Spin-orbital excitations encoding the magnetic phase transition in the van der Waals antiferromagnet FePS$_{3}$

TL;DR

This study uses resonant inelastic X-ray scattering to explore spin-orbital excitations in FePS₃, revealing their role in magnetic phase transitions and their robustness in few-layer forms, advancing understanding of 2D magnetic materials.

Contribution

It provides new insights into the spin-orbital excitations and their relation to magnetism in FePS₃, highlighting the effects of lattice distortion and charge transfer, and demonstrates RIXS's effectiveness in 2D magnetic studies.

Findings

Spectral enhancement of spin-orbital transitions at ~100 and ~220 meV in the antiferromagnetic phase.

Spectral profiles are robust down to few-layer limits, indicating persistent antiferromagnetic order.

Trigonal lattice distortion and charge-transfer effects are crucial for understanding excitations.

Abstract

In the rich phases of van der Waals (vdW) materials featuring intertwined electronic order and collective phenomena, characterizing elementary dynamics that entail the low-energy Hamiltonian and electronic degrees of freedom is of paramount importance. Here we performed resonant inelastic X-ray scattering (RIXS) to elaborate the spin-orbital ground and excited states of the vdW antiferromagnetic insulator FePS$_{3}$, as well as their relation to magnetism. We observed the spectral enhancement of spin-orbital multiplet transitions about $\sim$ 100 and $\sim$ 220 meV, as well as quasielastic response, when entering the zig-zag antiferromagnetic phase, where the spectral changes develop an order-parameter-like evolution with temperature. By comparing with ligand field theory calculations, we discovered the essential role of trigonal lattice distortion and negative metal-ligand charge-transfer to account for these emergent excitations. Such spectral profiles are further examined upon confinement by mechanical exfoliation. We reveal their spectral robustness down to the few atomic layer limit, in accordance with the persistent antiferromagnetic state previously reported in optical measurements. Our study demonstrates the versatile RIXS capability that resolves magneto-crystalline anisotropy and charge-transfer energetics. These provide the crucial insight to understand how the spontaneous magnetic symmetry-breaking stabilizes in the quasi-two-dimensional limit for the vdW magnet FePS$_{3}$.