Spin Textures Visualised Reveal New Magnetic Order

Researchers are increasingly focused on altermagnetism, a novel magnetic order characterised by zero net magnetisation yet exhibiting significant momentum-dependent spin splitting, and its potential for next-generation spintronics. Jiayu Liu from the Shanghai Institute of Microsystem and Information Technology, Chinese Academy of Sciences, Xun Ma, and Xinnuo Zhang from the National Synchrotron Radiation Laboratory and School of Nuclear Science and Technology, University of Science and Technology of China, alongside Wenchuan Jing, Zhengtai Liu from the University of Chinese Academy of Sciences, and Dawei Shen, working in collaboration with colleagues at all four institutions, demonstrate the crucial role of angle-resolved photoemission spectroscopy in directly observing the non-relativistic band splitting and spin textures inherent to altermagnets. This work systematically reviews key altermagnetic materials, including RuO2, KV2Se2O, Rb1-delta V2Te2O, MnTe, CrSb, and MnTe2, and establishes ARPES as a vital technique for understanding the symmetry-driven origins of spin polarisation, promising advancements in both fundamental materials science and applied device technologies.

Scientists are unlocking new routes to control electron spin without needing conventional magnetism. This discovery challenges established ideas about magnetic materials and opens possibilities for more energy-efficient technologies. Advanced spectroscopic techniques are now directly revealing the subtle symmetries governing these novel electronic states.

Scientists are now directly visualizing a newly understood form of magnetism called altermagnetism, revealing its potential for next-generation spintronic devices. Altermagnetism combines characteristics of both ferromagnets and antiferromagnets, exhibiting strong spin polarization without generating a net magnetic field. This unique combination circumvents limitations of conventional magnetic materials and opens avenues for low-power electronics and quantum technologies.

Researchers have employed advanced angle-resolved photoemission spectroscopy (ARPES) techniques to observe the intricate spin textures and band structures within altermagnetic materials, confirming theoretical predictions about their symmetry-driven behaviour. This work highlights the crucial role of ARPES, including spin-resolved (SARPES) and circular-dichroism (CD-ARPES) variants, in directly mapping the nonrelativistic band splitting and spin arrangements present in these materials.

Within a sophisticated spin-group framework, the team distinguished altermagnetic order from conventional ferromagnetic and antiferromagnetic arrangements, pinpointing the symmetry origins of spin polarization. Future developments in spectroscopic techniques, coupled with materials engineering approaches like strain manipulation and heterostructure design, promise to further unlock the potential of altermagnetism.

These advances are expected to drive progress in both fundamental research into correlated quantum phenomena and the development of innovative spintronic applications. The ability to control and harness spin without generating unwanted magnetic fields represents a significant step towards more efficient and versatile electronic devices.

Mapping electronic structure via photoemission and spectral function analysis

Angle-resolved photoemission spectroscopy (ARPES) serves as the primary experimental technique employed in this work to investigate the electronic structure of materials exhibiting altermagnetism. ARPES directly maps the electronic energy-momentum dispersion relations by measuring the kinetic energy and emission angle of photoelectrons emitted from a sample following the photoelectric effect.

The modern interpretation of ARPES relies on a quantum mechanical one-step model, describing photoemission as a coherent transition from an initial Bloch wavefunction within the solid to a detectable free-electron final state in vacuum. The intensity of the measured photocurrent, I(k, E), is proportional to the material’s one-particle spectral function, A(k, E), which directly reflects the electron self-energy and its energy-momentum dispersion relation.

This proportionality allows for the direct determination of the electronic band structure, E(k). Crucially, the transition matrix element, Mf,i, governs the probability of this transition and is influenced by experimental parameters such as photon energy and polarization. Systematic variation of the photon energy, hν, is employed to probe the electron momentum perpendicular to the sample surface, k⊥, and to mitigate the effects of matrix element modulation.

Specific combinations of photon energy and momentum can suppress or enhance signals from certain bands, necessitating photon energy-dependent measurements to obtain a complete picture of the electronic structure. To further refine the analysis, spin-resolved ARPES (SARPES) was implemented, incorporating a spin detector to analyse the spin polarization of the emitted photoelectrons.

This technique provides direct access to the spin texture of the electronic bands. Complementing SARPES, circular dichroism ARPES (CD-ARPES) utilizes circularly polarized light to enhance sensitivity to spin-dependent features. These advanced ARPES variants, alongside conventional ARPES, provide band-resolved, spin-resolved, and symmetry-resolved evidence for unambiguous signatures of altermagnetism.

D-wave and g-wave characteristics define altermagnetic spin textures in layered and topological materials

The research demonstrates that ruthenium dioxide (RuO2) exhibits a characteristic d-wave spin-splitting pattern in its Brillouin zone, confirmed through detailed analysis of its symmetry group as [C2||C4zt]. Investigations into g-wave altermagnets, specifically hexagonal manganese telluride (MnTe) and chromium antimonide (CrSb), reveal nuanced spin textures. Domain-tunable MnTe showcases the ability to manipulate spin configurations, while the topological compound CrSb presents a unique platform for exploring the interplay between topology and altermagnetism.

Analysis of the noncoplanar antiferromagnet manganese ditelluride (MnTe2) confirms the emergence of altermagnetic-type spin splitting, extending the observed phenomenon to a broader range of magnetic orders. ARPES methodologies, including conventional ARPES, spin-resolved ARPES (SARPES), and circular dichroism ARPES (CD-ARPES), were instrumental in verifying these findings.

SARPES directly maps momentum-space spin textures, while CD-ARPES extracts symmetry-imposed orbital and angular-momentum textures, providing complementary insights into the electronic structure. These spectroscopic techniques reveal spin-split bands and the characteristic d-, g-, or i-wave angular patterns of spin splitting, offering band-resolved, spin-resolved, and symmetry-resolved evidence for altermagnetism. Recent advancements, such as soft X-ray ARPES for kz mapping and micro/nano-beam ARPES, mitigate artifacts and allow for domain-selective measurements, enhancing the precision of the study.

Visualising spin textures unlocks definitive proof of altermagnetism

Scientists are increasingly focused on altermagnetism, a fascinating state of matter that challenges conventional understandings of magnetism. For decades, materials science has largely operated within the framework of ferromagnetism and antiferromagnetism, but altermagnetism, possessing zero net magnetisation yet exhibiting strong spin-splitting, demands a reassessment of these established paradigms.

The difficulty lies in both identifying materials that host this behaviour and then definitively proving its existence, disentangling it from more familiar magnetic orders. The power of angle-resolved photoemission spectroscopy (ARPES) to directly visualise the subtle spin textures within these materials is proving transformative. It’s not simply about detecting a signal, but about mapping the intricate relationship between a material’s electronic structure and its spin properties.

This capability is crucial for moving beyond theoretical predictions and establishing a firm experimental foundation for altermagnetism. Recent work demonstrates electrical switching of these magnetic states and strain-induced control, hinting at potential applications in novel spintronic devices. However, the field remains nascent. Many candidate materials are still under investigation, and the precise interplay between symmetry, electronic structure, and magnetic order is often complex.

The emergence of p-wave magnetism, a particularly intriguing variant, raises questions about its coexistence with superconductivity and the potential for entirely new quantum phenomena. Future progress will likely depend on combining advanced spectroscopic techniques with materials engineering, growing heterostructures and applying controlled strain, to tailor altermagnetic properties and unlock their full technological potential.