Control of Spin–Orbit Torques by Interface Engineering in Topological Insulator Heterostructures

Abstract

Bi 1−x Sb x) 2 Te 3 topological insulators (TIs) are gathering increasing attention owing to their large charge-to-spin conversion efficiency and the ensuing spin-orbit torques (SOTs) that can be used to manipulate the magnetization of a ferromagnet (FM). The origin of the torques, however, remains elusive, while the implications of hybridized 1 arXiv:2009.08215v1 [cond-mat.mes-hall] 17 Sep 2020 states and the strong material intermixing at the TI/FM interface are essentially unexplored. By combining interface chemical analysis and spin-transfer ferromagnetic resonance (ST-FMR) measurements, we demonstrate that intermixing plays a critical role in the generation of SOTs. By inserting a suitable normal metal spacer, material intermixing is reduced and the TI properties at the interface are largely improved, resulting in strong variations in the nature of the SOTs. A dramatic enhancement of a field-like torque, opposing and surpassing the Oersted-field torque, is observed, which can be attributed to the non-equilibrium spin density in Rashba-split surface bands and to the suppression of spin memory loss.

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

“…Motivated by these unique advantages, experimental studies began to focus on current-induced SOT in various TI/FM heterostructures. [101][102][103][104][105][106][107][108] Mellnik et al first studied currentinduced SOT in Bi 2 Se 3 /Py heterostructure by ST-FMR (Figure 6c). [30] As shown in Figure 6d, the ST-FMR signal exhibited a large symmetric peak, which suggests a considerable conventional damping-like SOT DL .…”

“…Therefore, different SOTs can be tuned via interface engineering. [108] In the heterostructures of high-resistive TI and low-resistive metal magnets, most of the electrical current is shunted through the magnetic metal layer, which leads to a weak SOT strength. By replacing the metal magnet with a magnetic insulator, the SOT strength can be strongly enhanced.…”

“…e,f) Reproduced with permission. [108] Copyright 2020, American Chemical Society. In all the rotation experiments, the B ext field magnitude is fixed at 2 T and the temperature is kept at 1.9 K. c) Damping-like SOT effective field with magnetic field angle in the yz-plane.…”

“…Motivated by these unique advantages, experimental studies began to focus on current‐induced SOT in various TI/FM heterostructures. [ 101 , 102 , 103 , 104 , 105 , 106 , 107 , 108 ] Mellnik et al. first studied current‐induced SOT in Bi 2 Se 3 /Py heterostructure by ST‐FMR (Figure 6c ).…”

Spin-Orbit Torques in Transition Metal Dichalcogenide/Ferromagnet Heterostructures

“…Crosssectional high-angle annular dark-field scanning transmission electron microscopy (HAADF-STEM) has indicated intermixing at the TaTe 2 /Py interface which is likely to affect the effective SOTs due to a change in the local electronic environment and the spin mixing conductance of the interface. Interestingly, a change in the SOTs in topological-insulator/ferromagnet devices due to intermixing at the interface has been recently reported (Bonell et al, 2020). Here we point out that in addition to the changes in the SOTs arising from the different electronic structures for devices using different FM layers (e.g., Py, Co, CoFeB), the materials intermixing should also be carefully considered and potentially quantified in order to obtain a more in-depth understanding of the microscopic mechanisms involved.…”