Phonon and magnon dynamics across the antiferromagnetic transition in the two-dimensional 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 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 T N . Furthermore, an additional mode with strong magnetic characteristics at ∼ 360 cm − 1 is identified and attributed to the magnon excitation of CrSBr.