Magnetic devices are a leading contender for the implementation of memory and logic technologies that are non-volatile, that can scale to high density and high speed, and that do not wear out. However, widespread application of magnetic memory and logic devices will require the development of efficient mechanisms for reorienting their magnetization using the least possible current and power. There has been considerable recent progress in this effort; in particular, it has been discovered that spin-orbit interactions in heavy-metal/ferromagnet bilayers can produce strong current-driven torques on the magnetic layer, via the spin Hall effect in the heavy metal or the Rashba-Edelstein effect in the ferromagnet. In the search for materials to provide even more efficient spin-orbit-induced torques, some proposals have suggested topological insulators, which possess a surface state in which the effects of spin-orbit coupling are maximal in the sense that an electron's spin orientation is fixed relative to its propagation direction. Here we report experiments showing that charge current flowing in-plane in a thin film of the topological insulator bismuth selenide (Bi2Se3) at room temperature can indeed exert a strong spin-transfer torque on an adjacent ferromagnetic permalloy (Ni81Fe19) thin film, with a direction consistent with that expected from the topological surface state. We find that the strength of the torque per unit charge current density in Bi2Se3 is greater than for any source of spin-transfer torque measured so far, even for non-ideal topological insulator films in which the surface states coexist with bulk conduction. Our data suggest that topological insulators could enable very efficient electrical manipulation of magnetic materials at room temperature, for memory and logic applications.

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The growth and elementary properties of p-type Bi 2 Se 3 single crystals are reported. Based on a hypothesis about the defect chemistry of Bi 2 Se 3 , the p-type behavior has been induced through low level substitutions (1% or less) of Ca for Bi. Scanning tunneling microscopy is employed to image the defects and establish their charge. Tunneling and angle resolved photoemission spectra show that the Fermi level has been lowered into the valence band by about 400 meV in Bi 1.98 Ca 0.02 Se 3 relative to the n-type material. p-type single crystals with ab plane Seebeck coefficients of +180 µV/K at room temperature are reported. These crystals show a giant anomalous peak in the Seebeck coefficient at low temperatures, reaching +120 µVK -1 at 7 K, giving them a high thermoelectric power factor at low temperatures. In addition to its interesting thermoelectric properties, p-type Bi 2 Se 3 is of substantial interest for studies of technologies and phenomena proposed for topological insulators.2

Understanding and control of spin degrees of freedom on the surfaces of topological materials are the key to future applications as well as for realizing novel physics such as the axion electrodynamics associated with time-reversal symmetry breaking on the surface. We experimentally demonstrate magnetically induced spin reorientation phenomena simultaneous with a Dirac-metal to gapped-insulator transition on the surfaces of manganese-doped Bi 2 Se 3 thin films. The resulting electronic groundstate exhibits unique hedgehog-like spin textures at low energies which directly demonstrates the mechanics of timereversal symmetry breaking on the surface. We further show that an insulating gap induced by quantum tunneling between surfaces exhibits spin texture modulation at low energies but respects time-reversal invariance. These spin phenomena and the control of their Fermi surface geometrical phase first demonstrated in our experiments pave the way for future realization of many predicted exotic magnetic phenomena of topological origin.Since the discovery of three dimensional topological insulators [1][2][3][4][5], topological order proximity to ferromagnetism has been considered as one of the core interests of the field [6][7][8][9][10][11][12][13][14][15][16]. Such interest is strongly motivated by the proposed time-reversal (TR) breaking topological physics such as quantized anomalous chiral Hall current, spin current, axion electrodynamics, and inverse spin-galvanic effect [9][10][11][12], all of which critically rely on finding a way to break TR symmetry on the surface and utilize the unique TR broken spin texture for applications. Since quantum coherence is essential in many of these applications, devices need to be engineered into thin films in order to enhance or de-enhance surface-tosurface coupling or the quantum tunneling of the electrons. The experimental spin behavior of surface states under the two extreme limits, namely the doped magnetic groundstate and ultra-thin film quantum tunneling groundstate, is thus of central importance to the entire field. However, surprisingly, it is not known what happens to the spin configuration under these extreme conditions relevant for device fabrications. Fundamentally, TR symmetry is inherently connected to the Kramers' degeneracy theorem which states that when TR symmetry is preserved, the electronic states at the TR invariant momenta have to remain doubly spin degenerate. Therefore, the establishment of TR breaking effect fundamentally requires measurements of electronic groundstate with a spin-sensitive probe. Here we utilize spin-3 resolved angle-resolved photoemission spectroscopy to measure the momentum space spin configurations in systematically magnetically doped, non-magnetically doped, and ultra-thin quantum coherent topological insulator films [17], in order to understand the nature of electronic groundstates under two extreme limits vital for magnetic topological devices. These measurements allow us to make definitive conclusions regarding magnetism on to...

Recent studies on the magneto-transport properties of topological insulators (TI) 1-7 have attracted great attention due to the rich spin-orbit physics and promising applications in spintronic devices. Particularly the strongly spinmoment coupled electronic states have been extensively pursued to realize efficient spin-orbit torque (SOT) switching. However, so far current-induced magnetic switching with TI has only been observed at cryogenic temperatures. It remains a controversial issue whether the topologically protected electronic states in TI could benefit spintronic applications at room temperature. In this work, we report full SOT switching in a TI/ferromagnet bilayer heterostructure with perpendicular magnetic anisotropy at room temperature. The low switching current density provides a definitive proof on the high SOT efficiency from TI. The effective spin Hall angle of TI is determined to be several times larger than commonly used heavy metals. Our results demonstrate the robustness of TI as an SOT switching material and provide a direct avenue towards applicable TI-based spintronic devices.Spin-orbit coupling has been extensively studied for the conversion between charge current and spin current 8 . When neighbored with a ferromagnet (FM), non-equilibrium spins induced by the spin-orbit coupling can exert torques onto magnetic moments (Fig.

The discovery of ferromagnetism in Mn doped

Electronic states in disordered conductors on the verge of localization are predicted to exhibit critical spatial characteristics indicative of the proximity to a metal-insulator phase transition. We have used scanning tunneling microscopy to visualize electronic states in Ga 1-x Mn x As samples close to this transition. Our measurements show that doping-induced disorder produces strong spatial variations in the local tunneling conductance across a wide range of energies. Near the Fermi energy, where spectroscopic signatures of electron-electron interaction are the most prominent, the electronic states exhibit a diverging spatial correlation length. Power-law decay of the spatial correlations is accompanied by log-normal distributions of the local density of states and multifractal spatial characteristics. Our method can be used to explore critical correlations in other materials close to a quantum critical point.Since Anderson first proposed that disorder could localize electrons in solids fifty years ago (1), studies of the transition between extended and localized quantum states have been at the forefront of physics (2). Realizations of Anderson localization occur in a wide range of physical systems from seismic waves to ultracold atomic gases, in

We report the observation of ferromagnetic resonance-driven spin pumping signals at room temperature in three-dimensional topological insulator thin films -Bi 2 Se 3 and (Bi,Sb) 2 Te 3 -deposited by molecular beam epitaxy on Y 3 Fe 5 O 12 thin films. By systematically varying the Bi 2 Se 3 film thickness, we show that the spin-charge conversion efficiency, characterized by the inverse RashbaEdelstein effect length (λ IREE ), increases dramatically as the film thickness is increased from 2 quintuple layers, saturating above 6 quintuple layers. This suggests a dominant role of surface states in spin and charge interconversion in topological insulator/ferromagnet heterostructures.Our conclusion is further corroborated by studying a series of Y 3 Fe 5 O 12 /(Bi,Sb) 2 Te 3 heterostructures. Finally, we use the ferromagnetic resonance linewidth broadening and the inverse RashbaEdelstein signals to determine the effective interfacial spin mixing conductance and λ IREE .