Spins Embrace a Twisted Path in a Kagome Metal
11 May 2026, Lynn
In the kagome metal CrRhAs, chromium spins arrange in a noncollinear spiral pattern, revealing unexpected ferromagnetic coupling between second-nearest neighbors.
Now here’s a discovery that’s pretty twisted. A team led by Peng Cheng at Renmin University of China has mapped the magnetic order of a kagome‑lattice material for the first time — and found that the chromium spins refuse to settle into any single direction. Instead, they arrange themselves in a noncollinear pattern, spiralling in ways that earlier theoretical calculations had not predicted. The work appears in a preprint (arXiv:2605.07540).
By this point, “noncollinear antiferromagnet” might sound like a term only a specialist would love. But it is really a snapshot of how atoms negotiate when their magnetic moments cannot all point the same way. Imagine a playground where every child has a different idea of which way to face; the result is not a neat alignment, but a compromise that stitches together many orientations in a repeating, orderly cell. That compromise, in a crystal, can generate exotic electronic behaviour and hint at strong electron‑electron interactions.
Kagome lattices — patterns of corner‑sharing triangles — are famous for frustrating magnetic order. When a magnetic ion sits at each vertex, it feels competing pulls from its neighbours, often preventing a simple ferromagnetic or antiferromagnetic arrangement. The new study zeroes in on CrRhAs, a compound in which chromium atoms occupy a distorted kagome net. Previous density‑functional‑theory calculations had suggested that the second‑nearest‑neighbour coupling in the kagome plane would be antiferromagnetic. The experiment, however, shows that it is actually ferromagnetic — a difference that reshapes the whole magnetic landscape.
To see the spins, Chenglin Shang and colleagues used powder neutron diffraction. As the authors write, “CrRhAs is an antiferromagnet with TN = 149 K.” That Néel temperature, about twice the chill of liquid nitrogen, is high enough to make the material a robust platform for studying correlations. Below that temperature, the neutron data reveal a magnetic structure with a propagation vector that encodes a larger repeating cell — one that spreads further in the plane and along the stacking axis than the chemical unit cell alone would dictate. The low‑temperature magnetic structure, settled through a careful Rietveld refinement, is a noncollinear arrangement where three inequivalent chromium positions carry moments canted in different directions.
Think of it like a piece of fabric that, when stretched unevenly, causes the threads to weave a new pattern. The applied distortion of the kagome lattice breaks the perfect triangular symmetry, giving certain interactions the upper hand. When the plane is punctured by a longer bond, you can think of the magnetic coupling splitting into distinct channels — each favoring a different tilt angle — so that the spins end up pointing in a mosaic of orientations rather than a single north. This is not a random jumble; it is a precise, recurring state that the team was able to model using a magnetic space group.
Magnetism, however, is only one part of the picture. The electrical transport properties of CrRhAs are even more telling. The resistivity shows a curious crossover: above the Néel temperature it is semiconducting‑like, but once the moments order, it turns metallic. That switch tells us that spin scattering is suppressing conduction in the paramagnetic phase, suggesting strong spin fluctuations that persist well above the ordering transition. Then there is the Hall coefficient. Rather than staying positive or negative, it flips sign near 70 K and again at room temperature. Two sign reversals are a clear signature of multiband effects — different families of charge carriers, electrons and holes, take turns dominating the current as the temperature changes.
The team dug deeper into the material’s heavy‑fermion‑like character using heat‑capacity measurements. The extracted linear term in the specific heat points to a large electronic density of states, and when combined with the resistivity data, it yields a Kadowaki‑Woods ratio. That indicator — a widely used barometer for electron‑electron correlations — comes out at about 34 µΩ·cm·mol²·K²/J². For comparison, ordinary transition metals like iron or nickel have values near 0.4, while strongly correlated systems can reach figures like this. The result places CrRhAs squarely in the camp of kagome metals where electron interactions are not a small perturbation but a central driver of the observed behaviour.
The finding that the second‑nearest‑neighbour exchange is ferromagnetic, contrary to theory, is a reminder that even well‑established computational methods can miss crucial details when a lattice is distorted and correlations are strong. It invites theorists to revisit the electronic structure of this family of compounds and to explore whether the anomalous transport — the semiconducting‑to‑metallic crossover, the double sign change in the Hall coefficient — can be explained by the same multiband interplay. The paper does not claim that CrRhAs is a topological semimetal or a superconductor, but it does map out a rich phase of strongly correlated magnetism that many condensed‑matter physicists are eager to understand.
What makes this study stand out is its combination of structural, magnetic, and transport probes on a single compound. It is not enough to know that a material has a kagome lattice; one must experimentally determine how the spins order and how they influence the electrons. The Renmin team has done that with precision, and in doing so they have provided a textbook example of how a distorted geometry can coax out a noncollinear magnetic ground state. The work also underscores the growing interest in kagome metals as a stage where topology, magnetism, and strong correlations can intersect — an intersection that, in other materials, has yielded phenomena such as anomalous Hall effects and charge‑density waves.
For the broader community, the take‑home is this: when a lattice deviates from perfect symmetry, the magnetic response can be far richer than simple antiferromagnetism or ferromagnetism. The noncollinear order in CrRhAs is not just a curiosity; it is a concrete snapshot of the compromises that frustrated interactions demand. As more distorted‑kagome compounds are synthesized and characterized, we may find that such twisted spin textures are more common than they appear. The Renmin team’s careful experiment on CrRhAs lights a path forward.
Lynn is an online editor of LoomSci
References
- Chenglin Shang et al., Noncollinear antiferromagnetic structure and physical properties of CrRhAs with distorted kagome lattice, arXiv:2605.07540