Conversion Of Spin Research Articles

МАГНИТОЭЛЕКТРИЧЕСКИЕ ЭФФЕКТЫ И НОВЫЕ ЛОГИЧЕСКИЕ УСТРОЙСТВА СПИНТРОНИКИ

The paper discusses new logic spintronic devices and the prospects for the use of perovskite-type multiferroics as working elements of magnetoelectric components. The principle of operation of the considered logical devices is based on the use of two components - a magnetoelectric, in which the magnetic state is recorded due to energy-efficient magnetoelectric interaction, and a spin-orbital component, in which information is read out based on the conversion of spin into charge due to the spin-orbital interaction of electrons; both components are interconnected by a nanoelectrode. When designing new logic spintronic devices, it is necessary to take into account the effectiveness of the mechanisms of ME interactions; features of spin - polarized currents and associated torques influencing magnetic moments; as well as other factors affecting the speed of switching magnetic states and the sensitivity of the device to external agents. Multiferroic materials that are promising for use as elements of ME components of new logic devices must meet a number of requirements, the most significant of which are the magnitude of the magnetoelectric coupling coefficient and the temperature at which ME effects occur. The paper considers representatives of multiferroics with a perovskite structure that meet these conditions, to some extent partially, these are high-temperature multiferroic bismuth ferrite (BiFeO3) and Ruddlesden-Popper structures, in which high-temperature ferroelectric effects are already realized and under certain conditions an ME effect is possible. The crystal structure of these compounds is considered, and the role of crystallographic distortions responsible for the manifestation of magnetoelectric properties is analyzed. Expressions are obtained for the tensor of the magnetoelectric effect as functions of magnetic order parameters, and the fundamental possibility of realizing ME effects in Ruddlesden-Popper structures containing magnetic cations is shown.

Spin–orbit interaction in non-paraxial Gaussian beams and the spin-only measurement of optical torque

A circularly polarized focussed Gaussian beam carries total angular momentum of per photon about the beam axis, but less than spin per photon, due to the focussing of the beam. The remainder of the angular momentum is carried as orbital angular momentum. When such beams are used to rotate microscopic birefringent particles in optical tweezers, the change in angular momentum can be optically measured. However, this measurement is made using the collimated transmitted beam, rather than the focussed beam. Therefore, the conversion of spin to orbital angular momentum by focussing or collimating the beam is expected to affect the measurement. We show that for the typical cases where rotating optical tweezers are used for such measurements, the error due to spin–orbit conversion is unimportant, but there exist cases where a spin-only torque measurement would be completely erroneous.

Metal-Organic Frameworks with Assembled Bifunctional Microreactor for Charge Modulation and Strain Generation toward Enhanced Oxygen Electrocatalysis.

Two-dimensional metal-organic frameworks (MOFs) have served as favorable prototypes for electrocatalytic oxygen evolution reaction (OER). Despite promising catalytic activity, their OER reaction kinetics are still limited by the sluggish four-electron transfer process. Herein, we develop a ferrocene carboxylic acid (FcCA) partially substituted cobalt-terephthalic acid (CoBDC) catalyst with a bifunctional microreactor composed of two species of Co active sites and ligand FcCA (CoBDC FcCA). Benefiting from the ultrathin nanosheet structure, CoBDC FcCA catalyst exhibits an excellent OER performance with a low overpotential of 280 mV to reach 10 mA cm-2 and a small Tafel slope of 53 mV dec-1. Structure characterization together with theoretical calculations directly unravel the coordination for two species of Co active moieties with FcCA forming a microreactor of tensile strain, leading to a conversion of the Co spin from a high spin state (t2g5eg2) to an intermediate spin state (t2g6eg1) to regulate antibonding states of Co 3d and O 2p orbital. In situ spectroscopic measurements for mechanistic understanding reveal that this CoBDC FcCA catalyst possesses an optimal OH* adsorption energy for propitious formation of O-O bonds in the OOH* intermediate, thus effectively decreasing the thermodynamic Gibbs free energy of the rate-determining step (O* → OOH*) to accelerate reaction kinetics for the whole OER process. When loaded on an integrated BiVO4 photoanode as a cocatalyst, CoBDC FcCA enables highly active solar-driven oxygen production from water splitting.

Interconversion of orbital and spin angular momenta of light beams in the CARS generation process in isotropic gyrotropic medium

We investigate the conversion of spin and orbital angular momentum (AM) of elliptically polarized light beams at frequencies ω 1 and ω 2, helical phase profiles, and arbitrary radial intensity distributions to AM of a signal beam at frequency resulting from four-wave mixing process in isotropic gyrotropic medium with spatial dispersion of cubic nonlinearity. Conservation of the total z-projections of both spin and orbital angular momenta of interacting waves is demonstrated. A simple classical and quantum interpretation of validity of the AM conservation law is given.

Conversion of spin and orbital angular momentum in the third harmonic generation process in the bulk of an isotropic medium

The interaction between spin and orbital components of the angular momentum of an elliptically polarized beam with helicoidal phase profile and arbitrary axially symmetric intensity profile in the third-harmonic generation process is investigated. The third-harmonic beam is generated in an isotropic medium with cubic nonlinearity within undepleted pump approximation. The conservation of the total projections of both the spin angular momentum and orbital angular momentum components in four-wave mixing is demonstrated for interacting waves.

Transformation of spin and orbital angular momentum in second-harmonic generation process at oblique incidence of light from the surface of an isotropic medium with spatial dispersion of quadratic nonlinearity.

We discuss key features of the conversion of spin and orbital angular momentum of electromagnetic waves in the process of second-harmonic generation from the surface of the isotropic medium at oblique incidence. Conservation of the projections of spin and orbital parts of angular momentum of interacting waves onto the normal to the surface is shown for an arbitrary case of polarization and mode structure of the incident light beam.

Terahertz Spin-to-Charge Conversion by Interfacial Skew Scattering in Metallic Bilayers.

The efficient conversion of spin to charge transport and vice versa is of major relevance for the detection and generation of spin currents in spin-based electronics. Interfaces of heterostructures are known to have a marked impact on this process. Here, terahertz (THz) emission spectroscopy is used to study ultrafast spin-to-charge-current conversion (S2C) in about 50 prototypical F|N bilayers consisting of a ferromagnetic layer F (e.g., Ni81 Fe19 , Co, or Fe) and a nonmagnetic layer N with strong (Pt) or weak (Cu and Al) spin-orbit coupling. Varying the structure of the F/N interface leads to a drastic change in the amplitude and even inversion of the polarity of the THz charge current. Remarkably, when N is a material with small spin Hall angle, a dominant interface contribution to the ultrafast charge current is found. Its magnitude amounts to as much as about 20% of that found in the F|Pt reference sample. Symmetry arguments and first-principles calculations strongly suggest that the interfacial S2C arises from skew scattering of spin-polarized electrons at interface imperfections. The results highlight the potential of skew scattering for interfacial S2C and propose a promising route to enhanced S2C by tailored interfaces at all frequencies from DC to terahertz.

Conservation of angular momentum in second harmonic generation from under-dense plasmas

Spin and orbital angular momentum of an optical beam are two independent parameters that exhibit distinct effects on mechanical objects. However, when laser beams with angular momentum interact with plasmas, one can observe the interplay between the spin and the orbital angular momentum. Here, by measuring the helical phase of the second harmonic 2ω radiation generated in an underdense plasma using a known spin and orbital angular momentum pump beam, we verify that the total angular momentum of photons is conserved and observe the conversion of spin to orbital angular momentum. We further determine the source of the 2ω photons by analyzing near field intensity distributions of the 2ω light. The 2ω images are consistent with these photons being generated near the largest intensity gradients of the pump beam in the plasma as predicted by the combined effect of spin and orbital angular momentum when Laguerre-Gaussian beams are used.

Contribution of pulsars to cosmic-ray positrons in light of recent observation of inverse-Compton halos

The hypothesis that pulsar wind nebulae (PWNe) can significantly contribute to the excess of the positron ($e^+$) cosmic-ray flux has been consolidated after the observation of a $\gamma$-ray emission at TeV energies of a few degree size around Geminga and Monogem PWNe, and at GeV energies for Geminga at a much larger extension. The $\gamma$-ray halos around these PWNe are interpreted as due to electrons ($e^-$) and $e^+$ accelerated and escaped by their PWNe, and inverse Compton scattering low-energy photons of the interstellar radiation fields. The extension of these halos suggests that the diffusion around these PWNe is suppressed by two orders of magnitude with respect to the average in the Galaxy. We implement a two-zone diffusion model for the propagation of $e^+$ accelerated by the Galactic population of PWNe. We consider pulsars from the ATNF catalog and build up simulations of the PWN Galactic population. In both scenarios, we find that within a two-zone diffusion model, the total contribution from PWNe and secondary $e^+$ is at the level of AMS-02 data, for an efficiency of conversion of the pulsar spin down energy in $e^\pm$ of $\eta\sim0.1$. For the simulated PWNe, a $1\sigma$ uncertainty band is determined, which is of at least one order of magnitude from 10 GeV up to few TeV. The hint for a decreasing $e^+$ flux at TeV energies is found, even if it is strongly connected to the chosen value of the radius of the low diffusion bubble around each source.

Generation of Nondiffracting Vector Beams with Ring-Shaped Plasmonic Metasurfaces

Optical nondiffracting vector beams, with their invariant transverse profiles and longitudinal polarization states, have drawn interest in many areas, from optical tweezers to imaging and metrology, but bulky optical components still limit their utilization. The authors design ultrathin, ring-shaped plasmonic metasurfaces to produce nondiffracting Bessel, Mathieu, and Weber vector beams across a broad wavelength range. These metasurfaces present a compact, effective platform for producing complex optical beams, and thus for advancing numerous applications related to conversion of spin and orbital angular momentum, optical manipulation, and optical communication.

On the structure and physical properties of methyl ammonium lead iodide perovskite thin films by the two-step deposition method

In the present work, we synthesized methyl ammonium lead iodide perovskite (CH3NH3PbI3) thin films using the two-step deposition technique. The method involves conversion of spin coated lead iodide (PbI2) thin films by dipping in a solution of CH3NH3I (MAI) followed by annealing. In this work, the samples were grown by varying the time of dipping and the concentration of MAI solution. The thin films formed were analyzed to determine their crystalline structure, morphology, elemental composition, chemical states and physical properties. The films showed tetragonal crystal structure with compact morphology. The elemental and the respective oxidation state implied the formation of CH3NH3PbI3 thin films. Also, these thin films exhibited direct optical absorption, and the band gap values were in the range of 1.5–1.6 eV. The dark conductivity value of the films varied from 10−6 (Ω cm)−1 to 10−4 (Ω cm)−1 and all the thin films were photoconductive.

Inverse spin Hall effects in Nd doped SrTiO3

Conversion of spin to charge current was observed in SrTiO3 doped with Nd (Nd:STO), which exhibited a metallic behavior even with low concentration doping. The obvious variation of DC voltages for Py/Nd:STO, obtained by inverting the spin diffusion direction, demonstrated that the detected signals contained the contribution from the inverse spin Hall effect (ISHE) induced by the spin dependent scattering from Nd impurities with strong spin-orbit interaction. The DC voltages of the ISHE for Nd:STO were measured at different microwave frequency and power, which revealed that spin currents were successfully injected into doped STO layer by spin pumping. The linear relation between the ISHE resistivity and the resistivity induced by impurities implied that the skew scattering was the dominant contribution in this case, and the spin Hall angle was estimated to be (0.17±0.05)%. This work demonstrated that extrinsic spin dependent scattering in oxides can be used in spintronics besides that in heavy elements doped metals.

Generation of perfect vectorial vortex beams.

We propose the concept of perfect vectorial vortex beams (VVBs), which not merely have intensity profile independent of the polarization order and the topological charge of spiral phase, but also have stable intensity profile and state of polarization (SoP) upon propagation. Utilizing a Sagnac interferometer, we approximately generate perfect VVBs with locally linear and elliptical polarizations, and demonstrate that such beams can keep their intensity profile and SoP at a certain propagation distance. These proposed VVBs can be expanded to encode information and quantum cryptography, as well as to enrich the conversion of spin and orbital angular momenta.

Longitudinal Spin Diffusion in a Nondegenerate Trapped ^{87}Rb Gas.

Longitudinal spin diffusion of two pseudospin domains is studied in a trapped ^{87}Rb sample above quantum degeneracy, and the effect of coherence in the domain wall on the dynamics of the system is investigated. Coherence in a domain wall leads to transverse-spin-mediated longitudinal spin diffusion that is slower than classical predictions, as well as altering the domains' oscillation frequency. The system also shows an instability in the longitudinal spin dynamics as longitudinal and transverse spin components couple, and a conversion of longitudinal spin to transverse spin is observed, resulting in an increase in the total amount of coherence in the system.

An Artificial Enzyme Made by Covalent Grafting of an Fe(II) Complex into β-Lactoglobulin: Molecular Chemistry, Oxidation Catalysis, and Reaction-Intermediate Monitoring in a Protein.

An artificial metalloenzyme based on the covalent grafting of a nonheme Fe(II) polyazadentate complex into bovine β-lactoglobulin has been prepared and characterized by using various spectroscopic techniques. Attachment of the Fe(II) catalyst to the protein scaffold is shown to occur specifically at Cys121. In addition, spectrophotometric titration with cyanide ions based on the spin-state conversion of the initial high spin (S=2) Fe(II) complex into a low spin (S=0) one allows qualitative and quantitative characterization of the metal center's first coordination sphere. This biohybrid catalyst activates hydrogen peroxide to oxidize thioanisole into phenylmethylsulfoxide as the sole product with an enantiomeric excess of up to 20 %. Investigation of the reaction between the biohybrid system and H2 O2 reveals the generation of a high spin (S=5/2) Fe(III) (η(2) -O2 ) intermediate, which is proposed to be responsible for the catalytic sulfoxidation of the substrate.

Control of the spin to charge conversion using the inverse Rashba-Edelstein effect

We show here that using spin orbit coupling interactions at a metallic interface it is possible to control the sign of the spin to charge conversion in a spin pumping experiment. Using the intrinsic symmetry of the “Inverse Rashba Edelstein Effect” (IREE) in a Bi/Ag interface, the charge current changes sign when reversing the order of the Ag and Bi stacking. This confirms the IREE nature of the conversion of spin into charge in these interfaces and opens the way to tailoring the spin sensing voltage by an appropriate trilayer sequence.

Mechanism of high pulsed magnetic field treatment of the plasticity of aluminum matrix composites

For aluminum matrix composite, the introduced particles will strengthen the matrix, but as the obstacles, the heterogeneous particles will hinder the dislocation movement, generate uneven material structure, and may become a source of stress concentration. Therefore, they are detrimental severely to the elongation and plasticity of composite. It is known that dislocations exhibit a paramagnetic behavior because they contain paramagnetic centers including localized electrons, holes, triplet excitons, ion radicals, etc. The initial radical pair of the dislocation-obstacle S (spin angular momentum) = ± 1/2 is in a singlet state, and the total spin of the radical pair is 0 and in the antiparallel spin direction, offsetting a magnetism of the radical pair. The magnetic field can change the spin direction from singlet state to triplet state. In the triplet state the electron spin is 1 and in the same spin direction. A strong bond of the dislocation-obstacle is formed only in the singlet state when the spins of the two electrons are antiparallel. So an obstacle is able to pin a dislocation only if the radical pair is in the singlet state. Under the condition of high pulsed magnetic field treatment (HPMFT) the conversion of electronic spin will be a fundamental cause of dislocation motion along a glide plane. The movement of pinned dislocations will change the material microstructure and influence the performance of material. By comparing the microstructural evolutions and the residual stresses of samples subjected to HPMFT with different values of magnetic induced density (B), the positive influence of magnetoplastic effect on the plasticity of aluminum matrix composite is investigated in this paper. The results show that the dislocation density is significantly increased when B changes from 2 T to 4 T. When B=4 T the dislocation density is enhanced by 3.1 times compared with that of the sample without HPMFT. Moreover, the residual stress is reduced apparently from 41 MPa (B=0) to -1 MPa (B=3 T). In the view of atomic scale, the high magnetic field leads to a magnetoplastic effect which contributes to the dislocation movement and promotes the dislocation depinning, thereafter, the number of movable dislocations increases up. From the viewing of the internal structure of composite, the magnetic field accelerates the releasing rate of internal stress and lowers the residual stress in material, which is beneficial to improving the plasticity of aluminum matrix composite.

Conversion of spin and charge currents in magnetic and non-magnetic systems

We formulate the law of conversion between spin current and charge current in several classes of materials including ferromagnetic, paramagnetic and spin-orbit coupled systems. We find that the interpretation of spin current is highly non-trivial, and we have identified pure and composite spin currents. For an interfacial Rashba spin-orbit coupled Hamiltonian, we show that the spin transport is much closer to that in ferromagnets than in non-magnetic materials. In particular, we show that an in-plane charge current in layered structures will generate a pure spin current flowing perpendicular to the Rashba spin-orbit coupled interface, and vice versa. Application of the spin-charge current conversion in several experimental realizations is discussed.

Spin-to-orbital angular momentum conversion for Bessel light beams in crystals

We have studied the spatial dynamics of spin-to-orbital angular momentum conversion for zero-order and higherorder circularly polarized Bessel light beams, propagating along the optic axes of uniaxial and biaxial crystals. We have established that interaction of the Bessel beam with a uniaxial crystal slab makes possible practically complete conversion of the optical spin [angular] momentum to [orbital] angular [momentum]. In a biaxial crystal slab, during such a conversion the angular momentum is partially transferred to the medium, and as a result the slab undergoes rotation.

Enhanced inverse spin-Hall effect in ultrathin ferromagnetic/normal metal bilayers

Electrically detected ferromagnetic resonance is measured in microdevices patterned from ultra-thin Co/Pt bilayers. Spin pumping and rectification voltages are observed and distinguished via their angular dependence. The spin-pumping voltage shows an unexpected increase as the cobalt thickness is reduced below 2 nm. This enhancement allows more efficient conversion of spin to charge current.