Phonon and magnon dynamics across antiferromagnetic transition in 2D layered van der Waals material CrSBr

We report temperature-dependent reflectivity spectra of the layered van der Waals magnet CrSBr in the far-infrared region. Polarization-dependent measurements resolve the vibrational modes along the E$\|a$- and b-axes and reveal the clear structural anisotropy. While the a-axis phonons notably harden on cooling, the b-axis phonon frequencies are almost temperature-independent. A phonon splitting due to the antiferromagnetic phase transition is observed for the 180~cm⁻¹ a-axis vibrational mode, accompanied by a phonon softening below TN. Furthermore, an additional mode with strong magnetic characteristics at ~360~cm⁻¹ is identified and attributed to the magnon excitation of CrSBr.