Ultrafast electron heating as the dominant driving force of photoinduced terahertz spin currents

Ultrafast spintronics strongly relies on the generation, transport, manipulation and detection of terahertz spin currents (TSCs). In F|HM stacks consisting of a ferromagnetic layer F and a heavy-metal layer HM, ultrafast spin currents are typically triggered by femtosecond optical laser pulses. A key open question is whether the initial step, optical excitation and injection of spin currents, can be controlled by tuning the photon energy of the femtosecond pulse. While many theoretical works suggest a marked impact of photon-energy and of highly excited non-thermal electrons, profound experimental evidence is lacking. Here, we use terahertz-emission spectroscopy to study TSCs triggered with two different photon energies of 1.5 eV and 3 eV. We study a wide range of magnetic systems covering metallic ferromagnets, ferrimagnetic insulators, half-metals, as well as systems including tunneling barriers, and rare-earth metallic alloys. We find that variation of the exciting photon energy does not change the dynamics and only slightly the amplitude of the induced TSC in all sample systems. Our results reveal that the ultrafast pump-induced heating of electrons is a highly efficient process for generating TSCs, whereas highly excited primary photoelectrons are of minor importance.