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 one 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.
Terahertz (THz) spin‐to‐charge conversion has become an increasingly important process for THz pulse generation and as a tool to probe ultrafast spin interactions at magnetic interfaces. However, its relation to traditional, steady state, ferromagnetic resonance techniques is poorly understood. Here, nanometric trilayers of Co/X/Pt (X = Ti, Au or an Au:W alloy) are investigated as a function of the “X” layer thickness, where THz emission generated by the inverse spin Hall effect is compared to the Gilbert damping of the ferromagnetic resonance. Through the insertion of the “X” layer it is shown that the ultrafast spin current injected in the non‐magnetic layer defines a direct‐spin‐conductance, whereas the Gilbert damping leads to an effective spin‐mixing‐conductance of the trilayer. Importantly, it is shown that these two parameters are connected to each other and that spin‐memory‐losses can be modeled via an effective Hamiltonian with Rashba fields. This work highlights that magneto‐circuit concepts can be successfully extended to ultrafast spintronic devices, as well as enhancing the understanding of spin‐to‐charge conversion processes through the complementarity between ultrafast THz spectroscopy and steady state techniques.
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