  Magnetic van der Waals (vdW) materials have opened new frontiers for
realizing novel many-body phenomena. Recently NiPS3 has received intense
interest since it hosts an excitonic quasiparticle whose properties appear to
be intimately linked to the magnetic state of the lattice. Despite extensive
studies, the electronic character, mobility, and magnetic interactions of the
exciton remain unresolved. Here we address these issues by measuring NiPS3 with
ultra-high energy resolution resonant inelastic x-ray scattering (RIXS). We
find that Hund's exchange interactions are primarily responsible for the energy
of formation of the exciton. Measuring the dispersion of the Hund's exciton
reveals that it propagates in a way that is analogous to a double-magnon. We
trace this unique behavior to fundamental similarities between the NiPS3
exciton hopping and spin exchange processes, underlining the unique magnetic
characteristics of this novel quasiparticle.
