Large spin current generation by the spin Hall effect in mixed crystalline phase Ta thin films

Abstract

Manipulation of the magnetization in heavy-metal/ferromagnetic bilayers via the spin-orbit torque requires high spin Hall conductivity of the heavy metal. We measure inverse spin Hall voltage using a co-planar wave-guide based broadband ferromagnetic resonance set-up in Py/Ta system with varying crystalline phase of Ta. We demonstrate a strong correlation between the measured spin mixing conductance and spin Hall conductivity with the crystalline phase of Ta thin films. We found a large spin Hall conductivity of −2439 ( /e) Ω −1 cm −1 for low-resistivity (68 µΩ-cm) Ta film having mixed crystalline phase, which we attribute to an extrinsic mechanism of the spin Hall effect.

Optical damage limit of efficient spintronic THz emitters

“…In addition to the opposite polarity THz signal, we also find much lower THz generation efficiency of the CoFeB/Ta than the CoFeB/Pt bilayers ( Figures 1 J and 1K). Both of these effects can be attributed to the opposite sign and much smaller spin-Hall angle, respectively, in the α-phase Ta as compared to Pt ( Kumar et al., 2018 ; Sinova et al., 2015 ).…”

“…It is noteworthy to note from the results discussed above that the demarcation line for the thickness of the α-phase Ta layer below which it can be used as a capping or buffer layer is ∼2 nm. However, it may vary depending on the different phases of the Ta ( Kumar et al., 2018 ). 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 ).…”

“…However, it may vary depending on the different phases of the Ta ( Kumar et al., 2018 ). 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 ).…”

“…The spin mixing conductance is a measure of the spin current injected from the FM layer into the NM layer. Assuming a simple precession motion, the spin current density is related to the spin mixing conductance through a relation given by ( Kumar et al., 2018 ; Mosendz et al., 2010 ): , where, are the electronic charge, the angular frequency of spin dynamics, the real part of spin mixing conductance, and the magnetization precession angle, respectively. Therefore, a higher value of ensures an enhanced THz emission through Equation (1) .…”

“…Moreover, the FM/NM spintronic heterostructures containing α-phase Ta have not been studied much in the literature. The α-phase of Ta is more metallic than the β-phase and hence can be better suited for the THz applications though it has comparatively a slightly smaller spin-Hall coefficient ( Kumar et al., 2018 ). The experimentally observed small relative change in the THz signal from CoFeB/Pt as compared to Fe/Pt ( Figure 5 A) seems consistent with the corresponding values of the .…”