(Submitted on 3 Sep 2017)
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 straightforwardly by heating an adjacent metal layer. Here, we study the ultimate speed of this spin Seebeck effect with 10-fs time resolution in prototypical bilayers of ferrimagnetic yttrium iron garnet and platinum. Upon exciting the metal by a laser pulse, the spin flow is measured using the inverse spin Hall effect and terahertz electrooptic sampling. The spin Seebeck current reaches its peak within ~200 fs, a hallmark of the photoexcited metal electrons approaching a Fermi-Dirac distribution. Analytical modeling shows the spin Seebeck response is virtually instantaneous because the ferrimagnetic spins react without inertia and the metal spins impinging on the interface have a correlation time of only ~4 fs. Novel applications for material characterization, interface probing, spin-noise detection and terahertz spin pumping emerge.
