Coherent Oscillations in a SrRuO₃/BiFeO₃ Superlattice

We investigated the ultrafast dynamics of a SrRuO₃/BiFeO₃ superlattice grown on a SrTiO₃ substrate using a near infrared pump-probe technique at various temperatures. The superlattice exhibits a ferromagnetic order inherited from the SrRuO₃ layer. The pump-induced changes in the reflectivity reveal periodic oscillations. We found that the oscillation frequency can be well explained by zone-folded acoustic phonon oscillations, whose dispersion depends on the sound velocity, density, and thickness within the supercell of each constituent layer. It is found that the observed oscillation frequency corresponds to the A₁ mode, which suggests that oscillations are excited due to pump-induced expansion of the SrRuO₃ layer that absorbs the pump photon. Temperature-dependent measurements reveal significant suppression of the oscillation amplitude in the ferromagnetic state. The suppressed amplitude is proportional to the square of the magnetization, M(T)². This phenomenon can be attributed to a strong magnetostriction effect of SrRuO₃ that suppresses lattice expansion upon pumping.