Spin Hall Current Research Articles

Spin Hall effect: Symmetry breaking, twisting, and giant disorder renormalization

Atomically thin materials based on transition-metal dichalcogenides and graphene offer a promising avenue for unlocking the mechanisms underlying the spin Hall effect (SHE) in heterointerfaces. Here we develop a microscopic theory of the SHE for twisted van der Waals heterostructures that fully incorporates twisting and disorder effects and illustrate the critical role of symmetry breaking in the generation of spin Hall currents. We find that an accurate treatment of vertex corrections leads to a qualitatively and quantitatively different SHE than that obtained from the popular iη and ladder approximations. A pronounced oscillatory behavior of skew-scattering processes with twist angle θ is predicted, reflecting a nontrivial interplay of Rashba and valley-Zeeman effects and yields a vanishing SHE for θ=30∘ and, for graphene-WSe2 heterostructures, an optimal SHE for θ≈17∘. Our findings reveal disorder and broken symmetries as important knobs to optimize interfacial SHEs. Published by the American Physical Society 2024

Inverse Spin-Hall Effect and Spin Swapping in Spin-Split Superconductors.

When a spin-splitting field is introduced to a thin film superconductor, the spin currents polarized along the field couples to energy currents that can only decay via inelastic scattering. We study spin and energy injection into such a superconductor where spin-orbit impurity scattering yields inverse spin-Hall and spin-swapping currents. We show that the combined presence of a spin-splitting field, superconductivity, and inelastic scattering gives rise to a strong enhancement of the ordinary inverse spin-Hall effect, as well as unique inverse spin-Hall and spin-swapping signals orders of magnitude stronger than the ordinary inverse spin-Hall signal. These can be completely controlled by the orientation of the spin-splitting field, resulting in a long-range charge and spin accumulations detectable much further from the injector than in the normal state. While the enhanced inverse spin-Hall signals offer a major improvement in spin detection sensitivity, the unique spin-swap signals can be utilized for designing devices where both the spin and current directions are controlled and altered throughout the geometry.

Light-induced nonlinear spin Hall current in single-layer WTe2

In this theoretical investigation, we analyze light-induced nonlinear spin Hall currents in a gated single-layer 1T ′ -WTe2, flowing transversely to the incident laser polarization direction. Our study encompasses the exploration of the second and third-order rectified spin Hall currents using an effective low-energy Hamiltonian and employing the Kubo’s formalism. We extend our analysis to a wide frequency range spanning both transparent and absorbing regimes, investigating the influence of light frequency below and above the optical band gap. Additionally, we investigate the influence of an out-of-plane gate potential on the system, disrupting inversion symmetry and effectively manipulating both the strength and sign of nonlinear spin Hall responses. We predict a pronounced third-order spin Hall current relative to its second-order counterpart. The predicted nonlinear spin currents show strong anisotropic dependence on the laser polarization angle. The outcomes of our study contribute to a generalized framework for nonlinear response theory within the spin channel will impact the development of emerging field of opto-spintronic.

Spin and orbital Hall currents detected via current-induced magneto-optical Kerr effect in V and Pt

Spin and orbital Hall currents detected via current-induced magneto-optical Kerr effect in V and Pt

Impact of inherent energy barrier on spin-orbit torques in magnetic-metal/semimetal heterojunctions.

Spintronic devices are based on heterojunctions of two materials with different magnetic and electronic properties. Although an energy barrier is naturally formed even at the interface of metallic heterojunctions, its impact on spin transport has been overlooked. Here, using diffusive spin Hall currents, we provide evidence that the inherent energy barrier governs the spin transport even in metallic systems. We find a sizable field-like torque, much larger than the damping-like counterpart, in Ni81Fe19/Bi0.1Sb0.9 bilayers. This is a distinct signature of barrier-mediated spin-orbit torques, which is consistent with our theory that predicts a strong modification of the spin mixing conductance induced by the energy barrier. Our results suggest that the spin mixing conductance and the corresponding spin-orbit torques are strongly altered by minimizing the work function difference in the heterostructure. These findings provide a new mechanism to control spin transport and spin torque phenomena by interfacial engineering of metallic heterostructures.

Isotropic Spin Hall Effect in an Epitaxial Ferromagnet.

We report the observation of the isotropic spin Hall effect in a ferromagnet. We show that the spin Hall effect in an epitaxially grown Fe_{3}Si generates a sizable spin current with a spin direction that is noncollinear with the magnetization. Furthermore, we find that the spin Hall current is independent of the relative orientation between its spin direction and the magnetization; the spin Hall effect is isotropic. This observation demonstrates that the intrinsically generated transverse spin component is protected from dephasing, providing fundamental insights into the generation and transport of spin currents in ferromagnets.

Moiré Dirac fermions in transition metal dichalcogenides heterobilayers

Abstract Monolayer group-VIB transition metal dichalcogenides (TMDs) feature low-energy massive Dirac fermions, which have valley contrasting Berry curvature. This nontrivial local band topology gives rise to valley Hall transport and optical selection rules for interband transitions that open up new possibilities for valleytronics. However, the large bandgap in TMDs results in relatively small Berry curvature, leading to weak valley contrasting physics in practical experiments. Here, we show that Dirac fermions with tunable large Berry curvature can be engineered in moiré superlattice of TMD heterobilayers. These moiré Dirac fermions are created in a magnified honeycomb lattice with its sublattice degree of freedom formed by two local moiré potential minima. We show that applying an on-site potential can tune the moiré flat bands into helical ones. In short-period moiré superlattice, we find that the two moiré valleys become asymmetric, which results in a net spin Hall current. More interestingly, a circularly polarized light drives these moiré Dirac fermions into quantum anomalous Hall phase with chiral edge states. Our results open a new possibility to design the moiré-scale spin and valley physics using TMD moiré structures.

Easy-plane spin Hall oscillator

Spin Hall oscillators (SHOs) based on bilayers of a ferromagnet (FM) and a non-magnetic heavy metal (HM) are electrically tunable nanoscale microwave signal generators. Achieving high output power in SHOs requires driving large-amplitude magnetization dynamics by a direct spin Hall current. Here we present an SHO engineered to have easy-plane magnetic anisotropy oriented normal to the bilayer plane, enabling large-amplitude easy-plane dynamics driven by spin Hall current. Our experiments and micromagnetic simulations demonstrate that the easy-plane anisotropy can be achieved by tuning the magnetic shape anisotropy and perpendicular magnetic anisotropy in a nanowire SHO, leading to a significant enhancement of the generated microwave power. The easy-plane SHO experimentally demonstrated here is an ideal candidate for realization of a spintronic spiking neuron. Our results provide an approach to design of high-power SHOs for wireless communications, neuromorphic computing, and microwave assisted magnetic recording.

Fully relativistic first-principles quantum transport simulation of noncollinear spin transfer and spin Hall current

In this work, we report the fully relativistic (FR) first-principles quantum transport simulation of noncollinear spin transfer and spin Hall current in the device structure. In this method, the noncollinear FR exact muffin-tin orbital method is combined with Keldysh's nonequilibrium Green's function approach and mean-field theory to account for the multiple disorder scattering. We adopt the Bargmann-Wigner polarization operator to define the appropriate FR spin current so that the current-induced spin transfer, in the noncollinear magnetic device or due to the spin Hall effect, can be studied from first principles. As applications, we calculate the spin transfer torque in noncollinear spin valves Co/Cu/FM/Cu (FM = Co, ${\mathrm{Ni}}_{0.8}{\mathrm{Fe}}_{0.2}$) and spin Hall angles in various ${\mathrm{Pt}}_{1\ensuremath{-}x}{Y}_{x}$ [$Y$ = vacancy (Va), Au, Ag, Pd] alloys. We find that our FR results agree well with previous theoretical simulations and experimental measurements. Moreover, it is found that the applied finite bias can significantly enhance the spin Hall angle in ${\mathrm{Pt}}_{1\ensuremath{-}x}{\mathrm{Va}}_{x}$, and PtAg alloy presents a much higher spin Hall angle than that of PtAu and PtPd alloys. Our implementation of the FR method provides an important first-principles tool for studying various nonequilibrium spin phenomena and the associated relativistic effects in realistic device structures with atomic disorders, including both current-induced spin transfer and spin-orbit torques.

Topological magnetotransport and electrical switching of sputtered antiferromagnetic Ir20Mn80

Topological magnetotransport in non-collinear antiferromagnets has attracted extensive attention due to the exotic phenomena such as large anomalous Hall effect (AHE), magnetic spin Hall effect, and chiral anomaly. The materials exhibiting topological antiferromagnetic physics are typically limited in special Mn3 X family such as Mn3Sn and Mn3Ge. Exploring the topological magnetotransport in common antiferromagnetic materials widely used in spintronics will not only enrich the platforms for investigating the non-collinear antiferromagnetic physics, but also have great importance for driving the nontrivial topological properties towards practical applications. Here, we report remarkable AHE, anisotropic and negative parallel magnetoresistance in the magnetron-sputtered Ir20Mn80 antiferromagnet, which is one of the most widely used antiferromagnetic materials in industrial spintronics. The ab initio calculations suggest that the Ir4Mn16 (IrMn4) or Mn3Ir nanocrystals hold nontrivial electronic band structures, which may contribute to the observed intriguing magnetotransport properties in the Ir20Mn80. Further, we demonstrate the spin–orbit torque switching of the antiferromagnetic Ir20Mn80 by the spin Hall current of Pt. The presented results highlight a great potential of the magnetron-sputtered Ir20Mn80 film for exploring the topological antiferromagnet-based physics and spintronics applications.

Spin and valley Hall effects induced by asymmetric interparticle scattering

The physics of spin currents has always attracted the attention of Professor Rashba who made seminal contributions to the field. Here, the authors develop the theory of the spin and valley Hall effects in systems of interacting fermions and bosons. In high-mobility two-dimensional semiconductors, the interparticle scattering dominates over the impurity and phonon scattering and controls the transport effects. The authors demonstrate theoretically that the collisions between the quasiparticles give rise to the spin and valley Hall currents via the skew-scattering effect. They also calculate the efficiency of such a process.

Noncollinear Spin Current for Switching of Chiral Magnetic Textures.

We propose a concept of noncollinear spin current, whose spin polarization varies in space even in nonmagnetic crystals. While it is commonly assumed that the spin polarization of the spin Hall current is uniform, asymmetric local crystal potential generally allows the spin polarization to be noncollinear in space. Based on microscopic considerations, we demonstrate that such noncollinear spin Hall currents can be observed, for example, in layered Kagome Mn_{3}X (X=Ge, Sn) compounds. Moreover, by referring to atomistic spin dynamics simulations we show that noncollinear spin currents can be used to switch the chiral spin texture of Mn_{3}X in a deterministic way even in the absence of an external magnetic field. Our theoretical prediction can be readily tested in experiments, which will open a novel route toward electric control of complex spin structures in noncollinear antiferromagnets.

Curvature-induced quantum spin-Hall effect on a Möbius strip

The quantum Hall effect has been predicted and discovered in various condensed-matter systems. A promising quantum material for such topological effects is graphene. We report the numerical observation of a curvature-induced spin-Hall effect in a monolayer graphene M\"obius strip. The solution of the Dirac equation on the nontrivial and non-Euclidean manifold reveals that despite the absence of a Hall current, a spin-Hall current is a natural consequence for such a topology, as predicted from symmetry arguments.

AC response of spin-pseudospin current in a double quantum well

The spin Hall effect due to the skew scattering is studied using the Boltzmann equation in a double quantum well when the inplane electric field with angular frequency ω is applied. The two wells have opposite signs of impurity potential so that the skew-scattering spin Hall current is antiparallel and carries a pseudospin, which is formed by ∣L〉 and ∣R〉, the ground states of the two wells. The pseudospin precession is induced by the interwell tunneling in the strength of ℏ ω SAS, the energy difference between the symmetric and antisymmetric states. It is found that the dynamics of the spin-pseudospin current, described by the pseudospin analogue of the Bloch equation, is equivalent in form to the classical cyclotron resonance. Consequently, the antiparallel spin Hall current exhibits the resonance peak at ω ∼ ω SAS. Such spin-pseudospin coupling is expected to be useful in controlling the spin polarization in many electronic systems.

Orientational dependence of intrinsic orbital and spin Hall effects in hcp structure materials

We investigate the orientation dependence of the intrinsic orbital Hall effect (OHE) and spin Hall effect (SHE) in hexagonal close-packed (hcp) structure materials. The symmetry constraints of the hcp structure restrict the orbital (spin) current to the orbital (spin) Hall current. It also shows that there exist three symmetry-wise independent orientations in the hcp structure. We calculate the orbital Hall conductivity (OHC) and the spin Hall conductivity (SHC) of three hcp materials (Sc, Y, and Zr). We find that all these materials have sizable OHC, while the SHC is orders of magnitude smaller. Especially, the OHCs of Sc and Zr have significant orientation dependence, which may be employed to experimentally probe the OHE.

Spin Hall effect in heavy-ion collisions

Spin Hall effect (SHE) is the generation of spin current due to an electric field, and has been observed in a variety of materials. In this work, we use linear response theory to verify that the analogous spin Hall current can be induced by chemical potential and temperature gradient, both of which are present in hot and dense nuclear matter created in heavy-ion collisions. We propose to measure ``directed spin flow'', the first Fourier coefficients of local spin polarization of $\mathrm{\ensuremath{\Lambda}}$ $(\overline{\mathrm{\ensuremath{\Lambda}}})$ hyperon, at central collisions to probe spin Hall current in heavy-ion collision experiments. We benchmark the magnitude of the induced ``directed spin flow'' at two representatively collisions energies, namely ${\sqrt{s}}_{NN}=200\text{ }\text{ }\mathrm{GeV}$ and ${\sqrt{s}}_{NN}=19.6\text{ }\text{ }\mathrm{GeV}$, by employing a phenomenologically motivated freeze-out prescription. At both beam energies, the resulting ``directed spin flow'' ranges from ${10}^{\ensuremath{-}4}$ to ${10}^{\ensuremath{-}3}$, and is very sensitive to the rapidity.

Spin-orbit torques induced by spin Hall and spin swapping currents of a separate ferromagnet in a magnetic trilayer

Spin-orbit torques (SOTs) have been investigated most widely in normal metal/ferromagnet bilayers where the spin Hall effect of normal metal is a main source of spin currents. Recently, ferromagnets are found to also serve as spin-current sources through spin-orbit coupling. In this work, we theoretically investigate SOT acting on ferromagnet2 in ferromagnet1/normal metal/ferromagnet2 trilayers, which is caused by the spin Hall and spin swapping effects of ferromagnet1. Our result provides an analytical expression of SOT in the trilayers, which may be useful for quantifying the spin Hall and spin swapping effects of ferromagnets and also for designing and interpreting SOT experiments where a ferromagnet is used as a spin-current source instead of a normal metal.

Optically induced spin current in monolayer NbSe2

Monolayer NbSe2 is a metallic two-dimensional (2D) transition-metal dichalcogenide material. Owing to the lattice structure and the strong atomic spin-orbit coupling (SOC) field, monolayer NbSe2 possesses Ising-type SOC which acts as effective Zeeman field, leading to the unconventional topological spin properties. In this paper, we numerically calculate spin-dependent optical conductivity of monolayer NbSe2 using Kubo formula based on an effective tight-binding model which includes $d_{z^2}$, $d_{x^2-y^2}$ and $d_{xy}$ orbitals of Nb atom. Numerical calculation indicates that the up- and down-spin have opposite sign of Hall current, so the pure spin Hall current can be generated in monolayer NbSe2 under light irradiation, owing to the topological nature of monolayer NbSe2, i.e., finite spin Berry curvature. The spin Hall angle is also evaluated. The optical induced spin Hall current can be enhanced by the electron doping and persists even at room temperature. Our results will serve to design opt-spintronics devices such as spin current harvesting by light irradiation on the basis of 2D materials.

Extrinsic spin Hall effect in inhomogeneous systems

Charge-to-spin conversion in inhomogeneous systems is studied theoretically. We consider free electrons subject to impurities with spin-orbit interaction and with spatially modulated distribution, and calculate spin accumulation and spin current induced by an external electric field. It is found that the spin accumulation is induced by the vorticity of electron flow through the side-jump and skew-scattering processes, and the differences of the two processes are discussed. The results can be put in a form of generalized spin diffusion equation with a spin source term given by the divergence of the spin Hall current. This spin source term reduces to the form of spin-vorticity coupling when the spin Hall angle is independent of impurity concentration. We also study the effects of long-range Coulomb interaction, which is indispensable when the process involves charge inhomogeneity and accumulation as in the present problem.

Dimensional crossover in spin Hall oscillators

Auto-oscillations of magnetization driven by direct spin current have been previously observed in multiple quasi-zero-dimensional (0D) ferromagnetic systems such as nanomagnets and nanocontacts. Recently, it was shown that pure spin Hall current can excite coherent auto-oscillatory dynamics in quasi-one-dimensional (1D) ferromagnetic nanowires but not in quasi-two-dimensional (2D) ferromagnetic films. Here we study the 1D to 2D dimensional crossover of current-driven magnetization dynamics in wire-based Pt/$\mathrm{Ni}_{80}\mathrm{Fe}_{20}$ bilayer spin Hall oscillators via varying the wire width. We find that increasing the wire width results in an increase of the number of excited auto-oscillatory modes accompanied by a decrease of the amplitude and coherence of each mode. We also observe a crossover from a hard to a soft onset of the auto-oscillations with increasing the wire width. The amplitude of auto-oscillations rapidly decreases with increasing temperature suggesting that interactions of the phase-coherent auto-oscillatory modes with incoherent thermal magnons plays an important role in suppression of the auto-oscillatory dynamics. Our measurements set the upper limit on the dimensions of an individual spin Hall oscillator and elucidate the mechanisms leading to suppression of coherent auto-oscillations with increasing oscillator size.