Frustrated magnetic materials provide a fertile ground for exploring novel quantum states, wherein the interplay between spin and lattice degrees of freedom plays a pivotal yet underexplored role. Here, we employ terahertz (THz) time-domain spectroscopy to investigate the spin-lattice coupling and short-range spin correlations in ${\mathrm{NaCrO}}_2$, an $S=3/2$ layered triangular-lattice Heisenberg antiferromagnet. Combined with superconducting quantum interference device magnetometry, our THz spectroscopy reveals the persistence of short-range magnetic order up to twice the long-range magnetic ordering temperature. Furthermore, several low-frequency phonon modes are identified in the THz spectra. Their temperature dependence manifests a strong spin-phonon coupling, characterized by abrupt changes in the intensity and frequency of the phonon absorption peaks at magnetic critical temperatures. Further analysis implies that this coupling is primarily driven by the exchange-interaction-induced modification of the Cr-O bonds. Our work provides direct evidence of spin-phonon coupling in a frustrated magnet and suggests new avenues for manipulating lattice dynamics in quantum materials.

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• Antiferromagnetism
• Spin-phonon coupling
• Antiferromagnets
• Noncollinear magnets
• Polycrystalline materials
• Terahertz techniques