Spin excitations and spin wave gap in the ferromagnetic Weyl semimetal Co3Sn2S2

Abstract

We report a comprehensive neutron scattering study on the spin excitations in the magnetic Weyl semimetal Co ₃Sn ₂S ₂ with a quasi-two-dimensional structure. Both in-plane and out-of-plane dispersions of the spin waves were revealed in the ferromagnetic state. Similarly, dispersive but damped spin excitations were found in the paramagnetic state. The effective exchange interactions were estimated using a semi-classical Heisenberg model to consistently reproduce the experimental T _C and spin stiffness. However, a full spin wave gap below E _g = 2.3 meV was observed at T = 4 K. This value was considerably larger than the estimated magnetic anisotropy energy (~0.6 meV), and its temperature dependence indicated a significant contribution from the Weyl fermions. These results suggest that Co ₃Sn ₂S ₂ is a three-dimensional correlated system with a large spin stiffness, and the low-energy spin dynamics can interplay with the topological electron states.