Ultrafast Spin–Phonon Coupling Dynamics in BiFeO₃-Bi_0.5K_0.5TiO₃ Single Crystal

Besides equilibrium behavior, exploring the spin-phonon coupling in multiferroic materials under non-equilibrium conditions is crucial for a deep understanding of the mechanisms as well as their high-frequency applications. Here, by utilizing time-resolved reflectance spectroscopy, we demonstrate ultrafast spin-phonon coupling dynamics in multiferroic 0.58BiFeO₃-0.42Bi_0.5K_0.5TiO₃ (BF-BKT) single crystals. With ultrafast laser pumping, coherent acoustic phonons with low damping are created in BF-BKT. Temperature-dependent results indicate that both the frequency and amplitude of laser-induced coherent phonons are sensitive to the emergence of antiferromagnetic order. Moreover, the spin state change driven by external magnetic fields can enhance the oscillation amplitude of the coherent acoustic phonons even above the magnetic Néel temperature. These findings experimentally confirm that spin-phonon coupling in multiferroic materials exists not only in the spin-ordered state but also in the spin-disordered state, and not only in the equilibrium state but also in the non-equilibrium state excited by ultrafast lasers, suggesting their promising applications in high-frequency devices.