All-optical probe of magnetization dynamics in exchange biased bilayers on the picosecond timescale

All-optical control of the magnetization of polycrystalline exchange bias bilayer systems is achieved using short picosecond laser pulses. Due to the photoexcitation, the spin temperature across the interface between the ferromagnetic and antiferromagnetic layer is elevated, resulting in a collapse of the interfacial exchange coupling. Thus, within the first 10 ps, a fast reduction of both the exchange bias field and the coercive field is observed for three different exchange bias systems comprising both different ferromagnets and antiferromagnets. The fast thermal unpinning is followed by a slower heat diffusion dominated relaxation process, which strongly depends on the thermal conductivity of the used buffer layers and substrates. The fast optical unpinning can be understood in terms of an internal anisotropy pulse field capable of triggering ultrafast precessional magnetization dynamics of the ferromagnetic layer, which makes heat-assisted coherent magnetization rotation feasible.