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.

Electric-field manipulation of magnetic order has proved of both fundamental and technological importance in spintronic devices. So far, electric-field control of ferromagnetism, magnetization and magnetic anisotropy has been explored in various magnetic materials, but the efficient electric-field control of spin-orbit torque (SOT) still remains elusive. Here, we report the effective electric-field control of a giant SOT in a Cr-doped topological insulator (TI) thin film using a top-gate field-effect transistor structure. The SOT strength can be modulated by a factor of four within the accessible gate voltage range, and it shows strong correlation with the spin-polarized surface current in the film. Furthermore, we demonstrate the magnetization switching by scanning gate voltage with constant current and in-plane magnetic field applied in the film. The effective electric-field control of SOT and the giant spin-torque efficiency in Cr-doped TI may lead to the development of energy-efficient gate-controlled spin-torque devices compatible with modern field-effect semiconductor technologies.

Strong spin−orbit interaction and time-reversal symmetry in topological insulators enable the spin-momentum locking for the helical surface states. To date, however, there has been little report of direct electrical spin injection/ detection in topological insulator. In this Letter, we report the electrical detection of spin-polarized surface states conduction using a Co/Al 2 O 3 ferromagnetic tunneling contact in which the compound topological insulator (Bi 0.53 Sb 0.47 ) 2 Te 3 was used to achieve low bulk carrier density. Resistance (voltage) hysteresis with the amplitude up to about 10 Ω was observed when sweeping the magnetic field to change the relative orientation between the Co electrode magnetization and the spin polarization of surface states. The two resistance states were reversible by changing the electric current direction, affirming the spin-momentum locking in the topological surface states. Angle-dependent measurement was also performed to further confirm that the abrupt change in the voltage (resistance) was associated with the magnetization switching of the Co electrode. The spin voltage amplitude was quantitatively analyzed to yield an effective spin polarization of 1.02% for the surface states conduction in (Bi 0.53 Sb 0.47 ) 2 Te 3 . Our results show a direct evidence of spin polarization in the topological surface states conduction. It might open up great opportunities to explore energy-efficient spintronic devices based on topological insulators. KEYWORDS: Topological insulator, spin polarization, surface states, spin-momentum locking, spin detection S ince the discovery of two-dimensional (2D) and threedimensional (3D) topological insulators (TIs), 1−5 they have attracted extensive research interest for their exotic physical properties that could lead to dissipationless transport in the quantum spin Hall state. 6−9 Recent studies have shown a giant spin−orbit torque in TI originating from the strong spin− orbit interaction, 10,11 which enabled the current-induced magnetization switching through spin-transfer torque with a low current density. The unique feature of 3D TI, for instance, is that it has both insulating bulk and gapless Dirac surface states. 8,9 Ternary TI compounds, such as (Bi x Sb 1−x ) 2 Te 3 , have been widely investigated for their tunability to achieve low bulk carrier density and manifest topological surface states conduction. 12,13 The presence of surface states is supported by extensive angle-resolved photoemission spectroscopy (ARPES) measurements and transport studies, 14−20 such as Shubnikov-de Haas (SdH) and Aharonov Bohm (AB) quantum oscillations. Because of the strong spin−orbital interaction in TI, direct back scatterings from nonmagnetic impurities are prohibited by the time-reversal symmetry. 8,9 More importantly, the spin-momentum locking naturally leads to a currentinduced spin polarization in surface states; 21 the surface states conduction is spin-polarized once an electric current is passed through a TI film, and this spin polarization can be...

We use time-resolved photoluminescence from single InP nanowires containing both wurtzite (WZ) and zincblende (ZB) crystalline phases to measure the carrier dynamics of quantum confined excitons in a type-II homostructure. The observed recombination lifetime increases by nearly 2 orders of magnitude from 170 ps for excitons above the conduction and valence band barriers to more than 8400 ps for electrons and holes that are strongly confined in quantum wells defined by monolayer-scale ZB sections in a predominantly WZ nanowire. A simple computational model, guided by detailed high-resolution transmission electron microscopy measurements from a single nanowire, demonstrates that the dynamics are consistent with the calculated distribution of confined states for the electrons and holes.

Introducing magnetic order in a topological insulator (TI) breaks time-reversal symmetry of the surface states and can thus yield a variety of interesting physics and promises for novel spintronic devices. To date, however, magnetic effects in TIs have been demonstrated only at temperatures far below those needed for practical applications. In this work, we study the magnetic properties of Bi2Se3 surface states (SS) in the proximity of a high Tc ferrimagnetic insulator (FMI), yttrium iron garnet (YIG or Y3Fe5O12). Proximity-induced butterfly and square-shaped magnetoresistance loops are observed by magneto-transport measurements with out-of-plane and in-plane fields, respectively, and can be correlated with the magnetization of the YIG substrate. More importantly, a magnetic signal from the Bi2Se3 up to 130 K is clearly observed by magneto-optical Kerr effect measurements. Our results demonstrate the proximity-induced TI magnetism at higher temperatures, an important step toward room-temperature application of TI-based spintronic devices.

No abstract

We have studied core-shell nanowires with widely different core and shell lattice constants, namely GaAs core with GaP shell nanowires. We use Raman scattering to probe strain in these nanowires and relate this strain to the observed photoluminescence. The Raman and photoluminescence measurements are in agreement with 8 band k.p calculations for these structures. These results suggest that the electronic properties of core-shall nanowires can be tuned over a wide range by using the strain generated by growing core/shell materials with different lattice constants and different core/shell thicknesses.

InP nanowires were grown on 111B InP substrates by metal-organic chemical vapour deposition in the presence of colloidal gold particles as catalysts. Transmission electron microscopy and photoluminescence measurements were carried out to investigate the effects of V/III ratio and nanowire diameter on structural and optical properties. Results show that InP nanowires grow preferably in the wurtzite crystal structure than the zinc blende crystal structure with increasing V/III ratio or decreasing diameter. Additionally, time-resolved photoluminescence (TRPL) studies have revealed that wurtzite nanowires show longer recombination lifetimes of approximately 2500 ps with notably higher quantum efficiencies.