Quasi-elastic scattering (QES) serves as a sensitive probe of slow spin fluctuations in low-dimensional magnetic systems, multiferroics, and frustrated quantum magnets. Here, we investigate spin and lattice dynamics in anion-substituted $\mathrm{NiP}{\mathrm{S}}_{3-x}\mathrm{S}{\mathrm{e}}_x$ using temperature- and polarization-dependent Raman spectroscopy. Above the Néel temperature, the Raman spectra in the cocircular polarization configuration show a QES response near zero frequency, which is described by a Lorentzian profile. This QES is linked to paramagnetic spin fluctuations and the associated energy density of spins, which are slightly suppressed with increasing Se content. Notably, even inside the magnetically ordered state (below 50 K), we observe a strong enhancement of this QES signal, indicative of persistent fluctuations of the ordered magnetic moments. Concurrently, spin-phonon coupling manifests as an anomalous renormalization of phonon self-energy in certain phonon modes that couple strongly to the magnetic ions, with their frequencies shifting systematically as Se concentration increases. These results suggest that spin fluctuations and lattice vibrations remain strongly intertwined in $\mathrm{NiP}{\mathrm{S}}_{3-x}\mathrm{S}{\mathrm{e}}_x$, and that chemical substitution provides a means to tune this spin-phonon coupling in van der Waals magnets.

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• Magnetism
• Phonons
• Spin-phonon coupling
• 2-dimensional systems
• Antiferromagnets
• Raman spectroscopy