Spin Accumulation Research Articles

Spin accumulation and spin Hall effect in a two-layer system with a thin ferromagnetic layer

Spin accumulation and spin current are phenomena that enhance the functionality of the devices operating with charge and spin. We calculated them for the system consisting of a ferroelectric barrier and a thin ferromagnetic layer when the current flows parallel to the interface. We assume Dresselhaus and Rashba spin–orbit coupling linear in electron wave number. We demonstrate that spin accumulation and spin current can be manipulated by changing the direction of the magnetization of the FM layer with respect to the crystallographic axes of the ferroelectric barrier.

Large Low-Magnetic-Field Magnetocapacitance Effect and Spin Accumulation in Graphene Oxide

We report large magnetocapacitance (MC) effect and spin accumulation in graphene oxide (GO) studied at low magnetic fields and low frequencies, synthesized using modified Hummer’s method. Raman spectroscopy, atomic force microscopy (AFM), vibrating sample magnetometry (VSM), impedance spectroscopy, and dc transport measurements are used to investigate the origin of spin accumulation in GO. It was observed that synthesized GO was flake-like in morphology with prominent <inline-formula xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink"> <tex-math notation="LaTeX">$G$ </tex-math></inline-formula> - and <inline-formula xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink"> <tex-math notation="LaTeX">$D$ </tex-math></inline-formula> -bands of vibrations, with large defect density. It was found to have ferromagnetism with coercivity of the order of <inline-formula xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink"> <tex-math notation="LaTeX">$\sim 134$ </tex-math></inline-formula> Oe. The GO showed MC of 20% at around 200 Hz and 100 Oe, where capacitive coupling is higher and 2.2% at 1.1 MHz where inductive coupling is higher in the capacitor under study. The relaxation process in GO was modeled using modified Havriliak–Negami (H–N) approach for inhomogeneous medium, which predicted the spin accumulation length of the order of <inline-formula xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink"> <tex-math notation="LaTeX">$187~\mu \text{m}$ </tex-math></inline-formula> . The spin accumulation in GO here is proposed to be due to weak localization and exchange interaction at lower fields below coercivity and above it is interface mediated and dipolar in nature. The results are supported with dc–magnetoresistance (MR) study showing the depletive nature of the defects dominating the magnetic scattering process. The observations in GO are compared with reduced GO (rGO) which did not show significant spin accumulation. The comparison of data yields a proof of defect-mediated MC in GO.

Spin component in the nernst–Ettingshausen effect in metals with different band structure

The spin contribution of the spin-orbit interaction to the Nernst–Ettingshausen effect in nonmagnetic metals with different band structures (Pt and W) under conditions of the coexistence of a nonzero gradient of the spin chemical potential and a temperature gradient has been studied by direct measurement. The dependence of the manifestation of the different origin spin effects (spin Nernst–Ettingshausen effect is of a thermal diffusion nature and spin Hall effect is of an electrical nature) on the band structure of the metal, which controls the profile of the spin magnetization established at the edges of the sample due to the spin accumulation is found.

Enhanced spin accumulation in nano-pillar-based lateral spin valve using spin reservoir effect

Lateral spin valves are ideal nanostructures for investigating spin-transport physics phenomena and promoting the development of future spintronic devices owing to dissipation-less pure spin current. The magnitude of the spin accumulation signal is well understood as a barometer for characterizing spin current devices. Here, we develop a novel fabrication method for lateral spin valves based on ferromagnetic nanopillar structures using a multi-angle deposition technique. We demonstrate that the spin-accumulation signal is effectively enhanced by reducing the lateral dimension of the nonmagnetic spin channel. The obtained results can be quantitatively explained by the confinement of the spin reservoir by considering spin diffusion into the leads. The temperature dependence of the spin accumulation signal and the influence of the thermal spin injection under a high bias current are also discussed.

Connections between spin-orbit torques and unidirectional magnetoresistance in ferromagnetic-metal–heavy-metal heterostructures

We investigate the connections between current-induced spin-orbit torques and unidirectional magnetoresistance (UMR) by performing second harmonic longitudinal resistance measurements on Co/Pt bilayer with magnetic-field ${\ensuremath{\mu}}_{0}H$ up to 10 T and temperature $T$ down to 2 K. The fieldlike torque ${h}_{\mathrm{FL}}$ changes sign with varying Co thickness ${t}_{\mathrm{Co}}$, which indicates that competing mechanisms, i.e., the spin Hall effect (SHE) and the inverse spin galvanic effect (iSGE), are responsible for the generation of ${h}_{\mathrm{FL}}$. The sign of ${h}_{\mathrm{FL}}$ coincides with the sign of UMR induced by spin-dependent scattering. However, the dampinglike torque ${h}_{\mathrm{DL}}$ is proportional to the inverse Co thickness, with no sign reversal for all measured temperatures and ${t}_{\mathrm{Co}}$, indicating that ${h}_{\mathrm{DL}}$ originates solely from the SHE. The generation of ${h}_{\mathrm{DL}}$ via iSGE can be further excluded because of the observation of an $H$-linear dependence of UMR, which, in turn, indicates the negligible exchange coupling between spin accumulation and ferromagnetic metal.

Ultrasensitive Protein Concentration Characterization Based on Weak Measurements

An optical rotation bio-sensor based on the photonic spin Hall effect was established and applied to detecting the concentration varieties of chiral molecules. The optical rotation, introduced by sample solutions, was exploited to modulate the postselected polarization of a weak measurement system. Much work has been done in the case of glucose and fructose. However, little attention has been paid for biomolecules, such as proteins and amino acids. With this modulation, the optical rotation can be determined through the direction and spin accumulation of light spots, thus mirroring the concentration of solutions. A resolution of 2×10−4 degree was achieved.

Spin-orbit induced equilibrium spin currents in materials

The existence of spin-currents in absence of any driving external fields is commonly considered an exotic phenomenon appearing only in quantum materials, such as topological insulators. We demonstrate instead that equilibrium spin currents are a rather general property of materials with non negligible spin-orbit coupling (SOC). Equilibrium spin currents can be present at the surfaces of a slab. Yet, we also propose the existence of global equilibrium spin currents, which are net bulk spin-currents along specific crystallographic directions of materials. Equilibrium spin currents are allowed by symmetry in a very broad class of systems having gyrotropic point groups. The physics behind equilibrium spin currents is uncovered by making an analogy between electronic systems with SOC and non-Abelian gauge theories. The electron spin can be seen as the analogous of the color degree of freedom and equilibrium spin currents can then be identified with diamagnetic color currents appearing as the response to an effective non-Abelian magnetic field generated by SOC. Equilibrium spin currents are not associated with spin transport and accumulation, but they should nonetheless be carefully taken into account when computing transport spin currents. We provide quantitative estimates of equilibrium spin currents for several systems, specifically metallic surfaces presenting Rashba-like surface states, nitride semiconducting nanostructures and bulk materials, such as the prototypical gyrotropic medium tellurium. In doing so, we also point out the limitations of model approaches showing that first-principles calculations are needed to obtain reliable predictions. We therefore use Density Functional Theory computing the so-called bond currents, which represent a powerful tool to understand the relation between equilibrium currents, electronic structure and crystal point group.

Observation of the spin-Hall effect in Pt/GaAs by circular polarized photoconductivity

Electrically generated spin accumulation due to the spin Hall effect of Pt/GaAs is detected by circular polarized photoconductivity (CPPC), which shows electron spins with different polarizations accumulated around opposite sample boundaries. An optical absorption model incorporating spin is used to explain these features. The detailed analysis of the observed degree of circular polarization of the photocurrent strongly suggests that Pt and GaAs have the same spin accumulation length in the Pt/GaAs heterostructure.

Исследование спектров отражения и пропускания массивов гетерогенных ферромагнитных нанопроволок в терагерцовом и дальнем инфракрасном диапазонах

In the frequency range from 16 to 50 THz, the transmission, reflection, and absorption spectra of arrays of heterogeneous Ni/Co, FeNi/Co, and Ni/Fe nanowires grown in track polymer membranes by the galvanic method have been studied. The absorption spectra showed that the fraction of the radiation power absorbed by the nanowires and its spectrum depend on the materials of the nanowires. The features of the spectra can be explained by the accumulation of nonequilibrium spin due to electron diffusion and its relaxation stimulated by external terahertz (THz) radiation, which can be used to create a THz radiation detector. In addition, negative absorption was found for FeNi/Co nanowires, which can be explained by the laser effect on spin-flip transitions, which can be used to create a THz radiation source operating at room temperature.

Study of reflection and transmission spectra of arrays of heterogeneous ferromagnetic nanowires in the terahertz and far infrared ranges

In the frequency range from 16 to 50 THz, the transmission, reflection, and absorption spectra of arrays of heterogeneous Ni/Co, FeNi/Co, and Ni/Fe nanowires grown in track polymer membranes by the galvanic method have been studied. The absorption spectra showed that the fraction of the radiation power absorbed by the nanowires and its spectrum depend on the materials of the nanowires. The features of the spectra can be explained by the accumulation of nonequilibrium spin due to electron diffusion and its relaxation stimulated by external terahertz (THz) radiation, which can be used to create a THz radiation detector. In addition, negative absorption was found for FeNi/Co nanowires, which can be explained by the laser effect on spin-flip transitions, which can be used to create a THz radiation source operating at room temperature. Keywords: nanowires, THz radiation, THz spectroscopy, spin-flip transitions, metamaterial.

Electric field modulation of spin transport

The finite spin lifetime in solids is often considered a major hindrance for the development of spintronic devices, which typically require cryogenic temperatures to mitigate this phenomenon. In this work, we show that this feature can instead be exploited to realize a scheme where spin transport is modulated at room temperature by a modest electric field. A field directed antiparallel (parallel) to the spin-diffusion velocity can, in fact, largely increase (decrease) the spin-transport length compared with the zero field case. We find that applying an electric field E = 24 V/cm along a 40 μm-long path in germanium results in about one order of magnitude modulation of the spin-polarized electrons entering into the detector. This work demonstrates that electric fields can be exploited for guiding spins over macroscopic distances and for realizing fast room temperature modulation of spin accumulation.

Conversion between spin and charge currents in topological-insulator/nonmagnetic-metal systems

The charge current in a topological insulator (TI) will induce a spin accumulation (Edelstein effect or EE), from which the spin current will be generated. Inversely, the spin current injection into the TI will induce a charge current called the inverse Edelstein effect (IEE). Some experimental and theoretical works have been done for the understanding of either EE or IEE. However, little experimental work incorporating both processes in the same TI sample has been done. In this work, we propose a phenomenological model to understand EE and IEE in the TI-based system. Based on this model, efficiencies of EE and IEE can be directly derived, which is consistent with previous theoretical work based on Boltzmann transport theory and obeying the energy conservation law. We also measure EE and IEE efficiencies experimentally in a TI/Ru/CoFeB system by spin-torque ferromagnetic resonance and spin pumping, respectively. The experimental results are consistent with our model, which proves that the spin-charge conversion in TI can be understood in the framework of (I)EE instead of (inverse) spin Hall effect. By combining theories and experiments, we find that enhancing interfacial transparency is crucial for enhancing EE efficiency, and avoiding metallic contact is crucial for enhancing IEE efficiency.

Charge-spin interconversion in graphene-based systems from density functional theory

We present a methodology to address, from first principles, charge-spin interconversion in two-dimensional materials with spin-orbit coupling. Our study relies on an implementation of density functional theory based quantum transport formalism adapted to such purpose. We show how an analysis of the $k$-resolved spin polarization gives the necessary insight to understand the different charge-spin interconversion mechanisms. We have tested it in the simplest scenario of isolated graphene in a perpendicular electric field where effective tight-binding models are available to compare with. Our results show that the flow of an unpolarized current across a single layer of graphene produces, as expected, a spin separation perpendicular to the current for two of the three spin components (out-of-plane and longitudinal), which is the signature of the spin Hall effect. Additionally, it also yields an overall spin accumulation for the third spin component (perpendicular to the current), which is the signature of the Rashba-Edelstein effect. Even in this simple example, our results reveal an unexpected competition between the Rashba and the intrinsic spin-orbit coupling. Remarkably, the sign of the accumulated spin density does not depend on the electron or hole nature of the injected current for realistic values of the Rashba coupling.

Valley and spin accumulation in ballistic and hydrodynamic channels

A theory of the valley and spin Hall effects and resulting accumulation of the valley and spin polarization is developed for ultraclean channels made of two-dimensional semiconductors where the electron mean free path due to the residual disorder or phonons exceeds the channel width. Both ballistic and hydrodynamic regimes of the electron transport are studied. The polarization accumulation is determined by interplay of the anomalous velocity, side-jump and skew scattering effects. In the hydrodynamic regime, where the electron–electron scattering is dominant, the valley and spin current generation and dissipation by the electron–electron collisions are taken into account. The accumulated polarization magnitude and its spatial distribution depend strongly on the transport regime. The polarization is much larger in the hydrodynamic regime as compared to the ballistic one. Significant valley and spin polarization arises in the immediate vicinity of the channel edges due to the side-jump and skew scattering mechanisms.

Irreversible thermodynamics at surfaces: inverse Edelstein effect for topological insulator

We employ Onsager’s irreversible thermodynamics to study the inverse Edelstein effect (IEE) for a non-magnetic material (NM) adjacent to a topological insulator (TI) with a strong spin–orbit interaction. The TI surface state region is treated as quasi two-dimensional (2D). For the IEE, the source is a 3D spin flux incident from the NM that converts, at the NM–TI interface, to a quasi-2D charge current in the TI. For the Edelstein effect (EE), the source is a quasi-2D charge flux incident from the TI that converts, at the interface, to a three-dimensional (3D) spin flux in the NM. For strong spin–orbit coupling, as considered here, when the 3D spin flux crosses to the 2D TI, the quasi-2D charge current is produced along with a quasi-2D spin accumulation. (For weak spin–orbit coupling, production of charge current and of spin accumulation are distinct processes.) We compute the associated rates of heating.

Direct spin accumulation quantification in ferromagnetic heterostructures using DC bias harmonic Hall measurement

A technique for spin accumulation quantification, specifically in the ferromagnetic layer of spin-orbit torque inducing heterostructures has been elusive. Here, we demonstrate an easy-to-implement technique to achieve this quantification by applying an additional DC bias during the harmonic Hall measurement in Si/${\mathrm{SiO}}_{2}$/Ta/Co/Pt. The spin accumulation arising from the DC bias generates an amplitude offset detectable in the first harmonic Hall magnetoresistance. By performing the first harmonic Hall magnetoresistance measurement under a fixed DC bias for two oppositely magnetized states, spin accumulation polarity set by the DC bias enhances the magnetoresistance if it aligns with the magnetization, and vice versa. Thus, the difference in the magnetoresistance amplitude provides the quantitative magnitude of spin accumulation relative to the ferromagnet's saturation magnetization, measured to be up to 0.29% in Si/${\mathrm{SiO}}_{2}$/Ta/Co/Pt. The strength of both spin accumulation and dampinglike efficiency increased with Ta thickness, further verifying our experimental technique.

Spin accumulation and dissipation excited by an ultrafast laser pulse

An ultrafast spin current can be induced by femtosecond laser excitation in a ferromagnetic (FM) thin film in contact with a nonmagnetic (NM) metal. The propagation of an ultrafast spin current into an NM metal has recently been found in experiments to generate transient spin accumulation. Unlike spin accumulation in equilibrium NM metals that occurs due to spin transport at the Fermi energy, transient spin accumulation involves highly nonequilibrium hot electrons well above the Fermi level. To date, the diffusion and dissipation of this transient spin accumulation has not been well studied. Using the superdiffusive spin transport model, we demonstrate how spin accumulation is generated in NM metals after laser excitation in an $\mathrm{FM}/\mathrm{NM}$ bilayer. The spin accumulation shows an exponential decay from the $\mathrm{FM}/\mathrm{NM}$ interface, with the decay length increasing to the maximum value and then decreasing until saturation. By analyzing the ultrafast dynamics of laser-excited hot electrons, the ``effective mean free path,'' which can be characterized by the averaged product of the group velocity and lifetime of hot electrons, is found to play a key role. The interface reflectivity modulates the spin accumulation near the $\mathrm{FM}/\mathrm{NM}$ interface and varies the spin decay length quantitatively.

Seebeck effect in nanomagnets

We present a theory of the Seebeck effect in nanomagnets with dimensions smaller than the spin diffusion length, showing that the spin accumulation generated by a temperature gradient strongly affects the thermopower. We also identify a correction arising from the transverse temperature gradient induced by the anomalous Ettingshausen effect and an induced spin-heat accumulation gradient. The relevance of these effects for nanoscale magnets is illustrated by ab initio calculations on dilute magnetic alloys.

Spin of primordial black holes in the model with collapsing domain walls

The angular momentum (spin) acquisition by a collapsing domain wall at the cosmological radiation-dominated stage is investigated. During the collapses, primordial black holes and their clusters can be born in various mass ranges. Spin accumulation occurs under the influence of tidal gravitational perturbations from the surrounding density inhomogeneities at the epoch when the domain wall crosses the cosmological horizon. It is shown that the dimensionless spin parameter can have the small values aS < 1 only for primordial black holes with masses M > 10-3M☉, whereas less massive black holes receive extreme spins aS ≃ 1. It is possible that primordial black holes obtain an additional spin due to the vector mode of perturbations.

Emergence of spin-orbit coupled ferromagnetic surface state derived from Zak phase in a nonmagnetic insulator FeSi.

FeSi is a nonmagnetic narrow-gap insulator, exhibiting peculiar charge and spin dynamics beyond a simple band structure picture. Those unusual features have been attracting renewed attention from topological aspects. Although the surface conduction was demonstrated according to size-dependent resistivity in bulk crystals, its topological characteristics and consequent electromagnetic responses remain elusive. Here, we demonstrate an inherent surface ferromagnetic-metal state of FeSi thin films and its strong spin-orbit coupling (SOC) properties through multiple characterizations of two-dimensional conductance, magnetization, and spintronic functionality. Terminated covalent bonding orbitals constitute the polar surface state with momentum-dependent spin textures due to Rashba-type spin splitting, as corroborated by unidirectional magnetoresistance measurements and first-principles calculations. As a consequence of the spin-momentum locking, nonequilibrium spin accumulation causes magnetization switching. These surface properties are closely related to the Zak phase of the bulk band topology. Our findings propose another route to explore noble metal–free materials for SOC-based spin manipulation.