John A. Schneeloch, Yu Tao, Yongqiang Cheng, Luke Daemen, Guangyong Xu, Qiang Zhang, Despina Louca
Bosonic Dirac materials are testbeds for dissipationless spin-based electronics. In the quasi two-dimensional (2D) honeycomb lattice of the halides CrX3 (X=Cl, Br, I), Dirac magnons have been predicted at the crossing of acoustic and optic spin waves. This results in an energy dispersion that resembles that of graphene, with Dirac magnon nodes at the K-point of the Brillouin zone (BZ) edge. The ground state is either ferromagnetic (FM), as in CrBr3 and CrI3 with an out-of-plane spin orientation, or antiferromagnetic (AFM) with an in-plane FM order that alternates in the perpendicular direction, as in the insulating CrCl3 . Here we show that, distinct from CrBr3 and CrI3, gapless Dirac magnons are present in bulk CrCl3, with the inelastic neutron scattering intensity approaching zero at the linear crossing of the magnon bands at the Dirac K point. Upon warming, magnon-magnon interactions with strong renormalization and decreased lifetimes, especially of the upper magnon band, are observed that destabilize the Dirac magnons. Moreover, spin-wave features are observed well above the magnetic transition, coinciding with spin-phonon coupling and a spin-induced negative thermal expansion, together indicating a higher onset temperature of the in-plane spin correlations.