Transverse and longitudinal magnons in the strongly anisotropic antiferromagnet FePSe3

${\mathrm{FePSe}}₃$ is a collinear honeycomb antiferromagnet with an easy-axis anisotropy and large spins $S=2$. It belongs to a family of magnetic van der Waals materials which recently attracted considerable attention. In this work we present an experimental magneto-optical study of the low-energy excitation spectrum in ${\mathrm{FePSe}}₃$, together with its theoretical description. The observed response contains several types of magnon excitations. Two of them are conventional transverse magnons described by a classical theory of antiferromagnetic resonance. Two other modes are identified as multimagnon hexadecapole excitations with an anomalous $g$ factor approximately equal to 4 times the $g$ factor of a single ${\mathrm{Fe}}²⁺$ ion. These quasiparticles correspond to full reversals of iron spins that coherently propagate in the up-down antiferromagnetic structure. They constitute a novel type of collective excitations in anisotropic magnetic solids, called longitudinal magnons. Comparison between theory and experiment allows us to estimate the microscopic parameters of ${\mathrm{FePSe}}₃$, including exchange coupling constants and the single-ion anisotropy.