Coupling of magnetic and lattice collective excitations in the 2D van der Waals antiferromagnet FePS$_{3}$

TL;DR

This study investigates the coupling between magnetic and lattice excitations in FePS3, revealing spin-phonon hybridization, magnetic field effects, and inversion-symmetry breaking in a 2D van der Waals antiferromagnet.

Contribution

It provides the first detailed mapping of magnetic and lattice collective excitations in FePS3 using infrared magneto-spectroscopy, highlighting spin-phonon interactions and symmetry breaking.

Findings

Identification of a magnetic excitation at 122 cm$^{-1}$ that splits with magnetic field.

Observation of spin-phonon hybridization near 129 cm$^{-1}$.

Detection of inversion-symmetry breaking below the N{é}el temperature.

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 N\'{e}el temperature ($T_\mathrm{N} \approx 118$ K), the phonon spectrum becomes strongly anisotropic, while a prominent excitation at 122 cm$^{-1}$ (15 meV) hardens on cooling and splits linearly with magnetic field,identifying its magnetic origin. From absolute transmission and Faraday angle measurements, we reconstruct the circular optical conductivities, revealing a pronounced dichroism of the field-split excitations. The upper branch near 129 cm$^{-1}$ shows an anomalously reduced dichroism, consistent with spin-phonon hybridization with a nearby infrared phonon. Several phonons exhibit sizable Faraday rotation, and Raman-active modes appear in the infrared spectra. Together with a broad mid-infrared band near 900 cm$^{-1}$ emerging only below $T_\mathrm{N}$, these observations point to inversion-symmetry breaking in the magnetic state. A weak Faraday feature near 320 cm$^{-1}$ ($\sim 40$ meV) further supports the presence of a higher-energy magnetic excitation. These findings underscore the intricate interplay between magnetic and lattice degrees of freedom in FePS$_3$ and establish its relevance for exploring spin-lattice interactions in two-dimensional antiferromagnetic materials.