The Gd(0001) surface is investigated by pump-probe experi ments using femtosecond laser pulses at 740-860 nm wave length. Employing optical second harmonic generation, spin and lattice dynamics are separated through the symmetry of optical field contributions which are even and odd with respect to magnetization reversal. A coherent phonon-magnon mode at a frequency of 3 THz which is excited through the exchange-split surface state is observed in the time domain. A magneto-elastic phonon-magnon interaction based on spin-orbit coupling is weak for Gd and a modulation of the exchange interaction mediated by the lattice vibration is proposed as a microscopic interaction mechanism of this coupled mode. In parallel, electron-electron and electron-phonon interaction and their magnetic counterparts lead to incoherent dynamics of the electron, lattice, and spin subsystems. Variation of the optical wave length shows that for longer wave lengths up to 860 nm the coherent mode dominates while for shorter ones ( > 740 nm) incoherent contributions prevail. This dependence indicates that selective depopulation of the occupied surface state component drives the coherent excitation. However, temperature dependent studies show that the oscillation amplitude of even and odd contributions scale with the spin polarization of the surface state suggesting that the spin dependence of the ion potentials contribute as well. Furthermore, the frequency of the coher ent mode presents a blue shift with delay of 0.17 THz/ps which saturates at the static frequency of the respective bulk phonon. This behavior is a consequence of equilibration of the screened ion potential at the surface subsequent to the intense laser excitation.
The publication is available at link doi:10.1007/s00339-005-3366-2