Terahertz spin-to-charge conversion by interfacial skew scattering in metallic bilayers

The efficient conversion of spin to charge transport and vice versa is of major relevance for the detection and generation of spin currents in spin-based electronics. Interfaces of heterostructures are known to have a marked impact on this process. Here, we use terahertz (THz) emission spectroscopy to study ultrafast spin-to-charge-current conversion (S2C) in about 40 prototypical F|N bilayers consisting of a ferromagnetic layer F (e.g. Ni81Fe19, Co or Fe) and a nonmagnetic layer N with strong (Pt) or weak (Cu, Al) spin-orbit coupling. Varying the structure of the F/N interface allows us to drastically change the amplitude and even invert the polarity of the THz charge current. Remarkably, when N is a material with small spin Hall angle, we find a dominant interface contribution to the ultrafast charge current whose magnitude amounts to as much as about 20% of that found in an F|Pt reference sample. Symmetry arguments and first-principles calculations strongly suggest that the interfacial S2C arises from skew scattering of spinpolarized electrons at interface imperfections. Our results highlight the potential of skew scattering for interfacial S2C and propose a promising route to enhanced S2C by tailored interfaces at all frequencies from DC to terahertz.