  Thermal transport is a pivotal probe for studying low-energy, charge-neutral
quasi-particles in insulating magnets. In this Letter, we report an observation
of large magneto-thermal conductivity and thermal Hall effect (THE) in a van
der Waals antiferromagnet FeCl2. The magneto-thermal conductivity reaches over
~700%, indicating strong magnon-phonon coupling. Furthermore, we find an
appreciable thermal Hall signal which changes sign concurrently with the
spin-flip transition from the antiferromagnetic state to the polarized
ferromagnetic state. Our theoretical calculations suggest that, in addition to
the Berry curvature induced at the anticrossing points of the hybridized magnon
and acoustic phonon modes of FeCl2, other mechanisms are needed to account for
the magnitude of the observed THE.
