Coherent spin dynamics in the THz domain driven by coherent phonon excitation is observed in the time-resolved second harmonic response of the Gd(0001) ferromagnetic metal surface. An LO phonon of 2.9 THz is excited by a transient charge displacement at the surface caused by resonant absorption of a femtosecond laser pulse in the exchange-split Gd surface state. This phonon modulates the interlayer distance and generates a periodic variation of the exchange interaction between spins in adjacent layers which leads to parametric excitation of optical magnons coherent with the lattice vibration.
also cited in: Virt. J. Ultra Science 2, 12 (Dec 2003)