Effects of molecular adsorption on the spin-wave spectrum and magnon relaxation in two-dimensional Cr₂Ge₂Te₆

In this work, we performed detailed first-principles calculation and theoretical analysis to investigate the effect of molecular adsorption on the spin-wave spectrum and magnon relaxation in a Cr2Ge2Te6 (CGT) monolayer. It is found that NH3, NO, and NO2 adsorption can enhance the exchange constant of CGT, which can result in a blue-shift in the spin-wave spectrum. At 30 K, by means of a thorough investigation of many possible lattice configurations excited by thermal fluctuation, we identify the magnon scattering rate from the intrinsic lattice vibrational modes, and find that the relaxation of optical and acoustic magnons exhibits a completely different wave vector dependence. Moreover, although the adsorption of NO2 and NH3 molecules has a negligible influence on the magnon-phonon interaction, the adsorption of NO molecules results in a significant increase in magnon scattering strength. In the long-wavelength limit, the interlayer vibrational modes induced by NO adsorption increase the magnon-phonon scattering strength by ∼12.7%. The remarkable interlayer magnon-phonon interaction is ascribed to the strong CGT-NO coupling and large molecular vibration amplitude. Considering the importance of magnon relaxation time in the application of spin devices, we suggest that both the impacts on the exchange interaction and scattering rate must be considered when manipulating two-dimensional magnets by surface functionalization.