Launching magnons at terahertz speed with the spin Seebeck effect

Transport of spin angular momentum is an essential operation in spintronic devices. In magnetic insulators, spin currents are carried by magnons and can be launched simply by heating an adjacent metal film. Here, we reveal the initial elementary steps of this spin Seebeck effect with 10 fs time resolution in prototypical bilayers of the ferrimagnet yttrium-iron garnet and platinum. Exciting the metal with an ultrashort laser pulse, the spin Seebeck current is measured all-optically using the inverse spin Hall effect and terahertz electrooptic sampling. It rises within ~200 fs, a hallmark of the photoexcited electrons in the metal approaching a Fermi-Dirac distribution. Model-supported analysis shows the spin Seebeck current follows the dynamics of the metal electrons quasi-instantaneously because they impinge on the interface with a correlation time of only ~4 fs and deflect the ferrimagnetic spins without inertia. Promising applications for material characterization, interface probing, spin-noise detection and terahertz spin pumping emerge.