Noncollinear kagome magnet driven by giant antisymmetric exchange

Kagome lattices host exotic phenomena such as quantum spin liquids, topological Weyl nodes, and flat bands. The kagome ferromagnet Co₃Sn₂S₂ has been a focus of debate due to conflicting reports on its magnetic character. Here, using weakly perturbative probes-primarily neutron scattering and Mössbauer spectroscopy-we determine its ground state and magnetic Hamiltonian. The spin wave spectrum is reproduced by a simple Hamiltonian dominated by a large in-plane nearest-neighbor Heisenberg interaction J and an antisymmetric Dzyaloshinskii-Moriya interaction that is roughly one third of J. This Hamiltonian yields a canted, noncollinear ferromagnetic ground state with an umbrella-type structure, which our data show to be the true ground state at all temperatures below T_C. Additionally, the spin susceptibility above T_C exhibits a reduced critical exponent, akin to other spin-anisotropic systems. Our findings explain the unconventional magnetism in Co₃Sn₂S₂, including the giant anomalous Hall effect and the origin of chiral phonons.