In magnetic Weyl semimetals, non-trivial topological properties emerge below the magnetic transition, offering a potential way to control topology through temperature or, more intriguingly, via magnetic order if it can be tuned by external parameters. The kagome lattice (figure (a)) is famous for its frustration giving rise to complex magnetic orderings, and possibly quantum spin liquids, in case of antiferromagnetic interactions between local moments. Although frustration is relieved by ferromagnetic (FM) interactions, local anisotropy or competing antiferromagnetic interactions may lead to many different magnetic states. The umbrella ones, which do not break the kagome symmetry, are sketched in the left part of figure (c). They encompass simple ferromagnetic order and in-plane non-collinear antiferromagnetic (AF) order, for θ=0 and θ=90°, respectively. A coexistence of these two orders was proposed in Co3Sn2S2 by a µSR study, with a growing proportion of the AF phase above 90 K, but not confirmed by neutron experiments. On the other hand, many anomalies appearing above T~130 K were attributed to changes in the domain wall structure. This unclear situation made it difficult to foresee the possible impact of a change in magnetic order on the topological properties.
In this study, researchers at LPS used ⁵⁹Co NMR to probe the magnetic order at a local scale. The Zero-Field NMR (ZF-NMR) lineshapes are highly sensitive to the orientation of local electronic moments at the three inequivalent Co sites in the kagome lattice. Accurate simulations of these lineshapes require precise knowledge of the magnetic and electric tensors that describe the interaction between nuclear spins and their environment. To determine these tensors, the researchers applied an original refined fitting approach in the paramagnetic phase, utilizing a genetic algorithm known as differential evolution to achieve high-precision results.
This analysis enabled the researchers to simulate the ZF-NMR spectra in the magnetic phase, shown in the figure (b), by assuming a set of internal magnetic fields (Bint) at the three Co sites. Here, θ represents the angle relative to the c-axis, and Bint is proportional to the electronic moment through a hyperfine tensor. Their findings reveal that below 90 K, the spectra remain symmetric, indicating that the three Co sites are equivalent. However, above 90 K, a shoulder appears on the low-frequency side, signaling an increasing inequivalence among the three Co sites. Crucially, this spectral evolution contradicts the coexistence of two distinct magnetic orders proposed by µSR and instead suggests a smooth evolution of the magnetic order. Additionally, this effect cannot be attributed to domain walls, as NMR would be insensitive to them in this case. The best-fit simulations were obtained by introducing an additional in-plane magnetic component (Bbr) to the low-temperature field (BLT). This extra in-plane component above 90K tilts the internal magnetic field (Bint) in a specific direction, breaking the equivalence of the three Co sites in a nematic-like manner. Near TC=172 К, Bbr reaches up to 1 T, though its precise orientation from one triangle to the next remains unknown. Interestingly, this new magnetic order breaks the mirror plane symmetry responsible for the formation of Weyl points. The origin of this symmetry breaking, its impact on magnetic behavior (such as domain wall motion), and its consequences for topology remain open questions. Since this evolution appears linked to the reduction of the ferromagnetic moment near TC, they could initiate further investigations in doped phases, where TC is reduced. These results, along with further discussion, have been published in Physical Review B
Contributors :
Team Spectroscopies of Quantum Materials
And SPEC\CEA Saclay
Reference : “NMR study of the local magnetic order in the kagome Weyl semimetal Co3Sn2S2”, I. Mukhamedshin, P. Wzietek, F. Bert, P. Mendels, A. Forget, D. Colson, V. Brouet, Phys. Rev. B 111, 045122 (2025)