Femtosecond formation dynamics of the spin Seebeck effect revealed by terahertz spectroscopy

Understanding the transfer of spin angular momentum is essential in modern magnetism research. A model case is the generation of magnons in magnetic insulators by heating an adjacent metal film. Here, we reveal the initial steps of this spin Seebeck effect with <27fs time resolution using terahertz spectroscopy on bilayers of ferrimagnetic yttrium-iron garnet and platinum. Upon exciting the metal with an infrared laser pulse, a spin Seebeck current $j_\textrm{s}$ arises on the same ~100fs time scale on which the metal electrons thermalize. This observation highlights that efficient spin transfer critically relies on carrier multiplication and is driven by conduction electrons scattering …