All-electrical Manipulation Research Articles

Field-free switching of perpendicular magnetization in a noncollinear antiferromagnetic Mn3Sn/[Pt/Co]4 heterostructure

Spin-orbit torque (SOT) manipulation of perpendicular magnetization is essential for realizing spintronic storage and logic devices. When a charge current flows through a heavy metal/ferromagnetic layer structure, a spin current can be generated by the heavy metal and injects into the ferromagnetic layer with perpendicular magnetic anisotropy and switch its magnetization via the SOT. However, in order to realize the deterministic switching, an in-plane external magnetic field is needed to break the inversion symmetry, a technological requirement that hampers the practical application of SOT. Pt/Co multilayer is a good magnetic recording media and, thus, the all-electric manipulation of its perpendicular magnetic moments is of practical significance. The noncollinear spin structure in Mn3Sn allows an unconventional spin polarization direction, and the associated SOT can switch the magnetic moments in the absence of an external field. In this work, we demonstrate the field-free SOT switching of a heterostructure utilizing Mn3Sn and Pt/Co multilayer as the spin source and the ferromagnetic layer with perpendicular magnetization, respectively. Our results are important for advancing the development of spintronic devices based on noncollinear antiferromagnets.

Resonant tunneling and quantum interference of a two-spin system in silicon tunnel FETs

We investigated the resonant tunneling of a two-spin system through the double quantum dots in Al–N-implanted silicon tunnel FETs (TFETs) by electrical-transport measurements and Landau–Zener–Stückelberg–Majorana interferometry with and without magnetic fields. Our experimental results revealed the coexistence of spin-conserving and spin-flip tunneling channels in the two-spin system in non-zero magnetic fields. Additionally, we obtained the spin-conserving/spin-flip tunneling rates of the two-spin system through the double quantum dots in the TFET. These findings will improve our understanding of the two-spin system in silicon TFET qubits and may facilitate the coherent control of quantum states through all-electric manipulation.

Undoped Strained Ge Quantum Well with Ultrahigh Mobility of Two Million.

We develop a method to fabricate an undoped Ge quantum well (QW) under a 32 nm relaxed Si0.2Ge0.8 shallow barrier. The bottom barrier contains Si0.2Ge0.8 (650 °C) and Si0.1Ge0.9 (800 °C) such that variation of Ge content forms a sharp interface that can suppress the threading dislocation density (TDD) penetrating into the undoped Ge quantum well. The SiGe barrier introduces enough in-plane parallel strain (ε∥ strain -0.41%) in the Ge quantum well. The heterostructure field-effect transistors with a shallow buried channel obtain an ultrahigh two-dimensional hole gas (2DHG) mobility over 2 × 106 cm2/(V s) and a very low percolation density of (5.689 ± 0.062) × 1010 cm-2. The fractional indication is also observed at high density and high magnetic fields. This strained germanium as a noise mitigation material provides a platform for integration of quantum computation with a long coherence time and fast all-electrical manipulation.

Zoo of silicon-based quantum bits

Zoo of silicon-based quantum bits

Hole spin in tunable Ge hut wire double quantum dot

Holes in germanium (Ge) exhibit strong spin–orbit interaction, which can be exploited for fast and all-electrical manipulation of spin states. Here, we report transport experiments in a tunable Ge hut wire hole double quantum dot. We observe the signatures of Pauli spin blockade (PSB) with a large singlet-triplet energy splitting of ∼1.1 meV and extract the g factor. By analyzing the PSB leakage current, we obtain a spin–orbit length of ∼40–100 nm. Furthermore, we demonstrate the electric dipole spin resonance. These results lay a solid foundation for implementing high quality tunable hole spin–orbit qubits.

All-electrical manipulation of magnetization in magnetic tunnel junction via spin–orbit torque

Besides spin-transfer torque, spin–orbit torque (SOT) provides us with another electrical way for developing magnetic random access memory (MRAM) based on magnetic tunnel junctions (MTJs). By using a CoFeB/Ta/CoFeB T-type magnetic structure as a synthetic free layer, we have realized a magnetic-field-free MTJ switchable by SOT. In the T-type structure, an in-plane CoFeB layer is coupled to a perpendicular CoFeB layer via a Ta spacer. The spacer layer not only mediates exchange coupling but also generates strong SOT, which drives sharp switching between the low and high resistance states of the MTJ without any applied magnetic fields. This work could provide an efficient way toward realization of field-free SOT-MRAM or logic devices.

Field-free control of exchange bias by spin Hall currents

A trilayer structure consisting of ferromagnetic alloy CoFeB/antiferromagnetic IrMn/heavy metal Pt layers was prepared, and the manipulation of its exchange bias effect was achieved without applying an external magnetic field. Spin polarized current, generated by the spin Hall effect of the electrical current through the Pt layer, was shown to be able to control the spin alignment state in the neighboring antiferromagnetic material as well as the corresponding exchange bias effect in the ferromagnet coupled to the antiferromagnet. The results demonstrated stable and reliable switching of the spin structure in antiferromagnets, providing a useful route to all-electrical manipulation of antiferromagnetic states without a need for the bulky global application of the external magnetic field. Therefore, with the results presented in the present work, precise control of magnetism including both ferromagnets and antiferromagnets on submicron to nanometer length scales can be expected.

Electron g -factor of valley states in realistic silicon quantum dots

We theoretically model the spin-orbit interaction in silicon quantum dot devices, relevant for quantum computation and spintronics. Our model is based on a modified effective mass approach with spin-valley boundary conditions, derived from the interface symmetry under presence of perpendicular to the interface electric field. The g-factor renormalization in the two lowest valley states is explained by the interface-induced spin-orbit 2D (3D) interaction, favoring intervalley spin-flip tunneling over intravalley processes. We show that the quantum dot level structure makes only negligible higher order effects to the g-factor. We calculate the g-factor as a function of the magnetic field direction, which is sensitive to the interface symmetry. We identify spin-qubit dephasing sweet spots at certain directions of the magnetic field, where the g-factor renormalization is zeroed: these include perpendicular to the interface magnetic field, and also in-plain directions, the latter being defined by the interface-induced spin-orbit constants. The g-factor dependence on electric field opens the possibility for fast all-electric manipulation of an encoded, few electron spin-qubit, without the need of a nanomagnet or a nuclear spin-background. Our approach of an almost fully analytic theory allows for a deeper physical understanding of the importance of spin-orbit coupling to silicon spin qubits.

Electric-field tuning of the valley splitting in silicon corner dots

We perform an excited state spectroscopy analysis of a silicon corner dot in a nanowire field-effect transistor to assess the electric field tunability of the valley splitting. First, we demonstrate a back-gate-controlled transition between a single quantum dot and a double quantum dot in parallel which allows tuning the device into corner dot formation. We find a linear dependence of the valley splitting on back-gate voltage, from 880 μeV to 610 μeV with a slope of −45 ± 3 μeV/V (or equivalently a slope of −48 ± 3 μeV/(MV/m) with respect to the effective field). The experimental results are backed up by tight-binding simulations that include the effect of surface roughness, remote charges in the gate stack, and discrete dopants in the channel. Our results demonstrate a way to electrically tune the valley splitting in silicon-on-insulator-based quantum dots, a requirement to achieve all-electrical manipulation of silicon spin qubits.

Landau-Zener-Stückelberg-Majorana Interferometry of a Single Hole.

We perform Landau-Zener-Stückelberg-Majorana (LZSM) spectroscopy on a system with strong spin-orbit interaction (SOI), realized as a single hole confined in a gated double quantum dot. Analogous to electron systems, at a magnetic field B=0 and high modulation frequencies, we observe photon-assisted tunneling between dots, which smoothly evolves into the typical LZSM funnel-shaped interference pattern as the frequency is decreased. In contrast to electrons, the SOI enables an additional, efficient spin-flip interdot tunneling channel, introducing a distinct interference pattern at finite B. Magnetotransport spectra at low-frequency LZSM driving show the two channels to be equally coherent. High-frequency LZSM driving reveals complex photon-assisted tunneling pathways, both spin conserving and spin flip, which form closed loops at critical magnetic fields. In one such loop, an arbitrary hole spin state is inverted, opening the way toward its all-electrical manipulation.

All-electrical manipulation of silicon spin qubits with tunable spin-valley mixing

We show that the mixing between spin and valley degrees of freedom in a silicon quantum bit (qubit) can be controlled by a static electric field acting on the valley splitting $\Delta$. Thanks to spin-orbit coupling, the qubit can be continuously switched between a spin mode (where the quantum information is encoded into the spin) and a valley mode (where the the quantum information is encoded into the valley). In the spin mode, the qubit is more robust with respect to inelastic relaxation and decoherence, but is hardly addressable electrically. It can however be brought into the valley mode then back to the spin mode for electrical manipulation. This opens new perspectives for the development of robust and scalable, electrically addressable spin qubits on silicon. We illustrate this with tight-binding simulations on a so-called "corner dot" in a silicon-on-insulator device where the confinement and valley splitting can be independently tailored by a front and a back gate.

Magnetic skyrmions: advances in physics and potential applications

Magnetic skyrmions are small swirling topological defects in the magnetization texture. Their stabilization and dynamics depend strongly on their topological properties. In most cases, they are induced by chiral interactions between atomic spins in non-centrosymmetric magnetic compounds or in thin films with broken inversion symmetry. Skyrmions can be extremely small, with diameters in the nanometre range, and behave as particles that can be moved, created and annihilated, which makes them suitable for ‘abacus’-type applications in information storage and logic technologies. Until recently, skyrmions had been observed only at low temperature and, in most cases, under large applied magnetic fields. An intense research effort has led to the identification of thin-film and multilayer structures in which skyrmions are now stable at room temperature and can be manipulated by electrical currents. The development of skyrmion-based topological spintronics holds promise for applications in the mid-term furure, even though many challenges, such as the achievement of writing, processing and reading functionalities at room temperature and in all-electrical manipulation schemes, still lie ahead. Magnetic skyrmions are topologically protected spin whirls that hold promise for applications because they can be controllably moved, created and annihilated. In this Review, the underlying physics of the stabilization of skyrmions at room temperature and their prospective use for spintronic applications are discussed.

Maximum density of quantum information in a scalable CMOS implementation of the hybrid qubit architecture

Scalability from single-qubit operations to multi-qubit circuits for quantum information processing requires architecture-specific implementations. Semiconductor hybrid qubit architecture is a suitable candidate to realize large-scale quantum information processing, as it combines a universal set of logic gates with fast and all-electrical manipulation of qubits. We propose an implementation of hybrid qubits, based on Si metal-oxide-semiconductor (MOS) quantum dots, compatible with the CMOS industrial technological standards. We discuss the realization of multi-qubit circuits capable of fault-tolerant computation and quantum error correction, by evaluating the time and space resources needed for their implementation. As a result, the maximum density of quantum information is extracted from a circuit including eight logical qubits encoded by the [[7, 1, 3]] Steane code. The corresponding surface density of logical qubits is 2.6 Mqubit/cm$$^2$$2.

All-electrical manipulation of magnetization dynamics in a ferromagnet by antiferromagnets with anisotropic spin Hall effects

We investigate spin-orbit torques of metallic CuAu-I-type antiferromagnets using spin-torque ferromagnetic resonance tuned by a dc-bias current. The observed spin torques predominantly arise from diffusive transport of spin current generated by the spin Hall effect. We find a growth-orientation dependence of the spin torques by studying epitaxial samples, which may be correlated to the anisotropy of the spin Hall effect. The observed anisotropy is consistent with first-principles calculations on the intrinsic spin Hall effect. Our work demonstrates large tunable spin-orbit effects in magnetically-ordered materials.

Direct observation of spin-polarized bulk bands in an inversion-symmetric semiconductor

The coupling between spin, valley and layer degrees of freedom in transition-metal dichalcogenides is shown to give rise to spin-polarized electron states, providing opportunities to create and manipulate spin and valley polarizations in bulk solids. Methods to generate spin-polarized electronic states in non-magnetic solids are strongly desired to enable all-electrical manipulation of electron spins for new quantum devices1. This is generally accepted to require breaking global structural inversion symmetry1,2,3,4,5. In contrast, here we report the observation from spin- and angle-resolved photoemission spectroscopy of spin-polarized bulk states in the centrosymmetric transition-metal dichalcogenide WSe2. Mediated by a lack of inversion symmetry in constituent structural units of the bulk crystal where the electronic states are localized6, we show how spin splittings up to ∼0.5 eV result, with a spin texture that is strongly modulated in both real and momentum space. Through this, our study provides direct experimental evidence for a putative locking of the spin with the layer and valley pseudospins in transition-metal dichalcogenides7,8, of key importance for using these compounds in proposed valleytronic devices.

All-electrical manipulation of electron spin in a semiconductor nanotube

A controlled manipulation of electron spin states has been investigated for a cylindrical two-dimensional electron gas confined in a semiconductor nanotube/cylindrical nanowire with the Rashba spin–orbit interaction. We present analytical solutions for the two limiting cases, in which the spin–orbit interaction results from (A) the radial electric field and (B) the electric field applied along the z-axis of the nanotube. In the case (A), we have found that only the superposition of bands with the same orbital momentum leads to the spin precession around the cylinder (nanotube) axis. In the case (B), we have obtained the damped oscillations of the z spin component with the period that changes as a function of the coordinate z. We have shown that the damped oscillations of the average value of the z spin component form beats localized along the nanotube axis. The position of the beats can be controlled by the bias voltage.

Nuclear spin coherence in a quantum wire

We have observed millisecond-long coherent evolution of nuclear spins in a quantum wire at 1.2 K. Local, all-electrical manipulation of nuclear spins is achieved by dynamic nuclear polarization in the breakdown regime of the integer quantum Hall effect combined with pulsed nuclear magnetic resonance. The excitation thresholds for the breakdown are significantly smaller than what would be expected for our sample and the direction of the nuclear polarization can be controlled by the voltage bias. As a four-level spin system, the device is equivalent to two qubits.

GENERATING and MANIPULATING SPINS IN SEMICONDUCTORS

Spin-orbit coupling in semiconductors relates the spin of an electron to its momentum, and provides a pathway for electrically initializing and manipulating electron spins for applications in spintronics and spin-based quantum information processing. This coupling can be regulated with strain in bulk semiconductors and quantum confinement in semiconductor heterostructures. We will provide an overview of optical studies exploring spin dynamics in conventional and magnetic semiconductors, followed by recent experiments probing all-electrical generation and manipulation of spins. Using Faraday and Kerr rotation magneto-optical spectroscopies with temporal and spatial resolution, current-induced spin polarization1 and the spin Hall effect2 have been observed using both spatially-resolved and temporally-resolved techniques in bulk semiconductors. More recently, we have investigated the spin Hall effect and current-induced spin polarization in two-dimensional electron gases confined in (110) AlGaAs quantum wells using Kerr rotation microscopy.3 In marked contrast to previous measurements in three dimensional systems, the spin Hall profile in two dimensions shows a surprisingly complex structure and the current-induced spin polarization is out-of-plane. The experiments map the strong dependence of the current-induced spin polarization to the crystal axis along which the electric field is applied, reflecting the anisotropy of the spin-orbit interaction. The effect of reducing feature sizes in electrical spintronic devices is relevant for future technological applications. To this end, we discuss related measurements probing electron spin dynamics in narrow two-dimensional InGaAs quantum well wires with widths ranging from the micron to the submicron scale.4 The data reveal a surprising slowing of the electron spin relaxation in reduced geometries. These results reveal opportunities for tuning a spin source using quantum confinement, strain and device engineering in non-magnetic materials. This work was supported by the ARO, DARPA, NSF and ONR. Note from Publisher: This article contains the abstract only.

All-Electrical Control of Single Ion Spins in a Semiconductor

We propose a method for all-electrical manipulation of single ion spins substituted into a semiconductor. Mn ions with a bound hole in GaAs form a natural example. Direct electrical manipulation of the ion spin is possible, because electric fields manipulate the orbital wave function of the hole, and through the spin-orbit coupling the spin is reoriented as well. Coupling ion spins can be achieved using gates to control the size of the hole wave function. Coherent manipulation of ionic spins may find applications in high-density storage and in scalable coherent or quantum information processing.