  CrI3 is a prototypical van der Waals ferromagnet with a magnetic honeycomb
lattice. Previous inelastic neutron scattering studies have suggested
topological nature of its magnetic excitations with a magnon gap at the Dirac
points, which are anticipated to give rise to magnon thermal Hall effect. Here
we report thermal transport properties of CrI3 and show that the long-sought
thermal Hall signal anticipated for topological magnons is fairly small. In
contrast, we find that CrI3 exhibits an appreciable anomalous thermal Hall
signal at lower temperature which may arise from magnon-phonon hybridization or
magnon-phonon scattering. These findings are anticipated to stimulate further
neutron scattering studies on CrI3 single crystal, which can shed light not
only on the intrinsic nature of magnetic excitations but also on the
magnon-phonon interaction.
