Coupling of magnetic and lattice collective excitations in the two-dimensional van der Waals antiferromagnet FePS₃

We combine polarized infrared magnetotransmission and Faraday angle rotation measurements to map the collective excitations of the van der Waals antiferromagnet FePS₃. Below the Néel temperature (T_N ≈ 118 K), the phonon spectrum becomes strongly anisotropic, reflecting the underlying zigzag antiferromagnetic order. In contrast, a prominent excitation at 122 cm⁻¹ (15 meV) is polarization-independent, hardens on cooling, and splits linearly with magnetic field, identifying its magnetic origin. From absolute transmission and Faraday rotation, we reconstruct the circular optical conductivities and reveal a pronounced dichroism of the field-split excitations. The upper branch near 129 cm⁻¹ exhibits a reduced dichroic response, consistent with hybridization with a nearby infrared phonon. Several phonon modes exhibit sizable Faraday rotation, providing evidence for spin-phonon coupling and demonstrating that lattice vibrations acquire magnetic-field-dependent optical activity. In addition, additional excitations appear in the infrared spectra and a broad midinfrared feature near 900 cm⁻¹ emerges only below T_N, consistent with a modified lattice response in the magnetic state. These results highlight the anisotropic nature of spin-phonon coupling in FePS₃ and establish polarization-resolved magneto-optical spectroscopy as a powerful probe of coupled spin and lattice dynamics in two-dimensional antiferromagnets.