  A systematic investigation of spin injection behavior in Au/FM (FM = Fe and
Ni) multilayers is performed using the superdiffusive spin transport theory. By
exciting the nonmagnetic layer, the laser-induced hot electrons may transfer
spin angular momentum into the adjacent ferromagnetic (FM) metals resulting in
ultrafast demagnetization or enhancement. We find that these experimental
phenomena sensitively depend on the particular interface reflectivity of hot
electrons and may reconcile the different observations in experiment.
Stimulated by the ultrafast spin currents carried by the hot electrons, we
propose the multilayer structures to generate highly spin polarized currents
for development of future ultrafast spintronics devices. The spin polarization
of the electric currents carried by the hot electrons can be significantly
enhanced by the joint effects of bulk and interfacial spin filtering. Meanwhile
the intensity of the generated spin current can be optimized by varying the
number of repeated stacking units and the thickness of each metallic layer.
