High-angular momentum excitations in collinear antiferromagnet FePS${}_3$

We report on magneto-optical studies of the quasi-two-dimensional van der Waals antiferromagnet FePS${}_3$. Our measurements reveal an excitation that closely resembles the antiferromagnetic resonance mode typical of easy-axis antiferromagnets, nevertheless, it displays an unusual, four-times larger Zeeman splitting in an applied magnetic field. We identify this excitation with an $\mathrm{\mid }{S}_z\mathrm{\mid }=4$ multipolar magnon – a single-ion 4-magnon bound state – that corresponds to a full reversal of a single magnetic moment of the Fe${}^{2+}$ ion. We argue that condensation of multipolar magnons in large-spin materials with a strong magnetic anisotropy can produce new exotic states.