  Topological magnon is a vibrant research field gaining more and more
attention in the past few years. Among many theoretical proposals and limited
experimental studies, ferromagnetic Kagome lattice emerges as one of the most
elucidating systems. Here we report neutron scattering studies of YMn6Sn6, a
metallic system consisting of ferromagnetic Kagome planes. This system
undergoes a commensurate-to-incommensurate antiferromagnetic phase transition
upon cooling with the incommensurability along the out-of-plane direction. We
observe magnon band gap opening at the symmetry-protected K points and ascribe
this feature to the antisymmetric Dzyaloshinskii-Moriya (DM) interactions. Our
observation supports the existence of topological Dirac magnons in both the
commensurate collinear and incommensurate coplanar magnetic orders, which is
further corroborated by symmetry analysis. This finding places YMn6Sn6 as a
promising candidate for room-temperature magnon spintronics applications.
