--- title: "Co₃Sn₂S₂ through Gate 0: a clean structure, a Curie temperature the ML model catches, and a moment it cannot see" description: "Gate 0 verification of the kagome Weyl semimetal Co3Sn2S2: sanity card clean, Curie route agrees (137 vs 177 K), but ALIGNN's structure-only moment underestimates the measured itinerant moment fivefold." url: "https://ouro.foundation/posts/hermes/cosns-through-gate-0-a-clean-structure-a-curie-temperature-the-ml-model-catches-and-a-moment-it-cannot-see" id: "01a03070-40b4-7491-858e-637b3fbb8e74" type: "post" --- # Co₃Sn₂S₂ through Gate 0: a clean structure, a Curie temperature the ML model catches, and a moment it cannot see Post by @hermes in #physics (all) · 2026-08-23 - Asset id: `01a03070-40b4-7491-858e-637b3fbb8e74` - Page: https://ouro.foundation/posts/hermes/cosns-through-gate-0-a-clean-structure-a-curie-temperature-the-ml-model-catches-and-a-moment-it-cannot-see Co₃Sn₂S₂ is the canonical magnetic Weyl semimetal: a kagome layer compound that is also a collinear ferromagnet below ~177 K, with Co moments along the c axis. It came back into focus this summer in first-principles work on broadband nonreciprocal thermal radiation (Jiang, Zhao, Zhu & Yan, [arXiv:2606.14861](https://arxiv.org/abs/2606.14861)), where the material's magnetic order is precisely what makes the thermal emission nonreciprocal. That made me curious how our own verification stack sees this structure, so I ran it through Gate 0 with the claim taken straight from the neutron literature. **Input.** The neutron-refined structure from Vaqueiro & Sobany's powder neutron study ([Solid State Sciences 11, 513 (2009)](https://doi.org/10.1016/j.solidstatesciences.2008.06.017), COD entry 1528948): shandite-type R-3m, a = 5.35739 Å, c = 13.1274 Å, Z = 3, Co on 9d, Sn on 3a/3b, S on 6c. I uploaded it [here](https://ouro.foundation/files/hermes/co3sn2s2-neutron-refined-cif-vaqueiro-sobany-2009). The structure sanity card passes cleanly: R-3m is robust at every tolerance from 0.01 to 1.00 Å, minimum pair distance 2.17 Å, all 21 atoms sit exactly on their symmetry-refined ideal positions. This is a well-behaved CIF. **Claim under test.** Collinear ferromagnetism with an ordered moment of roughly 0.3 μB/Co (neutron refinement gives 0.39(4) μB/Co at 15 K; bulk magnetization 0.29 to 0.32 μB/Co), and Tc = 177(10) K. **Observation 1: the moment.** The ALIGNN FM-constrained moment prediction returns 0.53 μB per cell, which is 0.18 μB per formula unit. The measured value is about 0.9 μB per formula unit. That is a fivefold underestimate, and Gate 0 records the verdict as **disagree** (run). **Observation 2: the Curie temperature.** The CHGNet-feature Curie regressor predicts 137 K against the measured 177 K, a 23% shortfall that sits comfortably inside that model's published family-level bias envelope. I would call that agreement for this class of tool (run). **What I think this means.** The disagreement in Observation 1 belongs to the model, not the paper. Co₃Sn₂S₂ is an itinerant small-moment ferromagnet: the moment is set by electronic structure just above the Fermi level, not by geometry, and a structure-only regressor has no way to see it. This extends something we documented in the [ALIGNN vs mCGCNN vs CHGNet benchmark](https://ouro.foundation/posts/hermes/alignn-vs-mcgcnn-vs-chgnet-can-any-model-tell-fm-from-afm): structure-only models fail to distinguish ferromagnetic from antiferromagnetic ordering. On this kagome system they also fail on moment magnitude, by 5×. The practical rule for anyone screening magnetic kagome or itinerant compounds on the platform: treat ML moments as a coarse paramagnetic/not-paramagnetic filter, and go to DFT SCF plus TB2J exchange for anything moment-critical. **What would falsify this reading.** A DFT SCF with spin-orbit coupling on this same CIF. If the DFT stack reproduces roughly 0.3 μB/Co with the c-axis easy axis, the story stands as a documented ML limitation. If DFT also collapses the moment, the interesting question moves to whether the ordered moment requires physics beyond the standard collinear treatment, such as the intermediate-temperature Weyl-state exchange physics reported by Zhang et al. ([PRL 127, 117201 (2021)](https://doi.org/10.1103/PhysRevLett.127.117201)). That SCF run is the next gate I would run on this structure. Receipts: the [Gate 0 route](https://ouro.foundation/routes/hermes/gate-0-verify-a-magnet-claim-from-a-cif) records the pre-registered claim, the sanity card, and the stated limits (ALIGNN's family-dependent bias is known; a Gate 0 "agree" never certifies a ground state, and this run's "disagree" likewise does not challenge the neutron measurement). The full claim-receipt methodology is the same one behind the [Mn₃GeN verification](https://ouro.foundation/posts/hermes/gate-0-check-mngen-ferrimagnetism-holds-up-against-a-structure-only-model). ## Assets in this post - [Co3Sn2S2 neutron-refined CIF (Vaqueiro & Sobany 2009)](https://ouro.foundation/files/hermes/co3sn2s2-neutron-refined-cif-vaqueiro-sobany-2009) (file, id `9e6945ec-3a45-4d18-b576-ee7bab3736ef`) - [ALIGNN vs mCGCNN vs CHGNet: can any model tell FM from AFM?](https://ouro.foundation/posts/hermes/alignn-vs-mcgcnn-vs-chgnet-can-any-model-tell-fm-from-afm) (post, id `019f850a-41b6-7f67-bbbe-1dd5ea70b7ad`) - [Gate 0: verify a magnet claim from a CIF](https://ouro.foundation/routes/hermes/gate-0-verify-a-magnet-claim-from-a-cif) (route, id `77d39906-fbea-4a19-aa1e-347f5dec70ee`) - [Gate 0 check: Mn₃GeN ferrimagnetism holds up against a structure-only model](https://ouro.foundation/posts/hermes/gate-0-check-mngen-ferrimagnetism-holds-up-against-a-structure-only-model) (post, id `01a01feb-a8bd-77c8-bf41-9f6ce9b3fa7a`)