Femtosecond spin dynamics in magnetic multilayers employing high harmonics of laser radiation

This work explores laser-induced, element-selective, femtosecond spin dynamics in ferromagnetic layered structures. As a model system, interlayer exchange coupled Fe/Ru/Ni layers were investigated in time- and element-selective, magneto-optical measurements at the 3p absorption edges of Fe (54 eV) and Ni (67 eV) in the transversal magneto-optical Kerr effect (T-MOKE) geometry. Spin dynamics was initiated by femtosecond laser excitation with 1.5 eV photon energy. The temporal evolution of magnetization was probed by sub-10 fs pulses of laser high harmonics generated in Ne gas. First, element-selective, temperature-dependent magnetization reversal of the Fe and Ni layers in external magnetic field is traced and the interlayer exchange coupling energy J$_{1}$ is derived for the antiferromagnetically coupled Fe and Ni layers from magnetometer measurements. In the next step, femtosecond dynamics of J$_{1}$ is studied in pump-probe experiments with 1.5 eV optical pump and both 3.0 eV and laser high harmonics probe. In time-resolved measurements of magnetic hysteresis, J$_{1}$ is transiently quenched on the femtosecond timescale by the action of the pump laser, following the demagnetization ofthe Ni layer. Moreover, time- and layer-resolved measurements for parallel and antiparallel magnetization orientation of the Fe and Ni layers and for various pump fluences were carried out. The central and surprising result is, that the (buried) Fe layer is transiently magnetized above its equilibrium magnetization prior to laser excitation $\textit{for parallel orientation}$ and demagnetized $\textit{for antiparallel orientation}$ for a defined pump fluence range. For the highest fluence reached in the experiment, the Fe magnetization is quenched for both parallel and antiparallel orientation, albeit, for antiparallel orientation, the quenching ishigher. Regardless of the relative orientation of the Ni and Fe magnetization and opticalpump fluence, the (top) Ni layer is always demagnetized. The magnetization orientationdependent spin dynamics in the Fe layer (magnetization for parallel, demagnetization for antiparallel orientation) is interpreted as a consequence of superdiffusive spin transport from the Ni to the Fe layer. The spin transport originates from the different mean free path of majority and minority electrons in a ferromagnet. Calculations based on the model for superdiffusive spin transport agree well with experimental data within a defined fluence range. The deviation between the model and measurements for the highest fluence reached in the experiment suggests, that spin-flip processes contribute to laser-induced spin dynamics on the femtosecond timescale