Ultrafast dynamics at lanthanide surfaces: microscopic interaction of the charge, lattice and spin subsystems

In solids the charge, lattice and spin degrees of freedom are coupled and the respective coupling strengths result in characteristic timescales on which excitations of one particular subsystem interact and equilibrate with the remaining subsystems. In ferromagnetic metals the respective elementary interaction processes occur on the pico- and femtosecond time scales. To elucidate these interaction mechanisms and the timescales we investigate the ultrafast dynamics of electrons, phonons and spins excited by an intense infrared optical pulse at Gd(0 0 0 1) and Tb(0 0 0 1) surfaces, employing complementary time-resolved methods of optical second harmonics generation and photoelectron spectroscopy. These surfaces exhibit rich dynamics including a collective response of the crystal lattice and the magnetization, manifested by a coupled coherent optical phonon-magnon mode at 3 THz. After a review of the earlier studies we present temperature- and fluence-dependent results of pump-probe experiments. The temperature dependence of the coherent phonon (CP) amplitude shows that a spin- and a charge-driven excitation mechanism can be separated. The spin-driven excitation dominates below the Curie temperature in agreement with ab initio model calculations that establish a displacement of the surface plane upon a change of the surface spin polarization. Analysis of the temperature-dependent CP damping shows an anomaly in the vicinity of the Curie temperature that is for Gd well described by phonon-magnon scattering. In the case of Tb an additional damping channel is required that could be related to local spin-flip excitations