charge-spin conversion and ii) the speed of SOT switching.Generally, SOT in a magnetic layer originates from the spin current injection from the adjacent layer with strong spin-orbit coupling (SOC). The chargespin conversion efficiency is vital and can be quantified as the θ = / SHE s 3D e 3D J J (dimensionless) or q J J t θ = = / / ICS s 3D e 2D SHE s , where s 3D J represents the 3D spin current density; e 3D J and e 2D
matter arising from spin-orbit interaction that is strong enough to change the band topology. [8][9][10][11][12][13] The polarized spins on the topological surface states (TSS) in TIs are topologically protected from nonmagnetic backscattering by time-reversal symmetry, offering great potential of larger SOT and higher switching efficiency. [14][15][16][17][18][19] However, though various techniques such as spin-torque ferromagnetic resonance (ST-FMR), [14,20] spin pumping, [21] and second harmonic measurement [15,22] have been used to determine the spin Hall angle θ SH in TIs, there are still large discrepancies among the reported values of θ SH . Such discrepancies exist even when the same measurement setup is employed for different TI samples. [23] Besides, a fundamental difference between TIs and heavy metals is that the spin polarization in TIs arises from the topologically protected TSS, which is reported to be quite temperature-sensitive, [24][25][26] but the temperature dependence of SOT from TSS still remains unclear.In this work, we comprehensively determined θ SH in a Crdoped Bi x Sb 2-x Te 3 /undoped Bi x Sb 2-x Te 3 (Cr-BST/BST) bilayer structure by both hysteresis loop shift measurement and SOT magnetometer based on the magneto-optic Kerr effect (MOKE), where consistent results were obtained by these two approaches. Moreover, θ SH from hysteresis loop shift measurements exhibits a drastic increase as the temperature decreases when the temperature is below 12 K. The sharp increase of θ SH could be attributed to a higher spin polarization ratio in TSS at lower temperatures. Measurement of SOT was also carried out in a uniformly doped Cr-BST film, where the devices were fabricated into a field-effect transistor (FET) geometry to tune the top TSS Fermi level (E F ). While the carrier concentration of the top TSS is tuned by electrostatic gating, the current-induced SOT effective field can be manipulated, revealing a competition between the top TSS and bottom TSS in the contribution to SOT. The strongly temperature-and carrier concentration-dependent SOT as well as the competition between the top and bottom TSS may help elucidate the wide discrepancies among the reported values of θ SH .The modulation-doped Cr-BST/BST bilayer film illustrated in Figure 1a was grown on an insulating GaAs (111) B substrate by molecular beam epitaxy (MBE). The bottom layer of the TI film is not magnetically doped and the thickness is 2 quintuple layers (QLs), while the top layer is doped with Cr and has a thickness of 5 QLs. When a DC current passes through the The topological surface states (TSS) in topological insulators (TIs) can exert strong spin-orbit torque (SOT) on adjacent magnetization, offering great potential in implementing energy-efficient magnetic memory devices. However, there are large discrepancies among the reported spin Hall angle values in TIs, and its temperature dependence still remains elusive. Here, the spin Hall angle in a modulation-doped Cr-Bi x Sb 2−x Te 3 (Cr-BST) film is quantitatively determin...
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