Optically excited spin pumping mediating collective magnetization dynamics in a spin valve structure

An alternative method for spin pumping -- injection of a pure spin current into a nonmagnetic material by the precessing magnetization in an adjacent ferromagnet -- is demonstrated without the need to use microwaves. Femtosecond laser pulses trigger the magnetization precession in adjacent (Fe,Ga) layers separated by a 5-nm Cu spacer. The spin currents emerging from the two layers couple their magnetization, leading to superposition of the two collective modes, observed in time-resolved Kerr rotation measurements. For resonant precession frequencies in the (Fe,Ga) layers, the authors achieve strongly split decay rates of the collective modes, indicating a purely dissipative coupling.