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.