High-Angular Momentum Excitations in Collinear Antiferromagnet FePS₃

Abstract

We report on magneto-optical studies of the quasi-two-dimensional van der Waals antiferromagnet FePS₃. 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 |<i>S</i>_<i>z</i>| = 4 multipolar magnon─a single-ion 4-magnon bound state─that corresponds to a full reversal of a single magnetic moment of the Fe²⁺ ion. We argue that condensation of multipolar magnons in large-spin materials with a strong magnetic anisotropy can produce new exotic states.