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

TL;DR

This study demonstrates that the two-dimensional ferromagnetic honeycomb magnet CrCl3 hosts massless Dirac magnons, confirmed by inelastic neutron scattering, with a spectrum well-described by a simple in-plane Heisenberg model, contrasting with CrI3.

Contribution

First experimental evidence of massless Dirac magnons in a 2D van der Waals honeycomb magnet CrCl3 using neutron scattering.

Findings

CrCl3 exhibits a spin wave spectrum consistent with a 2D Heisenberg model.

CrCl3 hosts massless Dirac magnons, not topological.

CrI3 shows broadening in magnon spectra, unlike CrCl3.

Abstract

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 CrCl$_3$, which exhibits ferromagnetic honeycomb layers with antiferromagnetic stackings along the $c$-axis. For comparison, polycrystal samples of CrI$_3$ with different grain sizes are also studied. We find that the powder-averaged spin wave spectrum of CrCl$_3$ 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 CrI$_3$. Based on these results, we conclude that the ferromagnetic phase of CrCl$_3$ hosts massless Dirac magnons and is thus not topological.