Spin voltage gradient is the driving force for ultrafast demagnetization and Terahertz spin transport

Laser-induced terahertz spin transport (TST) and ultrafast demagnetization (UDM) are central phenomena in femtomagnetism and terahertz spintronics, but so far disconnected. 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 observation suggests that UDM in F sample and TST in F|N stack arise from the same force. An analytical model quantitatively explains our measurements and reveals that the driving force is a generalized spin voltage, i.e., an excess of magnetization which is defined for arbitrary, nonthermal electron distributions. Our observation indicates that contributions due to a possible temperature difference between F and N, i.e., the spin-dependent Seebeck effect, are minor. Based on these findings, one can apply the vast knowledge of UDM to TST to significantly increase spin-current amplitudes and, thus, open up new pathways toward energy-efficient ultrafast spintronic devices.