  Laser-induced terahertz spin transport (TST) and ultrafast demagnetization
(UDM) are central but so far disconnected phenomena in femtomagnetism and
terahertz spintronics. Here, we show that UDM and TST are driven by the same
force: a generalized spin voltage, which is induced by the incident femtosecond
laser pulse. Using broadband terahertz emission spectroscopy, we find that the
rate of UDM of a single ferromagnetic film F has the same time evolution as the
flux of TST from F into an adjacent normal-metal layer N. 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
\~{\Delta}{\mu}_s, which is defined for arbitrary, nonthermal electron
distributions. Our findings open up unexpected synergies and new pathways
toward large-amplitude terahertz spin currents and, thus, energy-efficient
ultrafast spintronic devices.
