Massless Dirac magnons in the two dimensional van der Waals honeycomb magnet CrCl₃

Two dimensional van der Waals ferromagnets with honeycomb structures are expected to host the bosonic version of Dirac particles in their magnon excitation spectra. Using inelastic neutron scattering, we study spin wave excitations in polycrystalline CrCl3, which exhibits ferromagnetic honeycomb layers with antiferromagnetic stackings along the c-axis. For comparison, polycrystal samples of CrI3 with different grain sizes are also studied. We find that the powder-averaged spin wave spectrum of CrCl3 at T = 2 K can be adequately explained by the two dimensional spin Hamiltonian including in-plane Heisenberg exchanges only. The observed excitation does not exhibit noticeable broadening in energy, which is in remarkable contrast to the substantial broadening observed in CrI3. Based on these results, we conclude that the ferromagnetic phase of CrCl3 hosts massless Dirac magnons and is thus not topological.