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.

Spin‐to‐charge conversion (SCC) involving topological surface states (TSS) is one of the most promising routes for highly efficient spintronic devices for terahertz (THz) emission. Here, the THz generation generally occurs mainly via SCC consisting in efficient dynamical spin injection into spin‐locked TSS. In this work, sizable THz emission from a nanometric thick topological insulator (TI)/ferromagnetic junction—SnBi2Te4/Co—specifically designed to avoid bulk band crossing with the TSS at the Fermi level, unlike its parent material Bi2Te3 is demonstrated. THz emission time domain spectroscopy (TDS) is used to indicate the TSS contribution to the SCC by investigating the TI thickness and angular dependence of the THz emission. This work illustrates THz emission TDS as a powerful tool alongside angular resolved photoemission spectroscopy (ARPES) methods to investigate the interfacial spintronic properties of TI/ferromagnet bilayers.

Antiferromagnetic materials have been proposed as new types of narrowband THz spintronic devices owing to their ultrafast spin dynamics. Manipulating coherently their spin dynamics, however, remains a key challenge that is envisioned to be accomplished by spin-orbit torques or direct optical excitations. Here, we demonstrate the combined generation of broadband THz (incoherent) magnons and narrowband (coherent) magnons at 1 THz in low damping thin films of NiO/Pt. We evidence, experimentally and through modeling, two excitation processes of spin dynamics in NiO: an off-resonant instantaneous optical spin torque in (111) oriented films and a strain-wave-induced THz torque induced by ultrafast Pt excitation in (001) oriented films. Both phenomena lead to the emission of a THz signal through the inverse spin Hall effect in the adjacent heavy metal layer. We unravel the characteristic timescales of the two excitation processes found to be < 50 fs and > 300 fs, respectively, and thus open new routes towards the development of fast opto-spintronic devices based on antiferromagnetic materials.

Spintronic terahertz (THz) emitters based on the inverse spin Hall effect in ferromagnetic/heavy metal (FM/HM) heterostructures have become important sources for THz pulse generation. The design, materials, and control of these interfaces at the nanometer level have become vital to engineer their THz emission properties. In this work, we present studies of the optimization of such structures through a multi-pronged approach, taking advantage of material and interface engineering to enhance THz spintronic emission. This includes the application of multi-stacks of HM/FM junctions and their application to trilayer structures, the use of spin-sinks to simultaneously enhance the THz emitted fields and reduce the use of thick Pt layers to reduce optical absorption, and the use of semi-metals to increase the spin polarization and, thus, THz emission. Through these approaches, significant enhancements of the THz field can be achieved. Importantly, taking into account the optical absorption permits to elucidate novel phenomena such as the relation between the spin diffusion length and the spin-sink using THz spectroscopy, as well as possibly distinguishing between self- and interface-spin-to-charge conversion in semi-metals.

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The helicity of three‐dimensional (3D) topological insulator surface states has drawn significant attention in spintronics owing to spin‐momentum locking where the carriers' spin is oriented perpendicular to their momentum. This property can provide an efficient method to convert charge currents into spin currents, and vice‐versa, through the Rashba–Edelstein effect. However, experimental signatures of these surface states to the spin‐charge conversion are extremely difficult to disentangle from bulk state contributions. Here, spin‐ and angle‐resolved photo‐emission spectroscopy, and time‐resolved THz emission spectroscopy are combined to categorically demonstrate that spin‐charge conversion arises mainly from the surface state in Bi1 − xSbx ultrathin films, down to few nanometers where confinement effects emerge. This large conversion efficiency is correlated, typically at the level of the bulk spin Hall effect from heavy metals, to the complex Fermi surface obtained from theoretical calculations of the inverse Rashba–Edelstein response. Both surface state robustness and sizeable conversion efficiency in epitaxial Bi1 − xSbx thin films bring new perspectives for ultra‐low power magnetic random‐access memories and broadband THz generation.