We report time-resolved magneto-optic Kerr effect measurements of the longitudinal spin Seebeck effect in normal $\mathrm{metal}/{\mathrm{Y}}_3{\mathrm{Fe}}_5{\mathrm{O}}_{12}$ bilayers driven by an interfacial temperature difference between electrons and magnons. The measured time evolution of spin accumulation induced by laser excitation indicates transfer of angular momentum across normal $\mathrm{metal}/{\mathrm{Y}}_3{\mathrm{Fe}}_5{\mathrm{O}}_{12}$ interfaces on a picosecond time scale, too short for contributions from a bulk temperature gradient in an yttrium iron garnet. The product of spin-mixing conductance and the interfacial spin Seebeck coefficient determined is of the order of ${10}^8\mathrm{A}{\mathrm{m}}^{-2}{\mathrm{K}}^{-1}$.

• Received 25 July 2016

DOI:https://doi.org/10.1103/PhysRevLett.118.057201