigms and unlocking new technological horizons in spintronics and beyond.

Subject of Research: Experimental real-space imaging and manipulation of altermagnetic domains in RuO₂ using spin-dependent thermoelectric effects.

Article Title: Real-space thermal imaging and deterministic switching of altermagnetic domains in RuO₂ epitaxial films.

Web References: 10.1093/nsr/nwag203

Image Credits: ©Science China Press

Keywords

Altermagnetism, RuO₂, spin-dependent Peltier effect, spin caloritronics, magnetic domain imaging, antiferromagnetism, spintronics, thermal imaging, spin Seebeck effect, Néel vector, magnetic field cooling, epitaxial thin films

The essence of their work lies in the ability to reverse the magnetization states of tiny nanomagnets using radiofrequency signals to excite spin waves. This transformation is reminiscent of switching between binary states, akin to the fundamental mechanics of digital information storage. The overarching implications of this research transcend mere technological curiosity; it points toward a future where computing methods can mitigate the energy losses—often termed Joule heating—that plague traditional electronic devices. The researchers’ approach thus embodies a paradigm shift toward eco-friendly computing solutions, capable of revolutionizing information and communication technologies.

Despite these accomplishments, the prototype systems initially developed had limitations. The spin wave signals could not effectively reset the magnetic bits to allow the overwriting of existing data. This impediment tempered the enthusiasm surrounding the technology, emphasizing the need for further exploration and refinement to realize the full potential of spin-wave-based data encoding. However, the researchers remained undeterred, driven by curiosity and scientific inquiry to push the boundaries of what was possible with their findings.

The collaborative efforts between Grundler’s lab at EPFL and researchers at Beihang University in China led to a pivotal discovery: the exceptional properties of hematite, an iron oxide compound. This material is both earth-abundant and environmentally friendly, offering a sustainable alternative to materials traditionally employed in spintronics, such as yttrium iron garnet. The research team detailed their findings in a publication within "Nature Physics," shedding light on hematite’s capabilities, which extend beyond sustainability to the realms of high-frequency signal processing.

The unexpected nature of this discovery unfolded through the keen observations of EPFL alumnus Haiming Yu, currently a professor at the Fert Beijing Institute. Yu identified unusual electrical signals emanating from a nanostructured platinum stripe located on hematite. The peculiarities of these signals hinted at phenomena not previously documented in conventional magnetic materials, prompting Yu’s team to engage the expertise of Grundler’s group for further analysis. Such interdisciplinary collaboration illustrates the potency of collective scientific effort in unraveling the underlying principles governing new physical phenomena.

During the subsequent examination, Grundler’s team made an astute observation that would alter the trajectory of their research. They noted a distinct ‘wiggle’ in the spatial distribution of magnon signals. This observation served as a catalyst for the discovery of interference patterns between two separate excitation modes of spin waves, or magnon modes. The research conducted by EPFL PhD student Anna Duvakina utilized light scattering microscopy to discern that the strange signals correlated with these interference patterns. This critical turning point instigated a deeper understanding of magnon behavior within the hematite matrix.

The significance of having two magnon modes cannot be overstated; it enables spin currents to be manipulated more flexibly. This capability implies that devices could potentially switch back and forth between different polarizations while simultaneously controlling magnetization states of nanomagnets. The ability to dynamically reconfigure magnetic states paves the way for advanced data encoding and storage methodologies, allowing for unprecedented scalability and efficiency in information systems. It signals a step toward overcoming the challenges posed by existing methodologies in data management.

As the research elucidates, hematite’s magnetic properties, long deemed insufficient for practical applications, are now showcased as fundamentally advantageous in cutting-edge contexts. Its performance surpasses that of traditional materials optimized decades ago for microwave electronics. This revelation epitomizes the unpredictable nature of scientific inquiry, where established notions can be challenged and overturned by new insights. With hematite, the researchers can now present a material that is both sustainable and functional in the ever-evolving landscape of spintronics.

The implications of this development are far-reaching, as researchers contemplate the future of next-generation devices. These insights not only elevate the material’s significance but also hint at broader applications in advanced computing technologies. As the field of spintronics continues to mature, hematite stands as a symbol of innovation rooted in both practical application and environmental consciousness. The amalgamation of sustainability and performance in material science has never been more critical.

With this foundation laid, the next phase of research will involve the construction of nanomagnets onto hematite devices, thus testing the theoretical models proposed based on this intriguing interaction of magnon modes. The anticipation surrounding this next step encapsulates the thrill of scientific discovery—an endless pursuit fueled by curiosity, innovation, and the desire to uncover solutions to complex challenges. As the researchers embark on this phase, the scientific community watches with keen interest, eager for the advancements that lie ahead in the field of magnon-based computation.

The emergence of spin-wave computing is not merely a scientific evolution; it embodies a transformation with the potential to redefine the technological underpinnings of society. Researchers are optimistic that the advancements born from this wellspring of inquiry may lead to efficient and responsible methods of data encoding and storage, heralding a new era in computational capabilities. As they forge ahead, the collaborative spirit, resilience, and ingenuity of the scientific community remain at the forefront, ensuring that the future of technology is bright and vibrant.

Subject of Research: Spin wave-based computation using hematite
Article Title: Control of spin currents by magnon interference in a canted antiferromagnet
News Publication Date: 23-Apr-2025
Web References: Nature Physics
References: 10.1038/s41567-025-02819-7
Image Credits: © Anna Duvakina/LMGN EPFL

Keywords