  Laser-induced terahertz spin transport (TST) and ultrafast demagnetization
(UDM) are central but so far disconnected phenomena in femtomagnetism and
terahertz spintronics. Here, we use broadband terahertz emission spectroscopy
to reliably measure both processes in one setup. We find that the rate of UDM
of a single ferromagnetic metal film F has the same time evolution as the flux
of TST from F into an adjacent normal-metal layer N. This remarkable agreement
shows that UDM and TST are driven by the same force, which is fully determined
by the state of the ferromagnet. An analytical model consistently and
quantitatively explains our observations. It reveals that both UDM in F and TST
in the F|N stack arise from a generalized spin voltage, which is defined for
arbitrary, nonthermal electron distributions. We also conclude that
contributions due to a possible temperature difference between F and N are
minor and that the spin-current amplitude can, in principle, be increased by
one order of magnitude. In general, our findings allow one to apply the vast
knowledge of UDM to TST, thereby opening up new pathways toward large-amplitude
terahertz spin currents and, thus, energy-efficient ultrafast spintronic
devices.
