A team led by Qihang Liu and Xiaobing Chen of the State Key Laboratory of Quantum Functional Materials and the Department of Physics at Southern University of Science and Technology, together with the Quantum Science Center of the Guangdong–Hong Kong–Macao Greater Bay Area, has now turned that exercise into an automated workflow. Their open-source online platform, called FINDSPINGROUP, is reported in National Science Review. The tool places magnetic-moment arrangements and spin-orbit effects within a single, material-specific symmetry framework, allowing researchers to ask, in a consistent way, which responses a material’s magnetic geometry permits, and which additional responses emerge once spin-orbit coupling is switched on. The distinction is not academic. Knowing which mechanism drives a given response determines how that response will behave when the magnetic order is rotated, switched, or embedded in a different crystal environment, and it guides which materials deserve the expense of demanding calculations and experiments.

The conceptual heart of the platform is a reconciliation of two symmetry descriptions that have historically lived in separate regimes. In the absence of spin-orbit coupling, a magnetic material is described by a spin space group, in which operations acting on spins are not required to follow the corresponding rotations of the crystal lattice. Once spin-orbit coupling locks spin to lattice, the appropriate description becomes the magnetic space group, in which spin and lattice operations move together. Each map works well within its own regime, but the two are not automatically aligned. A spin space group is unchanged when the entire spin frame is rotated, so on its own it does not specify the physical orientation of spin operations relative to the crystal. A magnetic space group, by contrast, already assumes that this relationship is fixed. Different choices of unit cell, basis vectors, and origin add further complications, so comparing the two descriptions for a real material has meant identifying and realigning them by hand.

Liu and colleagues resolved this with the oriented spin space group, or OSSG. The OSSG selects the material-specific orientation of the spin operations relative to crystallographic directions, without itself introducing spin-orbit coupling. From this common starting point, two analyses branch out. The full OSSG provides symmetry constraints for the material without spin-orbit coupling, while the subset of operations that remain compatible with spin-lattice locking forms the corresponding magnetic space group and supplies the constraints that apply when spin-orbit coupling is included. Both branches generate crystallographic information and restrictions on magnetic and transport properties, and because both are expressed in the same coordinate frame, their predictions can be compared directly. “The important question is not only what symmetry a magnetic material has, but where each of its allowed responses comes from,” said Liu. “By placing magnetic geometry and spin-orbit effects in the same material-specific frame, the OSSG allows the two contributions to be compared directly.”

FINDSPINGROUP turns this framework into a practical pipeline. A user supplies a magnetic structure in CIF, MCIF, or the newly introduced SCIF format. The platform identifies the material-specific OSSG, converts the result to a standard crystallographic setting, matches it to a canonical database representative, and derives the corresponding magnetic space group in the same coordinate frame. It then interrogates the material with what amounts to two parallel questions: what does the magnetic geometry permit without spin-orbit coupling, and what additional responses become possible, or disappear, once spin-orbit coupling is included. To answer them, the software produces standard magnetic cells, spin Wyckoff positions, spin site-symmetry groups, spin Brillouin zones, and symmetry information at high-symmetry wave vectors. It evaluates whether magnetization, electric polarization, momentum-dependent spin polarization, spin texture, anomalous Hall conductivity, and nonlinear-response tensors are allowed in each regime.

Crucially, the platform returns symmetry constraints rather than numerical predictions. It does not calculate how large a Hall current or an electric polarization will be. Instead, it determines whether a response is symmetry-possible, which directions or tensor components are allowed, and whether its origin can be traced to magnetic geometry or to spin-orbit coupling. That separation of powers is what makes the tool useful as a screening instrument: it can eliminate candidates whose symmetry forbids a desired response before anyone invests in first-principles calculations or crystal growth, and it can flag candidates whose symmetry guarantees one. Combined with an online service, open-source code, and a searchable database, the workflow scales from the analysis of a single material to high-throughput searches across entire structural families.

The team demonstrated the approach on three materials, each posing a different puzzle. The first concerns spin splitting: does it always require spin-orbit coupling? The two-dimensional altermagnet V2Se2O shows that it does not. In this material, the magnetic geometry alone already permits momentum-dependent spin splitting of the electronic bands, a property once considered inseparable from spin-orbit physics. FINDSPINGROUP then tracks what changes when spin-orbit coupling is added: the symmetry is lowered, the allowed components of spin polarization change, and an additional magnetic response becomes permitted. The result is a complete symmetry account of how a purely geometric effect and a relativistic effect layer on top of one another in the same compound.

The second puzzle asks whether magnetic order can create electric polarization. In MnSe2, the arrangement of magnetic moments breaks the inversion symmetry of the underlying non-magnetic crystal, allowing an electric polarization to emerge. The same magnetic order also permits momentum-dependent spin splitting. Because multiple symmetry-related magnetic states satisfy the same constraints, FINDSPINGROUP identifies candidate switching endpoints, revealing whether the electric polarization and the spin-dependent responses can be reversed separately or must reverse together. That question bears directly on device design, since independent control of distinct order parameters is far more valuable than a single locked switch. The third puzzle concerns the anomalous Hall effect in the non-coplanar antiferromagnet CoNb3S6. There, the magnetic geometry permits a Hall response even without spin-orbit coupling, while forbidding spin polarization throughout the Brillouin zone. When spin-orbit coupling is included, additional spin polarization and magnetization become symmetry-allowed. Together, the three cases demonstrate why observing a ferromagnetic-like response does not, by itself, reveal its origin: superficially similar effects can obey different symmetry rules and respond differently to external control.

Reproducibility posed its own challenge, and the team addressed it with a new data standard. The spin crystallographic information file, or SCIF, extends the familiar CIF and MCIF formats by storing the crystal structure together with its spin-group assignment, symmetry operations, coordinate transformations, and metadata. Like its predecessors, SCIF is designed to make symmetry information portable between researchers, software packages, and databases. The team is working with the IUCr Commission on Magnetic Structures to develop SCIF as a community standard for exchanging spin-group information, a step that could spare future researchers the ambiguity of unit-cell choices and spin-frame conventions that has long complicated comparisons between published analyses.

FINDSPINGROUP is available as a web service at findspingroup.com, with source code hosted on GitHub, lowering the barrier for groups that lack specialized symmetry expertise. For the broader field of unconventional magnetism, the platform arrives at a moment of rapid expansion, as altermagnets, non-coplanar antiferromagnets, and magnetism-driven ferroelectrics compete for attention as next-generation spintronic materials. By making the paired symmetry analysis fast, standardized, and shareable, the tool shifts the field’s focus from cataloging exotic responses to classifying their mechanisms, a shift that could accelerate the search for materials whose magnetic geometry and spin-orbit physics can be controlled independently, and ultimately for devices that exploit each contribution on its own terms.

Subject of Research: Symmetry-based separation of magnetic-geometry and spin-orbit-coupling contributions to unconventional magnetic properties using oriented spin space groups

Article Title: How can scientists disentangle the origins of unconventional magnetic properties?

Article References: How can scientists disentangle the origins of unconventional magnetic properties?. (n.d.). Original publication

Image Credits: AI Generated

DOI: Not provided

Keywords: antiferromagnets, spin space group, oriented spin space group, magnetic space group, spin-orbit coupling, altermagnets, anomalous Hall effect, spin splitting, electric polarization, SCIF format, symmetry analysis, FINDSPINGROUP

The material at the heart of the study is chromium thiophosphate, CrPS4, an air-stable, layered magnetic semiconductor that has become a workhorse of two-dimensional magnetism research. On its own, CrPS4 behaves conventionally depending on thickness: few-layer samples can show ferromagnetic order, while bulk crystals adopt an antiferromagnetic arrangement. Neither of these ground states, however, exhibits the hallmark of altermagnetism, which is a finite splitting between electronic states of opposite spin that does not arise from spin-orbit coupling but instead from the symmetry of the crystal and its magnetic arrangement. The researchers reasoned that if two CrPS4 flakes were stacked with their crystal axes rotated by ninety degrees, the interlayer symmetry would be altered in precisely the way theory predicts should generate an altermagnetic state.

The idea traces back to theoretical work suggesting that twisted magnetic van der Waals bilayers constitute an ideal platform for altermagnetism. In an ordinary antiferromagnet, time-reversal symmetry combined with a lattice translation protects the degeneracy of spin-up and spin-down bands, so the electronic structure shows no net spin splitting. When layers are rotated relative to one another, that combined symmetry operation is broken, and the spin degeneracy is lifted in a momentum-dependent fashion. The result is a material whose spins cancel macroscopically, avoiding the stray fields that plague ferromagnetic devices, yet whose electronic bands are split in a way that can carry spin-polarized currents. For spintronics, this combination is extraordinarily attractive: fast switching, dense packing, and robust spin information without the crosstalk that limits conventional magnetic memory.

To test the prediction, the team fabricated orthogonally twisted CrPS4/CrPS4 homostructures, stacking four-layer flakes rotated by ninety degrees relative to each other. They then interrogated the samples with a battery of optical probes, beginning with polarized Raman spectroscopy. In the twisted configuration, the Raman spectra revealed a splitting of phonon modes that is absent in untwisted controls. This vibrational fingerprint, the researchers show, arises from the interlayer coupling unique to the altermagnetic arrangement: the same symmetry breaking that splits the electronic bands also modifies the lattice dynamics, producing a spectroscopic signature that distinguishes the twisted state from any simple superposition of ferromagnetic and antiferromagnetic layers.

The more decisive evidence came from magneto-optical spectroscopy. Measuring the photoluminescence of the twisted homostructure under an applied magnetic field, the researchers tracked the degree of circular polarization of the emitted light. The twisted sample displayed a magnetic-field dependence that resembles the response of a ferromagnet, a striking result given that the underlying spin order is compensated. Even more telling was the observation of a pronounced Zeeman splitting in the photoluminescence spectra, with the sigma-plus and sigma-minus emission peaks separating as the field increased. Crucially, neither ferromagnetic nor antiferromagnetic CrPS4 samples show this splitting in the same configuration. Its emergence only in the twisted geometry indicates that the stack has entered a genuinely distinct magnetic ground state, one that carries the fingerprints of altermagnetism.

First-principles calculations reinforced the interpretation. Using density functional theory with an on-site Coulomb correction to treat the correlated chromium d electrons, the team computed the electronic structure of the orthogonally twisted bilayer and found large spin-split bands in an antiferromagnetic configuration. The pattern of the splitting matches the theoretical expectations for altermagnets: opposite-spin bands separate in momentum space in a way dictated by crystal rotation symmetry, while the net magnetization remains zero. The agreement between the computed band structure and the optical measurements provides a coherent picture in which twisting acts as a symmetry-breaking knob that switches on spin splitting without introducing any net magnetic moment.

The significance of the result extends beyond confirming a theoretical prediction. Van der Waals homostructures, in which the same material is stacked with controlled twist angles, have already revolutionized research on graphene and transition metal dichalcogenides, giving rise to moiré physics, flat bands, and correlated electronic phases. The present work shows that the same stacking engineering can serve as a design principle for magnetism itself. Rather than searching for new chemical compounds that happen to be altermagnets, researchers can now, in principle, manufacture altermagnetic states from well-characterized magnetic layers by choosing the twist angle. Because the twist angle is a continuously tunable parameter, it opens a route to systematically exploring how altermagnetic spin splitting evolves with interlayer symmetry, something no fixed crystal structure can offer.

The experimental signatures reported here also add to a growing toolbox for identifying altermagnets. Earlier confirmations of altermagnetic band splitting relied on techniques such as spin-resolved and angle-resolved photoemission spectroscopy, x-ray magnetic circular dichroism, and magneto-optical Kerr effect measurements in compounds like CrSb, MnTe, and RuO2. The CrPS4 study demonstrates that circularly polarized photoluminescence and Raman spectroscopy, both accessible table-top optical methods, can detect altermagnetism in atomically thin devices. This accessibility matters: optical probes can be applied to microscopic samples inside cryostats, under magnetic fields, and across device geometries, accelerating the pace at which candidate altermagnetic structures can be screened and characterized.

For applications, the appeal of altermagnets lies in their potential to combine the speed and stability of antiferromagnetic spintronics with the readout convenience of ferromagnets. Antiferromagnetic memory elements are immune to external fields and can in principle switch at terahertz frequencies, but their vanishing net moment makes them hard to read. Altermagnets solve the readout problem because their spin-split bands allow spin-polarized transport and magneto-optical signals even without net magnetization. A twisted van der Waals altermagnet adds another dimension: the state exists in an atomically thin, air-stable semiconductor that can be integrated into heterostructures with other two-dimensional materials, potentially enabling spin filters, tunnel junctions, and valleytronic devices in which spin and momentum are locked by design.

Challenges remain before such devices materialize. The reported signatures are spectroscopic rather than transport-based, and future work will need to demonstrate electrical readout and manipulation of the altermagnetic state, quantify the magnitude and temperature stability of the spin splitting, and explore how different twist angles and layer numbers tune the effect. Still, the demonstration that a simple ninety-degree rotation of identical CrPS4 flakes produces a magnetic phase absent from either constituent marks a conceptual milestone. It establishes twisted van der Waals homostructures as an experimental platform for altermagnetism, transforming a theoretical proposal into a tangible, measurable state of matter and handing the spintronics community a new material class to engineer.

Subject of Research: Experimental observation of altermagnetism in orthogonally twisted CrPS4 van der Waals homostructures

Article Title: Altermagnetism in twisted van der Waals homostructures

Article References: Chen, J., Xie, X., Li, S., Zhang, S., Hou, S., Zhang, X., He, J., Liu, Z., Wang, J.-T., & Liu, Y. (2026). Altermagnetism in twisted van der Waals homostructures. Nature Physics. https://doi.org/10.1038/s41567-026-03440-y

Image Credits: AI Generated

DOI: 10.1038/s41567-026-03440-y

Keywords: altermagnetism, CrPS4, van der Waals heterostructures, twisted bilayers, spintronics, magneto-optical spectroscopy, Zeeman splitting, Raman spectroscopy, 2D magnets, spin splitting, antiferromagnetism, first-principles calculations