Current-induced Spin Polarization Research Articles

Current-induced spin polarization in chiral tellurium: A first-principles quantum transport study

Current-induced spin polarization in chiral tellurium: A first-principles quantum transport study

Bilinear magnetoresistance in 2DEG with isotropic cubic Rashba spin–orbit interaction

Bilinear magnetoresistance has been studied theoretically in 2D systems with isotropic cubic form of Rashba spin–orbit interaction. We have derived the effective spin–orbital field due to current-induced spin polarization and discussed its contribution to the unidirectional system response. The analyzed model can be applied to the semiconductor quantum wells as well as 2DEG at the surfaces and interfaces of perovskite oxides.

Antiparallel Spin Polarization Induced by Current in a Bilayer

The antiparallel current‐induced spin polarization (CISP) in an inversion‐symmetric double‐quantum‐well structure with the antiparallel Rashba effective magnetic field is calculated by solving the Boltzmann equation which is derived for the distribution operator in layer‐pseudospin space in the relaxation‐time approximation. The antiparallel CISP in an analytical form, obtained for constant momentum relaxation time in the limit of large Fermi energy, exhibits factor‐two increase with increasing the interlayer‐tunneling strength. This increase originates from the correction (to the semiclassical CISP) created by the subband mixing due to the momentum dependence of the Rashba effective magnetic field. The present interlayer‐coupling dependence of the antiparallel CISP is successfully explained as the layer‐pseudospin Hanle effect in which the spin in the original Hanle effect is replaced by the layer pseudospin. The present finding suggests that the correction by band mixing can be significant in the transport properties of various systems with the momentum‐dependent effective magnetic field.

Coupling of orbital and spin polarizations to interatomic hopping in a helical atomic chain.

Electron transport through a variety of helical structures has been shown to exhibit high-efficiency spin filtering, which is called chirality-induced spin selectivity (CISS). In this paper, we consider a helical chain of atomic p orbitals, which has been employed as a model in exploring the mechanism of CISS in previous theories, and show that the interatomic hopping along the helical chain induces an effective magnetic field (EMF) acting on the atomic orbital angular momentum (OAM). In chains where the curvature and torsion of the helix are small, we find that the EMF on the binormal component of the atomic OAM is created by the curvature, while that on the tangential component is produced by the torsion. We show that such coupling of the atomic OAM and the interatomic hopping leads to current-induced orbital and spin polarizations. We expect that the present coupling, which is expressed locally, can be used to estimate orbital and spin polarizations locally induced in molecules and solids.

Electric modulation of the Fermi arc spin transport via three-terminal configuration in topological semimetal nanowires

Spin–momentum locking is a key feature of the topological surface state, which plays an important role in spintronics. The electrical detection of current-induced spin polarization protected by the spin–momentum locking in nonmagnetic systems provides a new platform for developing spintronics, while previous studies were mostly based on magnetic materials. In this study, the spin transport measurement of Dirac semimetal Cd3As2 was studied by three-terminal geometry, and a hysteresis loop signal with high resistance and low resistance state was observed. The hysteresis was reversed by reversing the current direction, which illustrates the spin–momentum locking feature of Cd3As2. Furthermore, we realized the on–off states of the spin signals through electric modulation of the Fermi arc via the three-terminal configuration, which enables the great potential of Cd3As2 in spin field-effect transistors.

Current-induced spin polarization at metallic surfaces from first principles

We present the results of first-principles calculations based on density functional theory estimating the magnitude of the current-induced spin polarization (CISP) at the surfaces of the $5d$ transition metals with fcc and bcc crystal structures. We predict that the largest surface CISP occurs for W and Ta, whereas CISP is considerably weaker for Pt and Au surfaces. We then discuss how CISP emerges over a length scale equal to few atomic layers as opposed to the spin accumulation characteristic of the SHE, which is related to the materials' spin diffusion length. Finally, using our estimates for the CISP magnitude, we suggest that the spin density appearing near W surfaces in experiments is mostly due to CISP, whereas that at Pt surfaces stems from the Hall effect.

High-throughput screening giant bulk spin-split materials

Current-induced spin polarization or electric-field-controlled energy splitting shows great potential in the application of spintronic devices, such as spin–orbit torque devices, spin transistors, and so on. These mechanisms are often found in spin-split materials with strong spin-orbital coupling (SOC), which jointly trigger the Rashba, Dresselhaus, or Zeeman effect. The essential criterion to evaluate the strength of SOC is the spin-split energy. Thus, searching for bulk materials with large spin-split energy has attracted great interest. In this work, a high-throughput workflow was designed with the proposed three-point-spin-texture method, and 440 Rashba, 411 Dresselhaus, and 469 Zeeman-type candidate materials with large spin-split energy were screened out. Moreover, the spin-split energy of Rashba material KSnSb, Dresselhaus material TaSi2, and Zeeman-type material PtN2 achieve 0.19 eV, 0.82 eV, and 0.55 eV, respectively. This work significantly expands the materials database for spintronic devices and paves the way for further experimental research.

Time-Reversal-Even Nonlinear Current Induced Spin Polarization.

We propose a time-reversal-even spin generation in second order of electric fields, which dominates the current induced spin polarization in a wide class of centrosymmetric nonmagnetic materials, and leads to a novel nonlinear spin-orbit torque in magnets. We reveal a quantum origin of this effect from the momentum space dipole of the anomalous spin polarizability. First-principles calculations predict sizable spin generations in several nonmagnetic hcp metals, in monolayer TiTe_{2}, and in ferromagnetic monolayer MnSe_{2}, which can be detected in experiment. Our work opens up the broad vista of nonlinear spintronics in both nonmagnetic and magnetic systems.

Current-induced spin polarization in Janus WSSe monolayer

Janus engineering of transition metal dichalcogenide monolayers breaks the vertical mirror symmetry and induces huge built-in polarization and large Rashba spin-orbit coupling. By performing first-principles calculations, we find that the Rashba spin-orbit coupling induces sizable current-induced spin polarization in both the conduction and valence bands in the Janus WSSe monolayer, which is comparable to that in perovskite oxide interfaces. By constructing the low-energy $\mathbf{k}\ifmmode\cdot\else\textperiodcentered\fi{}\mathbf{p}$ Hamiltonian from the invariant theory, we find a sign change of the current-spin conductivity at chemical potential $\ensuremath{\mu}\ensuremath{\approx}\ensuremath{-}0.73$ eV, which indicates the flip of spin polarization and is attributed to the competition between the intrinsic Rashba spin-orbit coupling and the hexagonal warping of the valence bands. Our numerical and analytic results suggest that the current-induced spin polarization in the Janus WSSe monolayer is insensitive to temperature and impurity concentration but can be effectively tuned by external biaxial or uniaxial strain. The Janus WSSe monolayer could be a promising candidate to realize flexible two-dimensional spintronics devices.

Spin-current generation from local spin polarization induced by current through local inversion asymmetry: Double quantum well structure

The current-induced spin polarization (CISP) in a system without the inversion symmetry is known to efficiently generate the spin current. In this paper we propose another approach, generating the spin current from the CISP locally arising in a system with the inversion symmetry in the form of the antiparallel CISP in sublattice structure. In this approach the local CISP is extracted from one of sublattices by a selective contact between the sublattice and an electrode. As the simplest system with such antiparallel CISP, we consider a symmetric double-quantum-well structure (DQWS) and calculate the antiparallel CISP and the spin current to a metallic layer in parallel contact with one well of the DQWS using the Boltzmann equation in the relaxation time approximation. When the Fermi energy is large enough that the first-excited sub-band is occupied, the magnitude of the spin current, which is proportional to that of the antiparallel CISP, increases with the interwell tunnel coupling and reaches twice the value of the decoupled quantum well with the broken inversion symmetry. Such an estimate suggests that inversion-symmetric systems can be useful in generating the spin current if the inversion symmetry is locally broken.

Nonperturbative approach to interfacial spin-orbit torques induced by the Rashba effect

Current-induced spin-orbit torque (SOT) in normal metal/ferromagnet (NM/FM) bilayers bears great promise for technological applications, but the microscopic origin of purely interfacial SOTs in ultra-thin systems is not yet fully understood. Here, we show that a linear response theory with a nonperturbative treatment of spin-dependent interactions and impurity scattering potential predicts damping-like SOTs that are strictly absent in perturbative approaches. The technique is applied to a two-dimensional Rashba-coupled ferromagnet (the paradigmatic model of a NM/FM interface), where higher-order scattering processes encoding skew scattering from nonmagnetic impurities allow for current-induced spin polarization with nonzero components along all spatial directions. This is in stark contrast to previous results of perturbative methods (neglecting skew scattering), which predict a coplanar spin-polarization locked perpendicular to the charge current as a result of conventional Rashba-Edelstein effect. Furthermore, the angular dependence of ensuing SOTs and their dependence upon the scattering potential strength is analysed numerically. Simple analytic expressions for the spin-density--charge-current response function, and related SOT efficiencies, are obtained in the weak scattering limit. We find that the extrinsic damping-like torques driven by impurity scattering reaches efficiencies of up to 7% of the field-like (Rashba-Edelstein) torque. Our microscopic theory shows that bulk phenomena, such as the spin Hall effect, are not a necessity in the generation of the damping-like SOTs of the type observed in experiments on ultra-thin systems.

Charge to spin conversion in van der Waals metal NbSe2

Quantum materials with a large charge current-induced spin polarization are promising for next-generation all-electrical spintronic science and technology. Van der Waals metals with high spin–orbit coupling and spin textures have attracted significant attention for an efficient charge-to-spin conversion process. Here, we demonstrate the electrical generation of spin polarization in NbSe2 up to room temperature. Characterization of NbSe2 shows superconducting transition temperature, Tc ∼ 7 K. To probe the current-induced spin polarization in NbSe2, we used a graphene-based non-local spin-valve device, where the spin-polarization in NbSe2 is efficiently injected and detected using non-local spin-switch and Hanle spin precession measurements. A significantly higher charge-spin conversion in NbSe2 is observed at a lower temperature. Systematic measurements provide the possible origins of the spin polarization to be predominantly due to the spin Hall effect or Rashba–Edelstein effect in NbSe2, considering different symmetry-allowed charge-spin conversion processes.

Magnetic domain wall motion driven by spin-orbit torque based on non-collinear spin source

<p indent="0mm">Recently, the current-induced motion of magnetic domain walls has attracted much attention. It enables electrical control of domain wall motion without the application of an external magnetic field and thus has potential applications in high-performance magnetic racetrack memories, among others. The effective control of domain wall motion under zero magnetic field is one of the important topics in spintronics research. Current-driven magnetization switching via the spin-orbit torque effect is an important approach to realizing the field-free domain wall motion. The traditional research structure of spin-orbit torques is non-magnetic material/magnetic material (NM/FM) heterogeneous films, in which non-magnetic materials are used as the spin source layer to generate and provide spin current through charge-to-spin current conversion. Common non-magnetic materials mainly include heavy metals, such as Pt, β-Ta and β-W. However, in the system with heavy metals as spin sources, the conventional spin Hall effect requires the charge current, spin current and spin polarization directions to be perpendicular to each other, so it is necessary to apply an in-plane auxiliary magnetic field to break the symmetry, especially when driving the perpendicular magnetic moment switching, which increases extra power consumption and is not conducive to practical applications. Recently, it has been found that the spin current generated by the charge current in the outer semimetal WTe₂ is spin polarized in the <bold><italic>z</italic></bold>-direction. It has a unique in-plane symmetry breaking property, where the damping-like torque can contribute more to the domain wall motion of a ferromagnetic layer with perpendicular magnetic anisotropy. Moreover, a unique Rashba-like spin polarization has recently emerged in the noncollinear magnetic structure Mn₃GaN. A charge current is applied in the <bold><italic>x</italic></bold>-direction to generate a spin current in the <bold><italic>z</italic></bold>-direction, with spin polarization along the <bold><italic>x</italic></bold>-direction. The spin polarization along the <bold><italic>x</italic></bold>-direction produces damping-like torque (<bold><italic>τ</italic></bold>_{<italic>x</italic>}_,DL) and field-like torque (<bold><italic>τ</italic></bold>_{<italic>x</italic>}_,FL) in the adjacent in-plane and PMA ferromagnetic layers. The magnetization dynamics of the SOT mentioned above may be quite different. To realize the effectively current-induced spin-orbit torque driven domain wall motion, it is important to understand the spin source with unique physical properties of charge-to-spin conversion. In this work, we study the spin-orbit torque driven domain wall motion by micromagnetic simulations. We consider the mixture of non-collinear spin torques of Rashba-like <bold><italic>S</italic></bold><italic>_y</italic>, Dresselhaus-like <bold><italic>S</italic></bold><italic>_x</italic>, and out-of-plane <bold><italic>S</italic></bold><italic>_z</italic>. The effects of three different domain wall types: Bloch domain wall, Néel domain wall and head-to-head domain wall on motion velocity and the inclination angles of magnetic domain walls are demonstrated. The results for the same magnetic domain show that the current-induced spin polarization perpendicular to the membrane surface generates a damping-like torque perpendicular to the membrane surface, which can efficiently drive the motion of the domain wall. However, in the presence of multiple spin torques, the unconventional damping-like torques in the direction perpendicular to the film surface can be realized to drive domain wall motion at zero field. Our results may provide a new degree of freedom for understanding current-induced spin-orbit torque-driven domain wall motion from non-collinear spin sources for potential applications in spintronic devices.

Momentum-Space Spin Antivortex and Spin Transport in Monolayer Pb.

Nontrivial momentum-space spin texture of electrons can be induced by spin-orbit coupling and underpins various spin transport phenomena, such as current-induced spin polarization and the spin Hall effect. In this work, we find a nontrivial spin texture, spin antivortex, can appear at certain momenta on the Γ-K line in a 2D monolayer Pb on top of SiC. Different from spin vortex due to the band degeneracy in the Rashba model, the existence of this spin antivortex is guaranteed by the Poincaré-Hopf theorem and thus topologically stable. Accompanied with this spin antivortex, a Lifshitz transition of Fermi surfaces occurs at certain momenta on the K-M line, and both phenomena are originated from the anticrossing between the j=1/2 and j=3/2 bands. A rapid variation of the response coefficients for both the current-induced spin polarization and spin Hall conductivity is found when the Fermi energy is tuned around the spin antivortex. Our work demonstrates the monolayer Pb as a potentially appealing platform for spintronic applications.

Chirality-Induced Magnetoresistance Due to Thermally Driven Spin Polarization.

Chirality-induced current-perpendicular-to-plane magnetoresistance (CPP-MR) originates from current-induced spin polarization in molecules. The current-induced spin polarization is widely recognized as a fundamental principle of chiral-induced spin selectivity (CISS). In this study, we investigate chirality-induced current-in-plane magnetoresistance (CIP-MR) in a chiral molecule/ferromagnetic metal bilayer at room temperature. In contrast to CPP-MR, CIP-MR observed in the present study requires no bias charge current through the molecule. The temperature dependence of CIP-MR suggests that thermally driven spontaneous spin polarization in chiral molecules is the key to the observed MR. The novel MR is consistent with recent CISS-related studies, that is, chiral molecules in contact with a metallic surface possess a finite spin polarization.

Topological nature of higher-order hinge states revealed by spin transport

One-dimensional (1D) gapless hinge states are predicated in the three-dimensional (3D) higher-order topological insulators and topological semimetals, because of the higher-order bulk-boundary correspondence. Nevertheless, the topologically protected property of the hinge states is still not demonstrated so far, because it is not accessible by conventional methods, such as spectroscopy experiments and quantum oscillations. Here, we reveal the topological nature of hinge states in the higher-order topological semimetal Cd3As2 nanoplate through spin potentiometric measurements. The results of current induced spin polarization indicate that the spin-momentum locking of the higher-order hinge state is similar to that of the quantum spin Hall state, showing the helical characteristics. The spin-polarized hinge states are robust up to room temperature and can nonlocally diffuse a long distance larger than 5μm, further indicating their immunity protected by topology. Our work deepens the understanding of transport properties of the higher-order topological materials and should be valuable for future electronic and spintronic applications.

Current-Induced Spin Photocurrent in GaAs at Room Temperature.

Circularly polarized photocurrent, observed in p-doped bulk GaAs, varies nonlinearly with the applied bias voltage at room temperature. It has been explored that this phenomenon arises from the current-induced spin polarization in GaAs. In addition, we found that the current-induced spin polarization direction of p-doped bulk GaAs grown in the (001) direction lies in the sample plane and is perpendicular to the applied electric field, which is the same as that in GaAs quantum well. This research indicates that circularly polarized photocurrent is a new optical approach to investigate the current-induced spin polarization at room temperature.

Non-equilibrium spin polarization in magnetic two-dimensional electron gas with k-linear and k-cubed Dresselhaus spin–orbit interaction

The current-induced spin polarization (CISP) of charge carriers is one of the main mechanisms of spin-to-charge interconversion effects that can be used in new spintronics devices. Here, CISP is studied theoretically in symmetric quantum wells growing in [001] crystallographic direction, where both k -linear and k -cubed Dresselhaus spin–orbit interactions are present. The exchange interaction responsible for perpendicular to plane net magnetization is also taken into account. The main focus is on the influence of cubic Dresselhaus term on CISP and the interplay between spin–orbit interaction (SOI) and the exchange field. The analytical and numerical results are derived within the linear response theory and Matsubara Green’s function formalism. Apart from detailed numerical results, we also provide some analytical expressions that may be useful for interpretation of experimental results and for characterization of quantum wells with Dresselhaus SOI. Analytical expressions for the relevant Berry curvature are also derived, and it is shown that the Berry curvature in magnetic 2DEG with cubic Dresselhaus interaction oscillates in the k -space, while its averaged value is reduced. We also analyze the temperature behavior of CISP and calculate the low-temperature spin polarizability due to heat current. • The effects of the interplay of Dresselhaus spin–orbit interaction (SOI) (with both k -linear and k -cubed terms) and exchange interaction on the nonequilibrium current-induced spin polarization, have been analyzed. • The k -cubed term in Dresselhaus SOI leads to a decrease in nonequilibrium spin density with increasing chemical potential. • The cubic Dresselhaus SOI in magnetic 2DEG creates Berry curvature oscillations in the k -space and reduces the Berry curvature with increasing energy. • The out of plane magnetization leads to an additional component of spin polarization and also to anomalous Hall conductivity that is robust against impurities. • Thermally-induced spin-polarization is analyzed and is shown to be remarkable in a well-defined range of chemical potential.

Large enhancement of Edelstein effect in Weyl semimetals from Fermi-arc surface states

One remarkable feature of Weyl semimetals is the manifestation of their topological nature in the form of the Fermi-arc surface states. In a recent calculation by \cite{Johansson2018}, the current-induced spin polarization or Edelstein effect has been predicted, within the semiclassical Boltzmann theory, to be strongly amplified in a Weyl semimetal TaAs due to the existence of the Fermi arcs. Motivated by this result, we calculate the Edelstein response of an effective model for an inversion-symmetry-breaking Weyl semimetal in the presence of an interface using linear response theory. The scatterings from scalar impurities are included and the vertex corrections are computed within the self-consistent ladder approximation. At chemical potentials close to the Weyl points, we find the surface states have a much stronger response near the interface than the bulk states by about one to two orders of magnitude. At higher chemical potentials, the surface states' response near the interface decreases to be about the same order of magnitude as the bulk states' response. We attribute this phenomenon to the decoupling between the Fermi arc states and bulk states at energies close to the Weyl points. The surface states which are effectively dispersing like a one-dimensional chiral fermion become nearly nondissipative. This leads to a large surface vertex correction and, hence, a strong enhancement of the surface states' Edelstein response.

Current-induced hole spin polarization in a quantum dot via a chiral quasi bound state.

We put forward a mechanism for the current-induced spin polarization in semiconductor heterostructures, which is based on the complex structure of the valence band. It takes place for a light hole in a quantum dot side-coupled to a quantum wire with heavy holes. In stark contrast with the traditional mechanisms based on the linear in momentum spin-orbit coupling, an exponentially small bias applied to this structure is enough to create the 100% spin polarization in the quantum dot. Microscopically, this effect is related to the formation of the chiral quasi bound states and the spin-dependent tunneling of holes from the quantum wire to the quantum dot. This new concept is equally valid for the GaAs-, Si- and Ge-based nanostructures.