Terahertz time-domain spectroscopy to probe laser-excited spin currents in a Co/Gd system

Single shot all-optical switching of the magnetization by femtosecond laser pulses in rare-earth transition-metal ferrimagnetic materials is particularly promising for future ultrafast magnetic storage applications. Moreover, ultrafast laser-generated spin currents appear to play an important role in the switching process. Here, we try to separately detect the spin current from Gd in a Co/Gd bilayer system using terahertz time-domain spectroscopy. To this aim, we use different capping, buffer and embedded layers in order to tune the spin-to-charge and spin-current propagation and identify currents from each of the layers. We attribute the observed THz emission in all layers to the transition metal demagnetization induced spin currents, and detect no contribution from the Gd demagnetization. We attribute this absence of Gd-induced THz signal to the potentially slow demagnetization of Gd, which shift the emission spectra to lower frequencies, below our detection capabilities. These results highlight the limitations in using materials suffering from the so-called critical slowdown for the optimization of spintronic THz emitters.