  Femtosecond laser-induced photoexcitation of ferromagnet (FM)/heavy metal
(HM) heterostructures have attracted attention by emitting broadband terahertz
frequencies. The phenomenon relies on the formation of ultrafast spin current,
which is largely attributed to the direct photoexcitation of the FM layer.
However, we reveal that during the process, the FM layer also experiences a
secondary excitation led by the hot electrons from the HM layer that travel
across the FM/HM interface and transfer additional energy in the FM. Thus, the
generated secondary spins enhance the total spin current formation and lead to
amplified spintronic terahertz emission. The results also emphasize the
significance of the secondary spin current, which even exceeds the primary spin
currents when FM/HM heterostructures with thicker HM are used. An analytical
model is developed to provide deeper insights into the microscopic processes
within the individual layers, underlining the generalized ultrafast
superdiffusive spin-transport mechanism.
