Atomically Resolved Acoustic Dynamics Coupled with Magnetic Order in a van der Waals Antiferromagnet

TL;DR

This study uses ultrafast x-ray diffraction to reveal how atomic displacements and acoustic phonon modes in a van der Waals antiferromagnet are coupled with magnetic order, providing insights into magnetoelastic interactions at atomic scales.

Contribution

It provides the first atomic and femtosecond resolution characterization of acoustic phonon dynamics coupled with magnetic order in a vdW antiferromagnet.

Findings

Atomic displacements and phonon modes are coupled with magnetic order.

A transverse mode shows directional change across the Néel temperature.

Interlayer shear mode amplitude is significantly enhanced in the antiferromagnetic phase.

Abstract

Magnetoelastic coupling in van der Waals (vdW) magnetic materials enables a unique interplay between the spin and lattice degrees of freedom. Characterizing the elastic responses with atomic and femtosecond resolution across the magnetic transition is essential for guiding the design of magnetically tunable actuators and strain-mediated spintronic devices. Here, ultrafast x-ray diffraction employed at a free-electron laser reveals that the atomic displacements, wave vectors, and dispersion relations of acoustic phonon modes in a vdW antiferromagnet FePS$_3$ are coupled with the magnetic order, by tracking both in-plane and out-of-plane Bragg peaks upon optical excitation across the N\'eel temperature (T$_N$). One transverse mode shows that a quasi-out-of-plane atomic displacement undergoes a significant directional change across T$_N$. Its quasi-in-plane wave vector is derived by the comparison between the measured sound velocity and the first-principles calculations. The other transverse mode is an interlayer shear acoustic mode whose amplitude is strongly enhanced in the antiferromagnetic phase, exhibiting eight times stronger amplitude than the longitudinal acoustic mode below T$_N$. The atomically resolved characterization of acoustic phonon dynamics that couple with magnetic ordering opens opportunities for harnessing unique magnetoelastic coupling in vdW magnets on ultrafast timescales.