Spin Current Research Articles

Control of spin wave demultiplexing using spin current

The paper reports on the principle of signals demultiplexing in the structure ferromagnetic film/normal metal/magnonic crystal. It is discovered that in such a structure there is a formation of four band gaps - spin wave forbidden bands. It is shown that depending on the frequency, the signal comes out through different ports of the structure (ferromagnetic film or magnonic crystal), i.e. there is frequency division of channels. The influence of spin current on the signal coupling at frequencies inside the band gap significantly prevails over the effect of spin current at frequencies outside the band gap. The spin current allows effective control of the main line insertion loss of a directional coupler based on such a structure. The spin current has little effect on coupling factor and isolation.

Multi-functional spin-caloritronic device based on co-doped zigzag silicene nanoribbon heterojunction

The discovery of graphene has sparked significant interest in two-dimensional (2D) materials within the research community. Graphene’s discovery has led to the exploration of several 2D-materials with varied properties. Silicene, a silicon-based analogue of graphene, is one of the newer 2D-materials. Silicene is a promising material for electronic applications and novel fields like spintronics. In this study, we propose a spin-caloritronic device based on zigzag silicene nanoribbons (ZSiNRs). The device features a heterojunction between two ZSiNRs, one of which is doped with boron (B) and nitrogen (N) atoms. The ZSiNRs’ edges are terminated with hydrogen (H) atoms for both the nanoribbons in the heterojunction. We utilize the density functional theory (DFT) combined with non-equilibrium Green’s function (NEGF) to investigate the thermal-spin transport properties of this device under the influence of a thermal gradient. Notably, the device operates without any external bias, with spin transport driven solely by the thermal gradient. We find that a thermal colossal magnetoresistance (CMR) effect may be produced by modulating thermally generated spin currents by changing the magnetic configuration. Furthermore, the proposed device has superior transport properties, displaying phenomena including the spin-Seebeck diode effect (SSD), spin filtering effect (SFE), and spin-Seebeck effect (SSE). These characteristics position the proposed device as a strong candidate for multi-functional spin-caloritronic applications.

Observation of interface-induced nonlocal spin-torques from Gd/Pt interface

We investigate spin–orbit torques (SOTs) generated by Gd/Pt and Ti/Pt interfaces by performing spin-torque ferromagnetic resonance measurements at room temperature. We report a sizable spin-current created by the Gd/Pt interface, exhibiting the damping-like-torque efficiency per unit electric field, ξDLE=−8 ± 0.6×104ℏ2e(Ωm)−1 as observed in Gd(5 nm)/Pt(1.5 nm)/Py(3 nm-6 nm)/Pt(1.5 nm) heterostructures. This interface-induced spin-current is attributed to the vertical electric field produced by the broken structural inversion symmetry, with a work function difference of 2.75 eV between Gd and Pt. This is further substantiated by varying the thickness of the Gd-layer. We compare these results with the Ti/Pt interface, which has a work function difference of 1.3 eV, by implementing Ti(5 nm)/Pt(1.5 nm)/Py(3 nm-6 nm)/Pt(1.5 nm) heterostructures. We find that the spin-current induced by the Ti/Pt interface is too small to overcome the remnant SOTs caused by the asymmetrically grown Pt layers on both sides of Py. Our work reveals the role of interface-induced spin currents for practical applications, which can be leveraged to produce robust spin torques that complement the bulk spin-Hall torque.

Modern tensor–spinor symbolic algebra algorithms and computing non-closure geometry and holoraumy in 11D, 𝒩=1 supergravity

The Supersymmetry Genomics project aims to classify supermultiplets by properties like adinkras and holoraumy. The project’s protocol is: (1) set up the SUSY transformation rules (and action) with unknown numerical coefficients, (2) solve those coefficients and (3) compute desired properties, e.g. non-closure geometry (the non-closure functions in the anticommutator of supercovariant derivatives) and holoraumy (the commutator of supercovariant derivatives). This paper provides the first broadly applicable computer algorithms for completing these computations, comprising a new Cadabra module we call “SusyPy”. We provide a significant extension of the available tensor arithmetic/canonicalization to include spinor-indexed expressions and NW–SE convention, and we provide a new Fierz expansion algorithm for spinor-indexed expressions. On top of this new tensor–spinor symbolic algebra, we have built algorithms for solving for the coefficients of a multiplet and its action, and for computing holoraumy, for both off-shell and on-shell [Formula: see text] multiplets of any dimension. We apply our tools to assimilate linearized 11D, [Formula: see text] supergravity into the Genomics project. We demonstrate solving the 11D, [Formula: see text] multiplet, as in Cremmer, Julia and Scherk, in a few lines of code, and we provide a comprehensive picture of the multiplet’s non-closure geometry. Further, we provide the first-ever computation of the holoraumy of 11D, [Formula: see text] supergravity.

Spin-Hall effect in topological materials: evaluating the proper spin current in systems with arbitrary degeneracies

The spin-Hall effect underpins some of the most active topics in modern physics, including spin torques and the inverse spin-Hall effect, yet it lacks a proper theoretical description. This makes it difficult to differentiate the SHE from other mechanisms, as well as differentiate band structure and disorder contributions. Here, by exploiting recent analytical breakthroughs in the understanding of the intrinsic spin-Hall effect, we devise a density functional theory method for evaluating the conserved (proper) spin current in a generic system. Spin non-conservation makes the conventional spin current physically meaningless, while the conserved spin current has been challenging to evaluate since it involves the position operator between Bloch bands. The novel method we introduce here can handle band structures with arbitrary degeneracies and incorporates all matrix elements of the position operator, including the notoriously challenging diagonal elements, which are associated with Fermi surface, group velocity, and dipolar effects but often diverge if not treated correctly. We apply this method to the most important classes of spin-Hall materials: topological insulators, 2D quantum spin-Hall insulators, non-collinear antiferromagnets, and strongly spin-orbit coupled metals. We demonstrate that the torque dipole systematically suppresses contributions to the conventional spin current such that, the proper spin current is generally smaller in magnitude and often has a different sign. Remarkably, its energy-dependence is relatively flat and featureless, and its magnitude is comparable in all classes of materials studied. These findings will guide the experiment in characterizing charge-to-spin interconversion in spintronic and orbitronic devices. We also discuss briefly a potential generalization of the method to calculate extrinsic spin currents generated by disorder scattering.

Spintronic Pathways in a Nonconjugated Radical Polymer Glass

AbstractRadical chemistries have attracted burgeoning attention due to their intriguing technological applications in organic electronics, optoelectronics, and magneto‐responsive systems. However, the potential of these magnetically active glassy polymers to transport spin‐selective currents has not been demonstrated. Here, the spin‐transport characteristics of the radical polymer poly(4‐glycidyloxy‐2,2,6,6‐tetramethylpiperidine‐1‐oxyl) (PTEO) allow for sustained spin‐selective currents when incorporated into typical device geometries with magnetically polarized electrodes. Annealing thin films of PTEO above its glass transition temperature results in a giant magnetoresistance effect (i.e., an MR of ≈80%) at 4 K. Additionally, ferromagnetic resonance spin‐pumping results in a relatively large effective spin‐mixing conductance of 1.18 × 1019 m−2 at the NiFe/PTEO interface. Due to the large spin‐density and radical‐radical exchange interactions, there is effective propagation of pure spin currents through PTEO in the NiFe/PTEO/Pd multilayer devices. This results in the transport of spin current over long distances with a spin diffusion length of 90.4 nm. The spin diffusion length and spin mixing conductance values surpass those reported in inorganic and metallic systems and are comparable to conventional doped conjugated polymers. This is the first example of spin transport in a nonconjugated radical polymer, and these findings underscore the promising spin‐transporting potential of radical polymers.

Twisted Magnetic Van der Waals Bilayers: An Ideal Platform for Altermagnetism.

We introduce a universal methodology for generating and manipulating altermagnetism in two-dimensional (2D) magnetic Van der Waals (MvdW) materials through twisting. We find that a key in-plane twofold rotational operation can be achieved in a twisted bilayer of any 2D MvdW material, which takes one of all five 2D Bravais lattices, thereby inducing altermagnetism. By choosing the constituent MvdW monolayer with specific symmetry, our approach can tailor altermagnetism of any type, such as d wave, g wave, and i wave. Furthermore, the properties of our twisted altermagnetic materials can be easily engineered. Taking a transition-metal oxyhalide VOBr as an example, we find that by tuning the twist angle and Fermi level, a giant spin Hall angle can be obtained, much larger than the experimentally reported. This approach establishes a general, robust, and adjustable platform to explore altermagnetism and provides a new efficient way to generate and manipulate the spin current.

Voltage-Controlled Bimeron-Torques Switching of In-Plane Magnetization.

Controlling the out-of-plane magnetization in nanodevices via electric currents has played a pivotal role in advancing high-impact data storage and nonvolatile logic technologies within spintronic devices. However, the in-plane magnetization materials, which are more accessible than their out-of-plane counterparts, are less favored in terms of switching efficiency and magnetization stability. This is due to their larger switching barriers, resulting in a higher critical switching current in the commonly used spin-transfer torques or spin-orbit torques magnetic random-access memory. Here, we propose the use of voltage-controlled bimeron-torques to switch in-plane magnetization for ultralow energy consumption magnetic random-access memory. In this mechanism, magnetic bimerons act as spin-angular-momentum carriers as well as momentum transfer media rather than that of electrical spin currents in spin-transfer torques or spin-orbit torques. We address the microscopic origin of this mechanism and demonstrate its validity with two examples: Co(MoTe_{2})_{2} and HgInP_{2}O_{6} monolayers, respectively. The mechanism overcomes the restriction of perpendicular magnetization and avoids Joule heating, thereby providing additional pathways for electrical manipulation of magnetic materials.

Ferrospintronic Order in Noncentrosymmetric Antiferromagnets: An Avenue toward Spintronic‐Based Computing, Data Storage, and Energy Harvesting

A ferroic order, ferrospintronic (FSp) order, arising in some magnetic materials lacking inversion symmetry is reported on. Emerging from a macroscopic spontaneous symmetry breaking of both the time reversal T and the spatial inversion, while maintaining the symmetry, the order parameter is given by the dipolar moment of the spin density . Herein a model is advanced that fulfills the symmetry requirements mentioned earlier and its properties are investigated. With the aid of a mean‐field theory, its stability against thermal fluctuations is studied and evidence of truly ferroic states that result from breaking its symmetry is provided. Three predictions that can be checked experimentally to distinguish this class of materials from other compounds are provided. In the first place, it is illustrated that FSp systems host the potential for multiferroic behavior. Second, it is shown that the generic FSp system responds under strain by generating spin currents that can be used in spintronic devices, therefore showing a piezospintronic effect. Finally, it is proven that domain walls in the FSp order parameter lead to magnetoresistive effects. All of the findings mentioned earlier are a significant breakthrough in spintronics and multiferroic phenomena and they have wide‐ranging implications for advancing materials and technologies, particularly in computing and energy harvesting.

Time-dependent spin and charge transport in a strain-engineered graphene sheet coupled to a single-molecular magnet

Abstract Recent confirmation shows that graphene is the toughest substance on record and can withstand elastic and reversible deformations exceeding 20%. In this article, we focus on the effect of homogeneous axial tension on graphene-based junctions, where a molecular magnet is coupled to a graphene sheet connected to ferromagnetic leads. We demonstrate that a homogeneous longitudinal strain enhances the oscillatory variations in the spin and charge currents at the time, gate, and bias voltages. In particular, we demonstrate that applying strain makes it possible to control the switching time between the minimum and maximum spin and charge currents. These results can be employed in spintronic devices based on graphene to enhance the control of the device features.

Spin conduction and interfacial effects in La2/3Sr1/3MnO3/NiO/Pt heterostructures

In this work we report on the generation and transport of pure spin currents in La2/3Sr1/3MnO3(LSMO)/NiO/Pt heterostructures. Pure spin currents, generated by means of spin pumping in the LSMO layer, are transmitted through the NiO layer and detected by measuring the transverse voltage signal generated by the inverse spin Hall effect (ISHE) in the Pt layer. The spin current transmission is studied as a function of the NiO thickness and temperature. NiO layers grown at room temperature are polycrystalline and their microstructural and magnetic features clearly depend on thickness. Scanning transmission electron microscopy techniques revealed that homogeneous conformal coating of the LSMO layer is only achieved for NiO layer thicknesses above about 2 nm. Below this critical thickness the NiO layer is discontinuous and its main role would be to disturb the LSMO/Pt interface causing it to behave as something similar to an insulating barrier. Magnetic measurements indicate that NiO layers are paramagnetic (PM) well below room temperature. In agreement with these observations, the amplitude of the ISHE voltage signal decreases monotonically on lowering temperature and makes evident that there are two spin conduction regimes as a function of the NiO layer thickness with different spin diffusion lengths. For thin discontinuous NiO layers a spin diffusion length of λSDL≈0.8 nm is found, which is slightly larger than typical values found for insulating barriers, but definitely smaller than λSDL≈3.8 nm found for NiO layer thickness above 2 nm. The latter being similar to λSDL previously reported for epitaxial NiO layers. Our findings indicate that inserting a PM NiO insulating layer of optimal thickness improves the spin transparency of the LSMO/Pt interface. Additionally, the results suggest that spin conduction through the NiO layer is likely driven by magnetic correlations and short-range thermal magnons.

Extraction of two-magnon scattering and detection of inverse spin Hall effect up to 40 GHz in low-damping Fe65Co35/Pt bilayer thin film

Abstract We report an investigation of the inverse spin Hall effect (ISHE), as a measure of the pure spin current (J S ), induced by spin pumping in a Fe65Co35/Pt bilayer using coplanar waveguide ferromagnetic resonance (CPW-FMR) in the in-plane (IP) geometry over the broadband microwave frequency range of 8–40 GHz. From the IP-FMR measurements, the defect-induced two-magnon scattering (TMS) contribution is evaluated by analyzing the frequency dependence of FMR linewidth using Arias and Mills approach [Arias R and Mills D L (1999 Phys. Rev. B 60, 7395), Mills D L and Arias R (2006 Physica B 384 147–151)]. Here, we have determined a very low Gilbert damping constant (α G ) for Fe65Co35 bare film around 1.3 × 10−3, which is essential for the high spin current (>107 A/m2) generation. The enhanced value of α G ≈3.2 × 10−3 obtained for Fe65Co35/Pt bilayer film is due to spin pumping damping, α SP = 1.9 × 10−3, confirmed by the ISHE induced DC voltage (V DC ) signal measured across the edges of Pt layer. Out-of-plane (OP) angular ISHE measurements have been performed to disentangle spin pumping from spin rectification effects in the V DC signal. Further, we evaluated the effective spin-mixing conductance g eff ↑ ↓ = 3.33 × 1019 m−2, which is higher than the values reported in FeCo-alloy-based bilayer systems. The present study aims to detect the J S injected by low-damping Fe65Co35 thin films, leading to the realization of energy-efficient spintronic devices.

Damping of spin waves

We show that, in ideal-spin hydrodynamics, the components of the spin tensor follow damped wave equations. The damping rate is related to nonlocal collisions of the particles in the fluid, which enter at first order in ℏ in a semiclassical expansion. This rate provides an estimate for the timescale of spin equilibration and is computed by considering a system of spin-1/2 fermions interacting via a quartic self-interaction as well as via (screened) one-gluon exchange. It is found that the relaxation times of the components of the spin tensor can become very large compared to the usual dissipative timescales of the system. Our results suggest that the spin degrees of freedom in a heavy-ion collision may not be in equilibrium by the time of freeze-out, and thus should be treated dynamically. Published by the American Physical Society 2024

The effect of magnetic nanofilm morphology on spintronic terahertz emission performance

Femtosecond laser-driven spintronic terahertz (THz) emitters based on magnetic nanofilms are poised to be the next-generation mainstream THz radiation devices due to their low cost, high performance, ultra-broadband, and easy integration. The radiation performance of spintronic THz emitters is related to the material characteristics, heterostructure interfaces, pump laser, and magnetic field intensity. Additionally, the THz emission performance is greatly reliant on the material surface morphology. Here, we employed ultrafast THz scattering-type scanning near-field optical microscopy with nanoscale spatial resolution to obtain the static THz scattering nano-imaging of ferromagnetic/antiferromagnetic heterostructures (W/Co20Fe60B20/IrMn3). We established the relationship between surface morphology and THz scattering intensity. Utilizing laser-induced THz emission technology, we achieve injection and detection of nanoscale ultrafast spin current without the external magnetic field. The strong consistency of the THz emission nanoscopy with the atomic force microscopy topography demonstrates that the sample surface morphology is critical to the THz radiation performance. This study serves as a valuable reference for the further optimization of spintronic THz emitters and promotes the development of high-performance, strong-field spintronic THz sources.

Skyrmion motion monitoring based on a ferromagnetic nanodot chain

As one of the most promising information carriers, the generation, manipulation, and detection of magnetic skyrmions have emerged as a hot topic in the field of spintronics. However, a major bottleneck to their practical application lies in the existing limitations of detection technology, which fails to accurately locate skyrmions or monitor their real-time motion behavior. In this work, we propose a patterned heterostructure scheme comprising a nanodot chain (NDC) layer and a skyrmion nano-racetrack layer for precise monitoring of skyrmion motion. By exploiting the stray field generated by the moving skyrmion within the racetrack layer, magnetization changes are induced in nanodots within the NDC layer. These changes then translate into high-frequency magnetization oscillation signals that encode valuable information about the dynamic characteristics of driven skyrmions, such as speed and acceleration of the skyrmion, either by spin waves or spin currents. This scheme holds great potential for advancing spintronic devices based on a profound understanding of skyrmion dynamics.

Circular photocurrents in centrosymmetric semiconductors with hidden spin polarization

Centrosymmetric materials with site inversion asymmetries possess hidden spin polarization, which remains challenging to be converted into spin currents because the global inversion symmetry is still conserved. This study demonstrates the spin-polarized circular photocurrents in centrosymmetric transition metal dichalcogenide semiconductors at normal incidence without applying electric bias. The global inversion symmetry is broken by using a spatially-varying circularly polarized light beam, which could generate spin gradient owing to the hidden spin polarization. The dependence of the circular photocurrents on electrode configuration, illumination position, and beam spot size indicates an emergence of circulating electric current under spatially inhomogeneous light, which is associated with the deflection of spin-polarized current through the inverse spin Hall effect. The circular photocurrents is subsequently utilized to probe the spin polarization and the inverse spin Hall effect under different excitation wavelengths and temperatures. The results of this study demonstrate the feasibility of using centrosymmetric materials with hidden spin polarization and non-vanishing Berry curvature for spintronic device applications.

Enhanced mechanical and electrical properties of single-walled carbon nanotube fibers via electric field-assisted wet spinning

Researchers have been working on creating fibers from single-walled carbon nanotubes (SWCNTs) that have optimal properties at the molecular level. However, the prepared SWCNT fibers have showed much lower properties than the ones predicted by theory because the interactions between SWCNTs and SWCNT bundles were not strong enough due to the poor alignment of SWCNTs in the fibers. In this study we improved the mechanical and electrical properties of the SWCNT fibers by using an electric field during the wet spinning process. The electric field, which was applied to the top of the syringe plunger and the tip of the spinneret, oriented the SWCNTs in the chlorosulfonic acid dope solution in a nozzle system, and made them more compact. The SWCNT fibers produced through our process exhibited a significant improvement in their properties. Specifically, we observed a 117.5 % increase in tensile strength and a 140.5 % increase in electrical conductivity. Importantly, these enhancements were achieved without the need for any additional post-treatments. This clearly demonstrates the effectiveness of our current wet spinning method.

Continuum Mechanics Applied to the Biomechanics of Motion.

Continuum Mechanics is the discipline that studies the motion and deformation of material bodies, and is at the basis of both Solid Mechanics and Fluid Mechanics. This works aims at highlighting how a basic education in modern Continuum Mechanics can be of fundamental importance not only in the Biomechanics of Tissue, but also in the Biomechanics of Movement. The latter is largely based on Rigid Body Mechanics, which can be considered as a simple particular case of the general theory of Continuum Mechanics. As an example, a straightforward methodology for the extraction of the angular velocity from motion analysis data is illustrated. The method is based on a quantity that is more primitive than the angular velocity: the spin tensor.

Spin polarization and transport of hybrid interface states in π-stacked magnetic molecular junctions

Based on the first-principles method, the spin polarization and transport properties of the hybrid interface states (HIS) in π-stacked three-layer triphenylene molecular junction are studied theoretically. By rotating the torsion angle between two neighbor layers, the π-π interaction strength is adjusted. The results demonstrate that the HIS, generated by strong pz-d hybridization between molecules and ferromagnetic electrodes, can easily transfer to the central layer via a strong π-π interaction with similar spin polarization. A weak π-π interaction will block the transfer of the HIS, where the discrete molecular orbital states dominate the central molecule. However, an enhancement of the spin polarization for the molecular frontier orbital states is observed. The transport calculation shows that efficient transport of the HIS is realized in the π-stacked system, where the largest current appears at a strong π-π interaction but a significant current spin polarization appears at a weak π-π interaction. This work deepens our understanding of the role of the HIS in π-stacked organic systems.

Harnessing synergy of spin and orbital currents in heavy metal/ferromagnet multilayers

Spin-orbitronics, exploiting electron spin and/or orbital angular momentum, offers a powerful route to energy-efficient spintronic applications. Recent research on orbital currents in light metals broadens the scope of spin-orbit torque (SOT). However, distinguishing and manipulating orbital torque in heavy metal/ferromagnet (HM/FM) remains a challenge, limiting the promising synergy of spin and orbital currents. Here, we design a HM/FM/FMSOC heterostructure and experimentally separate orbital torque contribution from spin torque by utilizing the distinct diffusion length of spin and orbital currents. Furthermore, we achieve the synergy of spin and orbital torques by controlling their relative strength, and obtain a 110% improvement in torque efficiency compared to the representative Pt/Co bilayer. Our findings not only contribute to a deeper understanding of SOT mechanisms and orbital current transport in HM/FM multilayers, but also highlight the promising prospect of orbital and spin torque synergy for optimizing the efficiency of next-generation spintronic devices.