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 in a single simple ferromagnetic metal film F such as Co70Fe30 or Ni80Fe20 has the same time evolution as TST from F into an adjacent normal-metal layer N such as Pt or W. As this remarkable agreement refers to two very different samples, an F layer vs an F|N stack, it does not result from the trivial fact that TST out of F reduces the F magnetization at the same rate. Instead, our observation strongly suggests that UDM in F and TST in F|N are driven by the same force, which is fully determined by the state of the ferromagnet. An analytical model quantitatively explains our measurements and reveals that both UDM in the F sample and TST in the associated F|N stack arise from a generalized spin voltage, i.e., an excess of magnetization, which is defined for arbitrary, nonthermal electron distributions. We also conclude that contributions due to a possible temperature difference between F and N, i.e., the spin-dependent Seebeck effect, and optical intersite spin transfer are minor in our experiment. Based on these findings, one can apply the vast knowledge of UDM to TST to significantly increase spin-current amplitudes and, thus, open promising pathways toward energy-efficient ultrafast spintronic devices.
