Recent demonstrations of magnetization switching induced by in-plane current in heavy metal/ferromagnetic heterostructures (HMFHs) have drawn great attention to spin torques arising from large spin-orbit coupling (SOC). Given the intrinsic strong SOC, topological insulators (TIs) are expected to be promising candidates for exploring spin-orbit torque (SOT)-related physics. Here we demonstrate experimentally the magnetization switching through giant SOT induced by an in-plane current in a chromium-doped TI bilayer heterostructure. The critical current density required for switching is below 8.9 × 10(4) A cm(-2) at 1.9 K. Moreover, the SOT is calibrated by measuring the effective spin-orbit field using second-harmonic methods. The effective field to current ratio and the spin-Hall angle tangent are almost three orders of magnitude larger than those reported for HMFHs. The giant SOT and efficient current-induced magnetization switching exhibited by the bilayer heterostructure may lead to innovative spintronics applications such as ultralow power dissipation memory and logic devices.

Breaking of structural symmetries of nanomagnetic systems is of great interest for the development of ultralow-power spintronic devices. The structural asymmetry in various magnetic heterostructures has been engineered to reveal novel fundamental interactions between electric currents and magnetization, resulting in spin-orbit-torques (SOTs) on the magnetization [1][2][3][4][5][6] , which are both fundamentally important and technologically promising for device applications. Such SOTs have been used to realize current-induced magnetization switching [2][3][4]7 and domain-wall 3 motion [8][9][10] in recent experiments. Typical heterostructures exhibiting SOTs consist of a ferromagnet (F) with a heavy nonmagnetic metal (NM) having strong spin-orbit coupling on one side, and an insulator (I) on the other side (referred to as NM/F/I structures, shown schematically in Fig. 1a, which break mirror symmetry in the growth direction). In terms of device applications, the use of SOTs in NM/F/I structures allows for a significantly lower write current compared to regular spin-transfer-torque (STT) devices 4 . It can greatly improve energy efficiency and scalability [1][2][3][4][5]11 for new SOT-based devices such as magnetic random access memory (SOT-MRAM), going beyond state-of-the-art STT-MRAM.For practical applications, a critical requirement to achieve high-density SOT memory is the ability to perform SOT-induced switching without the use of external magnetic fields, in particular for perpendicularly-magnetized ferromagnets, which show better scalability and thermal stability as compared to the in-plane case 12 .However, there are currently no practical solutions that meet this requirement. In NM/F/I heterostructures studied so far, the form of the resultant current-induced SOT alone does not allow for deterministic switching of a perpendicular ferromagnet, requiring application of an additional external in-plane magnetic field to switch the perpendicular magnetization [2][3][4] . (This is a very general feature of SOT devices, which can be explained by symmetry-based arguments, as discussed below). In such experiments, the external field allows for each current direction to favor a particular orientation for the out-of-plane component of magnetization, thereby resulting in deterministic perpendicular switching. However, this external field is undesirable 4 from a practical point of view. For device applications, it also reduces the thermal stability of the perpendicular magnet by lowering the zero-current energy barrier between the stable perpendicular states, resulting in a shorter retention time if used for memory.This work provides a solution to eliminate the use of external magnetic fields, bringing SOT-based spintronic devices such as SOT-MRAM closer to practical application. We present a new NM/F/I structure, which provides a novel spin-orbit torque, resulting in zero-field current-induced switching of perpendicular magnetization. Our device consists of a stack of Ta/Co 20 Fe 60 B 20 /TaO x layers, but also has a...

Topological insulators display unique properties, such as the quantum spin Hall effect, because time-reversal symmetry allows charges and spins to propagate along the edge or surface of the topological insulator without scattering. However, the direct manipulation of these edge/surface states is difficult because they are significantly outnumbered by bulk carriers. Here, we report experimental evidence for the modulation of these surface states by using a gate voltage to control quantum oscillations in Bi(2)Te(3) nanoribbons. Surface conduction can be significantly enhanced by the gate voltage, with the mobility and Fermi velocity reaching values as high as ~5,800 cm(2) V(-1) s(-1) and ~3.7 × 10(5) m s(-1), respectively, with up to ~51% of the total conductance being due to the surface states. We also report the first observation of h/2e periodic oscillations, suggesting the presence of time-reversed paths with the same relative zero phase at the interference point. The high surface conduction and ability to manipulate the surface states demonstrated here could lead to new applications in nanoelectronics and spintronics.

We investigate the quantum anomalous Hall Effect (QAHE) and related chiral transport in the millimetersize (Cr 0.12 Bi 0.26 Sb 0.62 ) 2 Te 3 films. With high sample quality and robust magnetism at low temperatures, the quantized Hall conductance of e 2 /h is found to persist even when the film thickness is beyond the twodimensional (2D) hybridization limit. Meanwhile, the Chern insulator-featured chiral edge conduction is manifested by the non-local transport measurements. In contrast to the 2D hybridized thin film, an additional weakly field-dependent longitudinal resistance is observed in the 10 quintuple-layer film, suggesting the influence of the film thickness on the dissipative edge channel in the QAHE regime. The extension of QAHE into the three-dimensional thickness region addresses the universality of this quantum transport phenomenon and motivates the exploration of new QAHE phases with tunable Chern numbers.In addition, the observation of the scale-invariant dissipationless chiral propagation on a macroscopic scale makes a major stride towards ideal low-power interconnect applications.

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