Coupling of magnetic and lattice collective excitations in the 2D van der Waals antiferromagnet FePS${}_3$

Abstract

We combine polarized infrared magneto-transmission and Faraday angle rotation measurements to map the collective excitations of the van der Waals antiferromagnet FePS${}_3$. Below the Néel temperature ($T_{\mathrm{N}}\approx 118\mathrm{K}$), the phonon spectrum becomes strongly anisotropic, reflecting the underlying zigzag antiferromagnetic order. In contrast, a prominent excitation at $122{\mathrm{cm}}^{-1}$ ($15$~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 $129{\mathrm{cm}}^{-1}$ 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 mid-infrared feature near $900{\mathrm{cm}}^{-1}$ emerges only below $T_{\mathrm{N}}$, consistent with a modified lattice response in the magnetic state. These results highlight the anisotropic nature of spin--phonon coupling in FePS${}_3$ and establish polarization-resolved magneto-optical spectroscopy as a powerful probe of coupled spin and lattice dynamics in two-dimensional antiferromagnets.

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