Anomalous spin-lattice coupling in a 2D antiferromagnetic semiconductor revealed by surface acoustic Rayleigh waves

Magnetic order in van der Waals magnets can strongly influence their lattice dynamics, yet how it affects local and macroscopic manifestations of spin-phonon coupling remains unclear. Optical phonons probe bond-scale exchange modulation and short-range spin correlations, whereas long-wavelength acoustic modes couple to uniform strain fields and are sensitive to renormalization of the macroscopic elastic tensor associated with long-range magnetic order. Experimentally accessing these acoustic excitations in low-dimensional crystals is challenging due to their low energies and the small lateral dimensions of exfoliated samples. Here, we employ Brillouin-Mandelstam scattering spectroscopy to investigate surface acoustic phonons in exfoliated NiPS₃ thin films across the antiferromagnetic transition. Our results show a single Rayleigh surface mode whose phase velocity decreases significantly across the Néel temperature. This anomaly reflects a strong magnetoelastic renormalization of the elastic constants triggered by the onset of antiferromagnetic order. First-principles calculations, combined with continuum finite-element modeling reproduce the Rayleigh-wave dispersion and its magnetic-order-induced shift. The results reveal how microscopic exchange interactions govern the macroscopic elastic response of two-dimensional antiferromagnetic semiconductors, enabling magnetically tunable phononic, magnonic, and strain-mediated spintronic devices.