Strongly Surface State Carrier‐Dependent Spin–Orbit Torque in Magnetic Topological Insulators

Abstract

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...

Magnetic memory driven by topological insulators

“…Quantum materials such as topological insulators (TIs) inspire a promising route to overcome the limitation of charge-spin conversion efficiency θ SH = J s / J e < 1 in classical materials 22 – 34 , where J s and J e represent the spin current density and charge current density, respectively. In TIs, the topological surface states give rise to a large θ SH due to the spin-momentum locking of the surface Dirac electrons, where the bulk is insulating in the ideal case 25 , 27 , 35 . A great interest has been focused on the SOT in TI/FM bilayer structures, in which θ SH is found to be 1-2 orders of magnitude larger than that in HMs even at room temperature 28 – 33 .…”

Spin‐Orbit Torque in Van der Waals‐Layered Materials and Heterostructures

“…This strong temperature dependence of SOT in TSS was also recently demonstrated in TI/magnetic‐TI by transport and optical methods. [ 109 ] The maximum SOT effective field was obtained at a low temperature of 2.5 K, and the value drastically decreased with increasing temperature (Figure 7d ). Such temperature evolution is attributed to a higher spin generation efficiency in the TSS at a lower temperature.…”

“…Inset: Zoomed‐in figure of data points at 12 and 17 K. Reproduced with permission. [ 109 ] Copyright 2020, Wiley‐VCH.…”

“…Thus far, high SOT efficiencies have been observed in various TIs, including Bi 2 Se 3 , Bi x Se 1− x , (Bi 0.5 Sb 0.5 ) 2 Te 3 . [ 30 , 101 , 102 , 103 , 104 , 105 , 109 , 110 , 114 , 115 , 116 , 117 , 118 , 119 ] The largest SOT efficiency has been obtained in (Bi 0.5 Sb 0.5 ) 2 Te 3 , which is in excess of 400 at low temperatures. [ 114 ] However, the reported SOT efficiencies show large discrepancies.…”

Generation and manipulation of current-induced spin-orbit torques

“…This is the foundation of multiple different techniques developed to quantify the spin-orbit torques. 11 , 12 , 16 , 62 – 67 ) Among them, the spin-torque ferromagnetic resonance (ST-FMR) is widely used to quantify the spin-orbit torques in various systems. 11 , 12 )…”