Ultra-fast dynamics of coherent lattice and spin excitations at the Gd(0001) surface

Abstract

The Gd(0001) surface is investigated by pump–probe experiments using femtosecond laser pulses at 740–860 nm wavelength. Employing optical second-harmonic generation, spin and lattice dynamics are separated through the symmetry of optical field contributions that are even and odd with respect to magnetization reversal. A coherent phonon–magnon mode at a frequency of 3 THz that 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 interactions and their magnetic counterparts lead to incoherent dynamics of the electron, lattice, and spin subsystems. Variation of the optical wavelength shows that for longer wavelengths 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 scales with the spin polarization of the surface state, suggesting that the spin dependence of the ion potentials contributes as well. Furthermore, the frequency of the coherent mode presents a blue shift with a delay of 0.17 THz/ps that 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.