  Spintronic structures are extensively investigated for their spin orbit
torque properties, required for magnetic commutation functionalities. Current
progress in these materials is dependent on the interface engineering for the
optimization of spin transmission. Here, we advance the analysis of ultrafast
spin-charge conversion phenomena at ferromagnetic-transition metal interfaces
due to their inverse spin-Hall effect properties. In particular the intrinsic
inverse spin Hall effect of Pt-based systems and extrinsic inverse spin-Hall
effect of Au:W and Au:Ta in NiFe/Au:(W,Ta) bilayers are investigated. The
spin-charge conversion is probed by complementary techniques -- ultrafast THz
time domain spectroscopy in the dynamic regime for THz pulse emission and
ferromagnetic resonance spin-pumping measurements in the GHz regime in the
steady state -- to determine the role played by the material properties,
resistivities, spin transmission at metallic interfaces and spin-flip rates.
These measurements show the correspondence between the THz time domain
spectroscopy and ferromagnetic spin-pumping for the different set of samples in
term of the spin mixing conductance. The latter quantity is a critical
parameter, determining the strength of the THz emission from spintronic
interfaces. This is further supported by ab-initio calculations, simulations
and analysis of the spin-diffusion and spin relaxation of carriers within the
multilayers in the time domain, permitting to determine the main trends and the
role of spin transmission at interfaces. This work illustrates that time domain
spectroscopy for spin-based THz emission is a powerful technique to probe
spin-dynamics at active spintronic interfaces and to extract key material
properties for spin-charge conversion.
