  Femtosecond laser pulses can be used to induce ultrafast changes of the
magnetization in magnetic materials. Several microscopic mechanisms have been
proposed to explain the observations, including the transport of ultrashort
spin-polarized hot-electrons (SPHE). Such ultrafast spin currents find growing
interest because of the recent challenges in ultrafast spintronics however they
are only poorly characterized. One of the key challenges is to characterize the
spin-polarized ultrafast currents and the microscopic mechanisms behind SPHE
induced manipulation of the magnetization, especially in the case of
technologically relevant ferrimagnetic alloys. Here, we have used a combined
approach using time- and element-resolved X-ray magnetic circular dichroism and
theoretical calculations based on atomistic spin-dynamics simulations to
address the ultrafast transfer of the angular momentum from spin-polarized
currents into ferrimagnetic Fe74Gd26 films and the concomitant reduction of
sub-lattice magnetization. Our study shows that using a Co/Pt multilayer as a
polarizer in a spin-valve structure, the SPHE drives the demagnetization of the
two sub-lattices of the Fe74Gd26 film. This behaviour is explained based on two
physical mechanisms, i.e., spin transfer torque and thermal fluctuations
induced by the SPHE. We provide a quantitative description of the heat transfer
of the ultrashort SPHE pulse to the Fe74Gd26 films, as well as the degree of
spin-polarization of the SPHE current density responsible for the observed
magnetization dynamics. Our work finally characterizes the spin-polarization of
the SPHEs revealing unexpected opposite spin polarization to the Co
magnetization, explaining our experimental results.
