Spin-galvanic Effect Research Articles

Theoretical study of the dynamics of magnetization on the topological surface

We investigate theoretically the dynamics of magnetization coupled to the surface Dirac fermions of a three dimensional topological insulator, by deriving the Landau-Lifshitz-Gilbert (LLG) equation in the presence of charge current. Both the inverse spin-Galvanic effect and the Gilbert damping coefficient $\alpha$ are related to the two-dimensional diagonal conductivity $\sigma_{xx}$ of the Dirac fermion, while the Berry phase of the ferromagnetic moment to the Hall conductivity $\sigma_{xy}$. The spin transfer torque and the so-called $\beta$-terms are shown to be negligibly small. Anomalous behaviors in various phenomena including the ferromagnetic resonance are predicted in terms of this LLG equation.

Inverse Spin-Galvanic Effect in the Interface between a Topological Insulator and a Ferromagnet

When a ferromagnet is deposited on the surface of a topological insulator the topologically protected surface state develops a gap and becomes a two-dimensional quantum Hall liquid. We demonstrate that the Hall current in such a liquid, induced by an external electric field, can have a dramatic effect on the magnetization dynamics of the ferromagnet by changing the effective anisotropy field. This change is dissipationless and may be substantial even in weakly spin-orbit coupled ferromagnets. We study the possibility of dissipationless current-induced magnetization reversal in monolayer-thin, insulating ferromagnets with a soft perpendicular anisotropy and discuss possible applications of this effect.

Beyond spin-magnetism of magnetic nanowires in porous silicon

A ferromagnet–semiconductor (Si–Ni-) nanowire composite, electrochemically prepared inan n-doped (001) silicon wafer, is studied using SQUID magnetometry at magneticfields up to 7 T parallel and perpendicular to the Ni-wires (diameter 50 nm, length1 µm,4 × 108 wires mm−2). Apart from the conventional spin-magnetism of Ni which is saturatedat low field, an additional giant paramagnetism is observed at fields>1 T, which is nearly temperature independent, shows strict linear field dependence, stronganisotropy and a lack of saturation. Taking the difference of the orthogonal magnetizationsthis unconventional paramagnetism becomes obvious. It is based on mesoscopiccurrents driven by the symmetry breaking at the wire–silicon interface due tothe Rashba field (spin-galvanic effect). Spin-polarized carriers from the Ni-wiresare captured in low angular momentum quantum states . An attempt has been made to estimate the observed giant magnetic moment undersimplifying assumptions.

SPIN-GALVANIC EFFECT AND SPIN ORIENTATION BY CURRENT IN NON-MAGNETIC SEMICONDUCTORS

Lately, there is much interest in the use of the spin of carriers in semiconductor quantum well (QW) structures together with their charge to realize novel concepts like spintronics. The necessary conditions to develop spintronic devices are high spin polarizations in QWs and a large spin-splitting of subbands in k-space. The latter is important for the ability to control spins with an external electric field by the Rashba effect. Significant progress has been achieved recently in generating large spin polarizations, in demonstrating the Rashba splitting and also in using the splitting for manipulating the spins. At the same time as these conditions are fulfilled and spins are polarized in-plane of QW, it has been shown that the spin polarization itself drives a current resulting in the spin galvanic effect [1,2]. The spin-galvanic effect is due to asymmetric spin-flip scattering of spin polarized carriers and it is determined by the process of spin relaxation. In some optical experiments, where circularly polarized radiation is used to orient spins, the photocurrent may represent a sum of spin-galvanic and circular photogalvanic effects effects.2,3 Both effects provide methods to determine spin relaxation times and the relative strength of the Rashba/Dresselhaus spin-splitting in semiconductor quantum wells.2 The inverse spin-galvanic effect4 has also been detected demonstrating that electric current in non-magnetic but gyrotropic QWs results in a non-equilibrium spin orientation. Just recently a first direct experimental proof of this effect was obtained in semiconductor QWs5,6 as well as in strained bulk material.7 Microscopically the effect is a consequence of spin-orbit coupling which lifts the spin-egeneracy in k-space of charge carriers together with spin dependent relaxation. Note from Publisher: This article contains the abstract only.

MAGNETO-GYROTROPIC PHOTOGALVANIC EFFECTS IN SEMICONDUCTOR QUANTUM WELLS

The spin-orbit coupling provides a versatile tool to generate and to manipulate the spin degree of freedom in low-dimensional semiconductor structures. The spin Hall effect, where an electric current drives a transverse spin current and causes a nonequilibrium spin accumulation near the sample boundary,1,2 the spin-galvanic effect, where a nonequilibrium spin polarization drives an electric current3,4 or the reverse process, in which an electrical current generates a non-equilibrium spin-polarization,5–9 are all consequences of spin-orbit coupling. In order to observe a spin Hall effect a bias driven current is an essential prerequisite. Then spin separation is caused via spin-orbit coupling either by Mott scattering (extrinsic spin Hall effect) or by spin splitting of the band structure (intrinsic spin Hall effect). Recently an elementary effect causing spin separation which is fundamentally different from that of the spin Hall effect has been observed.10 In contrast to the spin Hall effect it does not require an electric current to flow: it is spin separation achieved by spin-dependent scattering of electrons in media with suitable symmetry. It is show that by free carrier (Drude) absorption of terahertz radiation spin separation is achieved in a wide range of temperatures from liquid helium temperature up to room temperature. Moreover the experimental results demonstrate that simple electron gas heating by any means is already sufficient to yield spin separation due to spin-dependent energy relaxation processes of non-equilibrium carriers. In order to demonstrate the existence of the spin separation due to asymmetric scattering the pure spin current was converted into an electric current. It is achieved by application of a magnetic field which polarizes spins. This is analogues to spin-dependent scattering in transport experiments: spin-dependent scattering in an unpolarized electron gas causes the extrinsic spin Hall effect, whereas in a spin-polarized electron gas a charge current, the anomalous Hall effect, can be observed. As both magnetic fields and gyrotropic mechanisms were used authors introduced the notation "magneto-gyrotropic photogalvanic effects" for this class of phenomena. The effect is observed in GaAs and InAs low dimensional structures at free-carrier absorption of terahertz radiation in a wide range of temperatures from liquid helium temperature up to room temperature. The results are well described by the phenomenological description based on the symmetry. Experimental and theoretical analysis evidences unumbiguously that the observed photocurrents are spin-dependent. Microscopic theory of this effect based on asymmetry of photoexcitation and relaxation processes are developed being in a good agreement with experimental data. Note from Publisher: This article contains the abstract only.

SPIN-GALVANIC EFFECT AND SPIN ORIENTATION BY CURRENT IN NON-MAGNETIC SEMICONDUCTORS

The spin-galvanic effect and the inverse effect, which yeilds current induced spin polarization, in low dimensional semiconductor structures are reviewed. Both effect are caused by asymmetric spin relaxation in systems with lifted spin degeneracy due to k-linear terms in the Hamiltonian.

Magneto-gyrotropic photogalvanic effects in GaN/AlGaN two-dimensional systems

Magneto-gyrotropic photogalvanic and spin-galvanic effects are observed in (0001)-oriented GaN/AlGaN heterojunctions excited by terahertz radiation. We show that free-carrier absorption of linearly or circularly polarized terahertz radiation in low-dimensional structures causes an electric photocurrent in the presence of an in-plane magnetic field. Microscopic mechanisms of these photocurrents based on spin-related phenomena are discussed. Properties of the magneto-gyrotropic and spin-galvanic effects specific for hexagonal heterostructures are analyzed.

New mechanism of the spin-galvanic effect

A mechanism of the spin-galvanic effect associated with spin-dependent scattering is proposed. The electrical current in a system of spin-polarized two-dimensional carriers is induced due to interference of spin-preserving scattering processes and spin relaxation processes. The spin-galvanic effect is studied for heterostructures with the Elliott-Yafet and Dyakonov-Perel spin relaxation mechanisms. The proposed contribution to the spin-galvanic current may be dominant in A3B5 asymmetric quantum wells.

Anisotropic current-induced spin accumulation in the two-dimensional electron gas with spin-orbit coupling

We investigate the magnetoelectric (or inverse spin-galvanic) effect in the two dimensional electron gases with both Rashba and Dresselhaus spin-orbit coupling using an exact solution of the Boltzmann equation for electron spin and momentum. The spin response to an in-plane electric field turns out to be highly anisotropic while the usual charge conductivity remains isotropic, contrary to earlier statements.

Rashba and Dresselhaus spin splittings in semiconductor quantum wells measured by spin photocurrents

The spin-galvanic effect and the circular photogalvanic effect induced by terahertz radiation are applied to determine the relative strengths of Rashba and Dresselhaus band spin-splitting in (001)-grown GaAs and InAs based two dimensional electron systems. We observed that shifting the $\delta$-doping plane from one side of the quantum well to the other results in a change of sign of the photocurrent caused by Rashba spin-splitting while the sign of the Dresselhaus term induced photocurrent remains. The measurements give the necessary feedback for technologists looking for structures with equal Rashba and Dresselhaus spin-splittings or perfectly symmetric structures with zero Rashba constant.

Spintronics aided by terahertz exposure

Generation, control, and detection of a non-equilibrium polarization of electron spin is the key challenge in the growing field of spintronics.1, 2 Optical orientation, in which circularly polarized photons transfer their angular momentum to electrons, is a versatile tool to prepare and investigate non-equilibrium spin polarization. In semiconductor quantum wells (QWs), this momentum transfer yields a spin-polarized electric current—a spin photocurrent—and the irradiated QW acts as a ‘spin battery’.3, 4 Because spin photocurrents occur in homogeneous, unbiased samples under uniform irradiation, the effect is very different from photovoltaic currents, which are caused by charge separation at potential barriers. These currents have been studied in various materials over a wide range of temperatures, including the technologically important room temperature. In contrast to higher-energy, inter-band excitations, the terahertz (THz) spectral range has the advantage that it involves only one kind of carrier, electrons or holes, yielding unipolar spin orientation. Two mechanisms by which THz excitation can generate a spin photocurrent have been investigated in detail: the circular photogalvanic effect (CPGE)4 and the optically induced spin-galvanic effect (SGE).5 A characteristic feature of a spin photocurrent, j, is that it reverses direction, j→ − j, when the radiation helicity is changed, for example from right-handed circular to left-handed circular, σ+ → σ−. The photocurrent is proportional to the helicity, Pcirc. 7 This dependence is illustrated in Figure 1 for p-GaAs QWs. The experimental arrangement is shown in the inset. In these low-symmetry, (113)-grown QWs, the spin photocurrent occurs for radiation at normal incidence, but in (001)-oriented samples a helicity-dependent signal is observed only at oblique incidence due to the higher symmetry. The spin photocurrent results from an asymmetric momentum distribution of photoexcited carriers, which in turn reflects Figure 1. Photocurrent, normalized to the incident power, P, of terahertz radiation, for p-GaAs quantum wells grown with (113) crystallographic orientation. The angleφ defines the helicity through Pcirc = sin 2φ. The radiation source was an NH3 molecular laser pumped by a TEA-CO2 laser emitting 100ns long pulses at 76μm at about P = 10kW peak power.

Zero-bias spin separation

Spin-orbit coupling provides a versatile tool to generate and to manipulate the spin degree of freedom in low-dimensional semiconductor structures. The spin Hall effect, where an electrical current drives a transverse spin current and causes a nonequilibrium spin accumulation observed near the sample boundary, the spin-galvanic effect, where a nonequilibrium spin polarization drives an electric current, or the reverse process, in which an electrical current generates a nonequilibrium spin polarization, are all consequences of spin-orbit coupling. In order to observe a spin Hall effect a bias driven current is an essential prerequisite. The spin separation is caused via spin-orbit coupling either by Mott scattering (extrinsic spin Hall effect) or by Rashba or Dresselhaus spin splitting of the band structure (intrinsic spin Hall effect). Here we provide evidence for an elementary effect causing spin separation which is fundamentally different from that of the spin Hall effect. In contrast to the spin Hall effect it does not require an electric current to flow: It is spin separation achieved by spin-dependent scattering of electrons in media with suitable symmetry. We show that by free carrier (Drude) absorption of terahertz radiation spin separation is achieved in a wide range of temperatures from liquid helium up to room temperature. Moreover the experimental results give evidence that simple electron gas heating by any means is already sufficient to yield spin separation due to spin-dependent energy relaxation processes of nonequilibrium carriers.

Terahertz Radiation Induced Spin Photocurrents in Non-Magnetic Low Dimensional Structures

Spin photocurrents generated by homogeneous excitation with terahertz radiation in low dimensional structures are reviewed. Three microscopic mechanisms are responsible for the occurrence of an electric current linked to a uniform spin polarization in a quantum well, the circular photogalvanic effect, the spin-galvanic effect and the magneto-gyrotropic effect. These phenomena provide several methods to investigate various spin properties in low dimensional structures like details of band structure and spin relaxation time.

Recombination mechanism of the spin-galvanic effect

The mechanism responsible for the spin-galvanic effect is considered. According to this mechanism, the current is generated as a result of the difference between the rates of spontaneous radiative transitions of charge carriers with oppositely directed spins. This difference arises when a spatially uniform nonequilibrium spin orientation of thermalized electrons (holes) is provided by any known method.

Theory of spin-charge-coupled transport in a two-dimensional electron gas with Rashba spin-orbit interactions

We use microscopic linear response theory to derive a set of equations that provide a complete description of coupled spin and charge diffusive transport in a two-dimensional electron gas (2DEG) with the Rashba spin-orbit (SO) interaction. These equations capture a number of interrelated effects including spin accumulation and diffusion, Dyakonov-Perel spin relaxation, magnetoelectric, and spin-galvanic effects. They can be used under very general circumstances to model transport experiments in 2DEG systems that involve either electrical or optical spin injection. We comment on the relationship between these equations and the exact spin and charge density operator equations of motion. As an example of the application of our equations, we consider a simple electrical spin injection experiment and show that a voltage will develop between two ferromagnetic contacts if a spin-polarized current is injected into a 2DEG, that depends on the relative magnetization orientation of the contacts. This voltage is present even when the separation between the contacts is larger than the spin diffusion length.

Experimental separation of Rashba and Dresselhaus spin splittings in semiconductor quantum wells.

The relative strengths of Rashba and Dresselhaus terms describing the spin-orbit coupling in semiconductor quantum well (QW) structures are extracted from photocurrent measurements on n-type InAs QWs containing a two-dimensional electron gas (2DEG). This novel technique makes use of the angular distribution of the spin-galvanic effect at certain directions of spin orientation in the plane of a QW. The ratio of the relevant Rashba and Dresselhaus coefficients can be deduced directly from experiment and does not relay on theoretically obtained quantities. Thus our experiments open a new way to determine the different contributions to spin-orbit coupling.

The infrared spin-galvanic effect in semiconductor quantum wells

The spin-galvanic effect generated by homogeneous optical excitation with infrared circularly polarized radiation in quantum wells (QWs) is reviewed. The spin-galvanic current flow is driven by an asymmetric distribution of spin-polarized carriers in k -space of systems with lifted spin degeneracy due to k -linear terms in the Hamiltonian. Spin photocurrents provide methods to investigate the spin-splitting of the band structure and to make conclusion on the in-plane symmetry of QWs.

Spin-galvanic effect due to optical spin orientation inn-type GaAs quantum well structures

Under oblique incidence of circularly polarized infrared radiation the spin-galvanic effect (SGE) has been unambiguously observed in (001)-grown n-type GaAs quantum well structures in the absence of any external magnetic field. Resonant intersubband transitions have been obtained making use of the tunability of the free-electron laser FELIX. A microscopic theory of the SGE for intersubband transitions has been developed, which is in good agreement with experimental findings.

Spin-Galvanic Effect in Quantum Wells

It is shown that a homogeneous nonequlibrium spin polarization in semiconductor heterostructures results in an electric current. The microscopic origin of the effect is an inherent asymmetry of spin-flip scattering in systems with lifted spin degeneracy caused by k-linear terms in the Hamiltonian.

Optical spin orientation under inter- and intra-subband transitions in QWs

It is shown that absorption of circularly polarized infrared radiation achieved by inter-subband and intra-subband (Drude-like) transitions results in a monopolar spin orientation of free carriers. The monopolar spin polarization in zinc-blende-based quantum wells (QWs) is demonstrated by the observation of the spin-galvanic and circular photogalvanic effects. It is shown that monopolar spin orientation in n-type QWs becomes possible if an admixture of valence band states to the conduction band wave function and the spin–orbit splitting of the valence band are taken into account.