Ultrafast spin transport and control of spin current pulse shape in metallic multilayers

We study the emission of femtosecond spin current pulses at Fe/Au interfaces in well-defined epitaxial Fe/Au(001) and Fe/Au/Fe(001) structures. Using the exceptional sensitivity of optical second harmonic generation to spin currents and transient spin density at interfaces, we perform direct measurements of the ultrafast spin transport. To link the optical data to microscopic material parameters, we use a simple model of electron emission from ultrathin Fe layers, which ignores the diffusive transport in Fe, and develop a description of superdiffusive spin transport in Au. This allows us to extract the electron velocity and scattering rates in Au, estimate the dynamics of secondary carrier population in Fe, and retrieve the spin current pulse shape from the experimental data. We study the variation of spin current pulse shape with increasing thickness of the Fe emitter layer. The observed dependency is well explained by including the diffusive spin transport in Fe. This example shows the potential of our approach to develop spin current emitters delivering desired pulse shapes.