Spin-orbit torques in topological insulator - two-dimensional ferromagnet heterostructures

Abstract

Topological insulators (TI) have gained much interest in the field of spintronics for the generation of pure spin currents. Indeed, three-dimensional TIs are predicted to host exotic properties like topologically protected surface states (TSS), which show Dirac-like band dispersion and spin-momentum locking [1]. One of the main strategies is to take advantages of the spin polarization of the TSSs to manipulate the magnetization of an adjacent ferromagnetic thin film (FM) using the spin-orbit torque (SOT) mechanism [2]. In the past few years, the community attempted to replace the traditional heavy metals by a TI in order to enhance the SOT efficiency with limited success. It now appears that the interface sharpness and the high chemical affinity between Bi-based TIs and classical 3d FMs is a major hurdle to reach the predicted breakthrough in magnetization switching power-efficiency [3]. The emergence of ferromagnetism in two dimensions in 2017, which started a new field in condensed matter physics, could bring a solution to this issue. The van der Waals (vdW) nature of the interaction between the TI and the 2D-FM should limit chemical reactions, interface intermixing and hybridization of state between the two layers.Our approach focuses on stacking and growing 2D-FM (Fe3GeTe2) onto high-quality compensated TIs ((Bi0.4Sb0.6 )2Te3) grown by molecular beam epitaxy. We use both low and high field second harmonic magnetotransport measurement to characterize the SOT generation, finding a strong temperature dependence of the harmonic signals. We also reports the SOT current-induced magnetization switching operation at low critical current density (Jc < 1010 A/m2) in the vdW heterostructure.  Fig.1: Second harmonic spin-orbit torque measurement in BST/FGT bilayers. a) Measurement geometry. b) Anomalous Hall effect (AHE) recorded by applying the magnetic field along z. c) AHE recorded by sweeping the field along y. d) Damping-like torque second harmonic measurements. e) Field-like torque second harmonic measurements. The sample temperature is 70 K and the applied current density is 1.3 × 1010 A.m-2.