Circular Optical Conductivity and Spin–Phonon Interactions in the van der Waals Antiferromagnet FePS₃

We present a comprehensive magneto-optical study of the two-dimensional Ising antiferromagnet FePS₃ that reveals a strong coupling between magnetic and lattice collective excitations. Using polarized infrared transmission and full-protocol Faraday rotation spectroscopy, we map the evolution of the 15 meV (122 cm⁻¹) excitation associated with the magnetic order. This mode hardens below the Néel temperature (T_N ≈ 118 K), splits linearly with magnetic field, and produces a large Faraday rotation, confirming its magnetic origin. By combining absolute transmission and Faraday angle data, we reconstruct the circular optical conductivities and uncover pronounced dichroism between σ⁺ and σ⁻. The upper branch exhibits an anomalously reduced dichroism, indicating hybridization with a nearby infrared-active phonon. Several phonons show sizable Faraday rotation, and Raman-active modes appear in the infrared spectra, together with a broad mid-infrared band near 900 cm⁻¹, signatures of inversion-symmetry breaking in the magnetic phase. These results demonstrate how magneto-infrared spectroscopy can directly probe spin–lattice coupling and circular dichroism in two-dimensional antiferromagnets, establishing FePS₃ as a benchmark platform for correlated spin–phonon phenomena.