Non-equilibrium THz-phonon spin coupling in CrI3

Manipulating magnetism at the THz timescale in atomically thin ferromagnets by exploiting the interactions of spins with optical phonon modes presents an innovative idea for THz spintronics and magnonics. Utilizing the coupling of phonon modes to the magnetization could lead to new ways of generating and controlling spin wave excitations in future applications. We use femtosecond optical laser pulses to generate excitons, bound electron-hole pairs, in bulk-like ferromagnet CrI3 flakes and probe the subsequent charge and spin dynamics with optical pump-probe spectroscopy. In CrI3, exciton formation is known to drive coherent optical phonon modes with 2.4 and 3.9 THz frequencies corresponding to the bending and stretching of the Cr-I bonds. We show that both phonon modes also lead to magnetization oscillations. This establishes spin-phonon coupling for both modes, contrary to previous observations that only report the 3.9 THz phonon mode can influence the CrI3 magnetization.