Interplay between magnetic and lattice excitations and emergent multiple phase transitions in MnPSe 3 − x S x

The intricate interplay between spin and lattice degrees of freedom in two-dimensional magnetic materials plays a pivotal role in modifying their magnetic characteristics, engendering hybrid quasiparticles, and implementing functional devices. Herein, we present our comprehensive and in-depth investigations on magnetic and lattice excitations of ${\mathrm{MnPSe}}_{3\ensuremath{-}x}{\mathrm{S}}_x$ $(x=0, 0.63, \text{and} 1.68)$ alloys, utilizing temperature- and polarization-dependent Raman scattering. Our experimental results reveal the occurrence of multiple phase transitions, evidenced by notable changes in phonon self-energy and the appearance or splitting of phonon modes. These emergent phases are tied to the development of long and short-range spin-spin correlations, as well as to spin reorientations or magnetic instabilities. Our analysis of two-magnon excitations as a function of temperature and composition showcases their hybridization with phonons whose degree weakens with increasing $x$. Moreover, the suppression of spin-dependent phonon intensity in chemically most-disordered ${\mathrm{MnPSe}}_{3\ensuremath{-}x}{\mathrm{S}}_x$ $(x=1.68)$ suggests that chalcogen substitution offers a control knob of tuning spin and phonon dynamics by modulating concurrently superexchange pathways and a degree of trigonal distortions.