Photoinduced nonthermal reduction of the coercive field in FePt and FePt$_{0.84}$Rh$_{0.16}$ epitaxial thin films in the $L1_0$ phase

The time-resolved magneto-optical Kerr effect in epitaxial thin films of the FePt compound and the FePt$_{0.84}$Rh$_{0.16}$ solid solution with the perpendicular magnetic anisotropy on MgO (001) substrates has been studied. The evolution of hysteresis loops at short (100 fs–1 ns) and long (1–20 ms) time scales after the excitation by a femtosecond light pulse has been studied. Long-lived nonthermal reduction of the coercive field has been detected. The coercive field is recovered in several milliseconds. It has been proposed to explain the observed phenomenon by the excitation of high-$Q$-factor acoustic resonances in the substrate/film system and to the strong magnetoelastic interaction in FePt and FePt$_{0.84}$Rh$_{0.16}$ films.