  FePS3 is a layered van der Waals (vdW) Ising antiferromagnet that has
recently been studied in the context of true 2D magnetism, and emerged as an
ideal material platform for investigating strong spin-phonon coupling, and
non-linear magneto-optical phenomena. In this work, we demonstrate an important
unresolved role of spin-orbit coupling (SOC) in the ground state and
excitations of this compound. Combining first principles calculations with
Linear Flavor Wave Theory (LFWT), we find strong mixing and spectral overlap of
different spin-orbital single-ion states. As such, the low-lying excitations
form complex mixtures of local degrees of freedom most accurately viewed as
hybrid spin-orbit exciton-magnon modes. Complete parameterization of the
resulting low-energy model including all such degrees of freedom requires
nearly half a million coupling constants. Despite this complexity, we show that
such a model can be inexpensively derived using local many-body-based
approaches, which yield quantitative agreement with recent experiments. The
results highlight the importance of SOC even in first row transition metals,
and provide essential insight into the properties of 2D magnets with unquenched
orbital moments.
