Photoinduced spin dynamics in La_0.6Sr_0.4MnO₃ observed by time-resolved magneto-optical Kerr spectroscopy

Photoinduced effect in a ferromagnetic perovskite La_0.6Sr_0.4MnO₃ was investigated by femtosecond pump-probe spectroscopy. By utilizing magneto-optical Kerr rotation as a probe, the photoinduced dynamics of the spins was observed independently of the quasiparticle relaxation dynamics. The low-energy optical conductivity decreases immediately after photoexcitation without changing magnetization, and subsequently the demagnetization occurs in a strongly temperature-dependent manner. The photoinduced demagnetization shows critical slowing down with the time constant up to 1.4 ns when the spin temperature after photoexcitation reaches near the Curie temperature. Such a slow response of a spin system indicates good thermal insulation between quasiparticles and a spin system, suggesting small spin-orbit interaction.