Novel schemes for controlling the ferromagnetic state at the femtosecond time scale by pulsed laser excitation have received great current interest recently. Driving systems into the strongly non-equilibrium regime, it has been shown possible not only to quench magnetic order by femtoseond laser pulses, but also to drive systems through a ferromagnetic phase transition, and even switch the magnetic moment by single pulses of circularly polarized light. More recently, it has been proposed that pulsed laser excitation can also induce spin currents over several to tens of nanometers, which can act as an additional source of sub-picosecond magnetization dynamics. In this presentation I will focus on two different processes in which spin currents induced by femtosecond laser pulses in especially engineered multilayered magnetic thin films are of relevance: emission of THz spin waves and all-optical switching of magnetization.
I will start with a brief review of the field of fs control of the magnetic state by pulsed laser excitation. Proposed mechanisms for ultrafast loss of magnetic order upon fs laser heating as well as all-optical switching will be discussed. Next, different processes that give rise to laser-induced spin currents will be distinguished. In particular I will address recent experiments that have demonstrated laser-induced spin transfer torque on a free magnetic layer, using a collinear multilayer configuration consisting of a free in-plane layer on top of a PMA injection layer and separated by a nonmagnetic spacer. As it will be shown, these non-collinear fs spin currents are absorbed within a few nanometers, and thereby provide ideal conditions for exciting THz spin waves, their quantum mechanical manifestation called magnons. This allowed us in recent experiments to map out the dispersion of the frequency and the Gilbert damping of thin Co and CoB layers.
In the final part of this presentation, it will be shown how the magnetization of synthetic ferromagnetic thin films systems can be reversed using single fs pulses. Although toggle switching of magnetization in ferromagnetic transition metal/rare earth alloys is known now for almost a decade, for synthetic (multilayered) systems only all-optical switching in a multi pulse scenario was found. Here we show fully deterministic single pulse toggle switching of Pt/Co/Gd trilayers. Exposing the system to an even number of pulses results in the original magnetization, while an odd number of pulses leads to the reversed magnetic state in a reducible way and even after thousands of laser pulses. Threshold fluences are determined as a function of Co thickness and record low efficiencies corresponding to below 50 fJ needed to switch a 50x50 nm2 are found. It is argued that also in this case fs laser induced spin currents may provide the driving force for the exciting dynamics.