Strongly Surface State Carrier-Dependent Spin–Orbit Torque in Magnetic Topological Insulators

The topological surface states (TSS) in topological insulators (TIs) can exert strong spin–orbit torque (SOT) on adjacent magnetization, offering great potential in implementing energy-efficient magnetic memory devices. However, there are large discrepancies among the reported spin Hall angle values in TIs, and its temperature dependence still remains elusive. Here, the spin Hall angle in a modulation-doped Cr-Bi_xSb_2−xTe₃ (Cr-BST) film is quantitatively determined via both transport and optic approaches, where consistent results are obtained. A large spin Hall angle of ≈90 in the modulation-doped Cr-BST film is demonstrated at 2.5 K, and the spin Hall angle drastically decreases to 0.3–0.5 as the temperature increases. Moreover, by tuning the top TSS carrier concentration, a competition between the top and bottom TSS in contributing to SOT is observed. The above phenomena can account for the large discrepancies among the previously reported spin Hall angle values and reveal the unique role of TSS in generating SOT.     

Current-Induced Spin–Orbit Torques for Spintronic Applications

Control of magnetization in magnetic nanostructures is essential for development of spintronic devices because it governs fundamental device characteristics such as energy consumption, areal density, and operation speed. In this respect, spin–orbit torque (SOT), which originates from the spin–orbit interaction, has been widely investigated due to its efficient manipulation of the magnetization using in-plane current. SOT spearheads novel spintronic applications including high-speed magnetic memories, reconfigurable logics, and neuromorphic computing. Herein, recent advances in SOT research, highlighting the considerable benefits and challenges of SOT-based spintronic devices, are reviewed. First, the materials and structural engineering that enhances SOT efficiency are discussed. Then major experimental results for field-free SOT switching of perpendicular magnetization are summarized, which includes the introduction of an internal effective magnetic field and the generation of a distinct spin current with out-of-plane spin polarization. Finally, advanced SOT functionalities are presented, focusing on the demonstration of reconfigurable and complementary operation in spin logic devices.     

Vortex–Antivortex-Pair Lattices in Spin–Orbit Coupled Bose–Einstein Condensates

We investigate theoretically the ground states of Bose–Einstein condensates with Rashba spin–orbit coupling in optical lattices within mean-field framework. We obtain numerically the Bloch states and energy spectrum for the single particle Hamiltonian, meanwhile the analytical solution of Bloch states is also presented. For a spin–orbit coupling Bose–Einstein condensates with a weak interaction, we show the existence of the vortex–antivortex-pair lattices state by simulating the Gross–Pitaevskii equation.     

Spin Manipulation Using Magnetic II–VI Semiconductors

Recently, efficient spin injection, being the first step towards semiconductor spin electronics, by using BeMnZnSe as a spin filter was accomplished. Such a spin filter made it possible to align the spin orientation of conduction electrons and subsequently inject them into GaAs. However, controlling spin orientation of conduction electrons by an external voltage would be very desirable for semiconductor-based magnetoelectronics. This can be accomplished by using spin switch structures, based on resonant tunneling through magnetic quantum wells, with two separate spin-up and spin-down resonances. Here we summarize both our recent results on spin injection as well as on spin aligner and magnetic resonant tunneling structures. For accomplishing the latter, we have developed magnetic resonant tunneling diodes based on BeTe–ZnMnSe–BeTe structures. Resonant tunneling diode is meant to serve as a spin switch because of the existence of two separate spin-up and spin-down resonances. The tunneling carriers have subsequently been injected into a nonmagnetic GaAs p–i–n light emitting diode. Circular polarization of the emitted light is an indicator of the spin polarization of injected electrons. At constant magnetic field and current, degree of spin polarization could be changed from 81% to 38% by only varying the voltage across the magnetic resonant tunneling device.     

Very Large Rashba Spin–Orbit Splitting in HgTe Quantum Wells

A large Rashba spin splitting has been observed in the first conduction subband of n-type modulation doped HgTe quantum wells with an inverted band structure via an investigation of Shubnikov–de Haas oscillations as a function of gate voltage. Self-consistent Hartree calculations of the band structure based on an 8 × 8 k p model quantitatively describe the experimental results. It has been shown that the heavy-hole nature of the H1 conduction subband greatly influences the spatial distribution of electrons in the quantum well and also enhances the Rashba spin splitting at large electron densities. These are unique features of type III heterostructures in the inverted band regime. The k ³ dispersion predicted by an analytical model is a good approximation of the self-consistent Hartree calculations for small values of the in-plane wave vector k . This is in contrast to the commonly used k dispersion for the conduction subband in type I heterojunctions.     

Enhancement of Rashba Spin–Orbit Interaction Due to Wave Function Engineering

We have demonstrated an enhancement of Rashba spin–orbit interaction (SOI) in In_0.53Ga_0.47As/In_0.7Ga_0.3As/In_0.53Ga_0.47As double-step structure in comparison with In_0.53Ga_0.47As normal quantum well. In the double-step structure, high electron probability density is located on the In_0.53Ga_0.47As/In_0.7Ga_0.3As heterointerface to enhance the interface contribution of Rashba SOI. The double-step structure is designed based on k⋅p formalism considering field and interface contributions separately. The Rashba parameter α calculated by the k⋅p formalism shows good agreement with the experimental value by analyzing weak antilocalization. The large carrier density dependence of α is due to the In_0.53Ga_0.47As/In_0.7Ga_0.3As heterointerface contribution as well as the energy-band bending in the In_0.7Ga_0.3As quantum well. The results of this study suggest that the precise control of interface and field contributions in Rashba SOI will make its application to semiconductor spintronics.     

Tailoring Bloch-type Stripe Domain Wall by Spin–orbit Torque for Reconfigurable Magnonic Waveguides

Journal of Superconductivity and Novel Magnetism - Efficient manipulation of magnetic textures by spin–orbit torque is of great significance to spintronic and magnonic technologies. Here,...     

Site-Controlled Uniform Ge/Si Hut Wires with Electrically Tunable Spin–Orbit Coupling

Semiconductor nanowires have been playing a crucial role in the development of nanoscale devices for the realization of spin qubits, Majorana fermions, single photon emitters, nanoprocessors, etc. The monolithic growth of site-controlled nanowires is a prerequisite toward the next generation of devices that will require addressability and scalability. Here, combining top-down nanofabrication and bottom-up self-assembly, the growth of Ge wires on prepatterned Si (001) substrates with controllable position, distance, length, and structure is reported. This is achieved by a novel growth process that uses a SiGe strain-relaxation template and can be potentially generalized to other material combinations. Transport measurements show an electrically tunable spin–orbit coupling, with a spin–orbit length similar to that of III–V materials. Also, charge sensing between quantum dots in closely spaced wires is observed, which underlines their potential for the realization of advanced quantum devices. The reported results open a path toward scalable qubit devices using nanowires on silicon.     

Resistive Switching of Perovskite-Type Oxides Using the Hebb–Wagner Polarization Method

Resistive switching, the change between a high-resistive OFF state and a low-resistive ON state, is well known for thin film oxides sandwiched between two ion-blocking electrodes. Herein, the possibility for resistive switching in perovskite-type oxides using the Hebb–Wagner polarization setup that uses an ion-blocking and a reversible electrode is investigated. The resistive switching behavior is simulated numerically in terms of a defect chemical and transport model that describes the polarization of the model system SrTiO₃ between two different electrodes by applying an ac voltage. Corresponding experiments are also performed and the experimental results are compared with the simulation results. It is shown that the Hebb–Wagner setup allows the bulk resistive state to change not only for thin films but also for large sample thicknesses using low maximum voltage values, without the need for initially high voltages to form filaments. The effect of other parameters and phenomena on the switching behavior, like maximum voltage, surface exchange coefficient, sample thickness, and a Schottky contact between an electrode and the semiconducting oxide, is also determined. The experimental results show a high agreement with the numerical simulations, demonstrating that the Hebb–Wagner polarization setup enables bulk resistive switching of perovskite-type oxides.     

The Role of the Spin–Orbit Couplings and Optical Phonons on the Anisotropic Resistivity

We have investigated the influence of the electron–phonon interaction on magnetoelectric properties and spin-related transport effects of a two dimensional electron gas in the presence of the spin–orbit couplings. We have employed a semiclassical method that has been extended to include the anisotropic effects of band structure in the presence of the spin–orbit couplings. We found that resistivity and anisotropic resistance (AR) can be controlled by Rashba spin–orbit coupling.     

The Influence of Electron–Phonon Coupling and Lattice Vibrations on the Rashba Coupling Induced Domain Wall Spin Current and Spin Torque

In the current work, a detailed calculation has been presented for spin-current and spin torque in a domain wall (DW). Specifically, we analyze both spin and momentum relaxation that are relevant to the spin-transport process. It was shown that the vibrational induced relaxations play important roles in the amount of the spin-current generated by the Rashba interaction.     

Spin Injection and Spin Accumulation in Permalloy–Copper Mesoscopic Spin Valves

We study the electrical injection and detection of spin currents in a lateral spin valve device, using permalloy (Py) as ferromagnetic injecting and detecting electrodes and copper (Cu) as nonmagnetic metal. Our multiterminal geometry allows us to experimentally distinguish different magnetoresistance signals, being (1) the spin valve effect, (2) the anomalous magnetoresistance (AMR) effect, and (3) Hall effects. We find that the AMR contribution of the Py contacts can be much larger than the amplitude of the spin valve effect, making it impossible to observe the spin valve effect in a conventional measurement geometry. However, these contact magnetoresistance signals can be used to monitor the magnetization reversal process, of the spin injecting and detecting Py contacts. In a nonlocal spin valve measurement we are able to completely isolate the spin valve signal and observe clear spin accumulation signals at T = 4.2 K as well as at room temperature. We obtain spin diffusion lengths in Cu of 1 m and 350 nm at T = 4.2 K and room temperature respectively.     

Spontaneous Magnetization of a Metal–Insulator Interface

Spontaneous magnetization of an eutectic In_0.15Ga_0.85-sapphire interface is discovered. Magnetic moment is directed from an insulator to a metal, its density corresponds to 1.8×10¹⁸ m⁻² localized spin-1/2 states.     

Deterministic and probabilistic creep–fatigue–oxidation crack growth modeling

Fatigue, creep, oxidation or their combinations have long been recognized as the principal mechanisms in many high-temperature failures in power plant components, turbine engines, and exhaust systems in vehicles. Depending on the specific materials and loading conditions and temperature, the role of each damage mechanism may change significantly, ranging from independent development to competing and combined creep–fatigue, fatigue–oxidation, and creep–fatigue–oxidation. In this paper a new linear superposition theory is proposed to model the cycle-dependent and time-dependent creep–fatigue–oxidation crack growth phenomena. The model can be reduced to creep–fatigue and fatigue–oxidation crack growth models previously developed by the authors as well as, under some assumptions, the current widely used linear superposition theory. The limits of the current superposition theory and the advantages of the new theory are clearly demonstrated with several worked examples. A general probabilistic analysis procedure is also proposed by introducing the uncertainties of parameters in fatigue, creep, and oxidation crack growth laws with the help of the Monte Carlo simulation.     

Infinite Spin Diffusion Length of Any Spin Polarization Along Direction Perpendicular to Effective Magnetic Field from Dresselhaus and Rashba Spin–Orbit Couplings with Identical Strengths in (001) GaAs Quantum Wells

In this note, we show that the latest spin grating measurement of spin helix by Koralek et al. (Nature 458:610, 2009) provides strong evidence of the infinite spin diffusion length of any spin polarization along the direction perpendicular to the effective magnetic field from the Dresselhaus and Rashba spin–orbit couplings with identical strengths in (001) GaAs quantum wells, predicted by Cheng et al. (Phys. Rev. B 75:205328, 2007).     

Current-Induced Magnetic Switching in an L10 FePt Single Layer with Large Perpendicular Anisotropy Through Spin–Orbit TorqueOA

In this study, current-induced partial magnetization-based switching was realized through the spin–orbit torque(SOT) in single-layer L1₀ FePt with a perpendicular anisotropy(K_u⊥) of 1.19 × 10⁷erg·cm^-3(1erg·cm^-3= 0.1 J·m^-3), and its corresponding SOT efficiency(β_DL) was 8 × 10^-6Oe·(A·cm^-2)^-1(1Oe = 79.57747 A·m^-1), which is several times higher than that of the traditional Ta/Co...     

Charge–Spin Coupling at a Ferromagnet–Nonmagnet Interface

The topic of electrical spin injection from a ferromagnetic to a nonmagnetic material is presently attracting great interest and attention. A thermodynamic study of spin injection across a ferromagnetic–nonmagnetic material interface is presented. Using an entropy production calculation, the linear dynamic equations for interfacial transport of charge, heat, and spin magnetic moment are derived. A general equation for the fractional polarization of injected current is developed by matching boundary conditions at the interface. Polarization efficiency is sensitive to the intrinsic interface resistance, and to the resisivities and spin diffusion lengths of both materials. The physics of nonequilibrium spin diffusion across the interface is discussed, and the limiting case where resistance mismatch is important is identified. Example systems of interest are spin injection from a ferromagnetic metal to a nonmagnetic metal and from a ferromagnetic metal to a semiconductor. Charge–spin coupling and spin diffusion in one dimension, compared with higher dimension, are also discussed.