Ultrafast spin-currents and charge conversion at 3d-5d interfaces probed by time-domain terahertz spectroscopy

Abstract

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.

Modeling ultrafast demagnetization and spin transport: the interplay of spin-polarized electrons and thermal magnons

“…(45). This is in agreement with the results of the previous section because Pt has a very short spin-flip scattering time of τ s,N ∼ 0.02 ps [60,61]. In the opposite case, if the receiving material displays inefficient spin-flip scattering, other relations may arise.…”

“…Despite the difficulty of observing the behavior of Eq. ( 47), the analysis emphasizes that by modifying the properties of the nonmagnetic material the bandwidth of the spin current can be tuned [3,60]. Although compositions other than Ni/Cu might not yield the ideal comparison as in Figs.…”

Optical damage limit of efficient spintronic THz emitters

“…Besides the fact that the spin Hall angle in Ta (all phases) is opposite in sign to that in the conventionally used Pt layer, depending on the phase, its spin Hall angle and resistivity both span over a large range ( Kim et al., 2015 ; Bansal et al., 2017 ; Kumar et al., 2018 ; Niimi and Otani, 2015 ). The THz generation efficiency through the ISHE is directly proportional to the spin Hall angle, spin-mixing conductance, and spin diffusion length, while it is inversely proportional to the resistivity of the material layer ( Dang et al., 2020 ; Seifert et al., 2016 ). All these parameters need to be optimized so as to constitute a correct figure of merit of the most efficient spintronic THz emitter.…”

“…Another highlight from Figure 5 A is that for the same α-phase Ta as the NM layer in the heterostructures, the THz signal from Fe(3)/Ta(3) is stronger by ∼300% than that in CoFeB(3)/Ta(3). This observation indicates that the spin injection efficiency in the CoFeB(3)/Ta(3) might have got severely affected by the seemingly larger crystalline mismatch and roughness at the FM/NM interface, both of which essentially affect the spin mixing conductance and the spin-Hall angle at the interface ( Dang et al., 2020 ).…”

Spin to charge conversion at Rashba-split SrTiO$_3$ interfaces from resonant tunneling

“…The interconversion between spin and charge currents mediated by Rashba interactions 1 , observed in a wide range of systems (Ag/Bi interfaces 2 , SrTiO 3 two-dimensional electron gases (STO 2DEGs) [3][4][5][6][7][8] CoFeB/MgO 9 , Fe/Ge, α-Sn 2,10 ), offers huge technological opportunities from spin orbit torque and magnetic commutation to THz wave generation [11][12][13][14][15] . In Rashba systems, a flow of charge results in an out-of equilibrium spin density arising from the uncompensated spin-texture at the Fermi surfaces, the so-called Rashba-Edelstein effect (REE) [16][17][18] .…”