Ultrafast Nanoimaging of Spin-Mediated Shear Waves in an Acoustic Cavity

Strong spin-lattice coupling in van der Waals (vdW) magnets shows potential for innovative magneto-mechanical applications. Here, nanoscale and picosecond imaging by ultrafast electron microscopy reveal heterogeneous spin-mediated coherent acoustic phonon dynamics in a thin-film cavity of the vdW antiferromagnet FePS₃. The harmonics of the interlayer shear acoustic modes are observed, in which the even and odd harmonics exhibit distinct nanoscopic dynamics. Corroborated by acoustic wave simulation, the role of defects in forming even harmonics is elucidated. Above the Néel temperature (T_N), the interlayer shear acoustic harmonics are suppressed, while the in-plane traveling wave is predominantly excited. The dominant acoustic dynamics shifts from the out-of-plane shear to the in-plane traveling wave across T_N, demonstrating that magnetic properties can influence phonon scattering pathways. The spatiotemporally resolved structural characterization provides valuable nanoscopic insights for interlayer-shear-mode-based acoustic cavities, opening up possibilities for magneto-mechanical applications of vdW magnets.