Spin Current Research Articles

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

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.

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.

Bias-induced magnetic to nonmagnetic transition in polyacene junctions

By means of the first-principles method, the bias effect on the magnetism of polyacene (n-acene) connected to gold electrodes is investigated. A magnetic to nonmagnetic transition for the polyacene (n > 6) is observed when the bias exceeds a critical value. The mechanism is explored as the bias-induced variation of electronic localization, which leads to the exchange of dominant mechanism for molecular magnetism from Columbic interaction between electrons to electron hopping rate. A significant enhancement of the differential conductance and suppression of current spin polarization for the molecular device are also obtained accompanied by the transition of molecular magnetism. This work proposes a feasible way to manipulate the magnetism of polyacene via electric method and reveals the relation between molecular magnetism and its conductance.

Surface plasmon resonance enhanced terahertz spin current emission in Au nanoparticles/ferromagnet thin film heterostructure

We present a design of an ultrafast spin current emitter utilizing the Au nanoparticles/Co/Pt heterostructures, which facilitate significant enhancement in terahertz radiation. Using Au nanoparticles synthesized by thermal annealing and embedded in Co/Pt bilayers, we show that the Au nanoparticles fulfill surface plasmon resonance conditions under the illumination of femtosecond laser pulses, and enhanced terahertz spin currents are generated in the Co thin film. The terahertz signal amplitude of Au nanoparticles/Co/Pt sample exhibits an enhancement of 70% and 45% compared to that observed in Au thin film/Co/Pt and Co/Pt samples, respectively. By integrating experimental analysis and numerical calculations, we ascribe the enhancement of terahertz emission amplitude to the contribution of spin current generated by surface plasmon resonance in the Co thin film, which may result from the magnetization enhancement induced by surface plasmon-excited electromagnetic fields at the Au nanoparticles/Co interface, and the plasmon-induced spin current density is approximately 1 × 1029 electrons s−1 m−2. This plasmon-induced spin current establishes a connection between the fields of plasmonic and spintronic physics, thereby promoting the development of spintronic terahertz emitters.

Realizing Kirchhoff’s superposition law of light-irradiated pure spin currents in paralleled graphene nanoribbons

Light-irradiated pure spin current has been achieved via several methods. In this work, we aim to realize its superposition law by reducing the quantum interference effect between parallel circuits via first principles. As an example, a single-layer zigzag graphene nanoribbon (ZGNR) and a two-layer ZGNR constituted two-probe structures are chosen as our model. It is found that the lowest total energy of the system constructed by bilayer ZGNR occurs at an interlayer distance of about d=4Å, which indicates that the quantum interference between the two layer ZGNRs can be neglected when the interlayer distance exceeds 4Å. In our anticipation, the total light-irradiated pure spin current in the two-layer ZGNR constituted two-probe structures is twice that in the single-layer ZGNR constituted two-probe structure when the interlayer distance is set to 6Å, satisfying the superposition law. While when the interlayer distance is 2Å, the total light-irradiated pure spin current in the two-layer ZGNR constituted two-probe structures is not bigger but smaller than that in the single-layer ZGNR constituted two-probe structure, which may be induced by the quantum coherent cancellation. So the superposition law of photogalvanic pure spin currents can be realized in ZGNR-based devices by manipulating the layer distances. In addition, the combination type of light irradiation area won’t affect the realization of the superposition law of photogalvanic pure spin currents. The results provide a reliable method for strengthening the photogalvanic pure spin currents in two-dimensional graphene materials and also for the integration of optoelectronic devices.

Unraveling the Spin-to-Charge Current Conversion Mechanism and Charge Transfer Dynamics at the Interface of Graphene/WS2 Heterostructures at Room Temperature.

We report experimental investigations of spin-to-charge current conversion and charge transfer (CT) dynamics at the interface of the graphene/WS2 van der Waals heterostructure. Pure spin current was produced by the spin precession in the microwave-driven ferromagnetic resonance of a permalloy film (Py=Ni81Fe19) and injected into the graphene/WS2 heterostructure through a spin pumping process. The observed spin-to-charge current conversion in the heterostructure is attributed to the inverse Rashba-Edelstein effect (IREE) at the graphene/WS2 interface. Interfacial CT dynamics in this heterostructure was investigated based on the framework of the core-hole clock (CHC) approach. The results obtained from spin pumping and CHC studies show that the spin-to-charge current conversion and charge transfer processes are more efficient in the graphene/WS2 heterostructure compared to isolated WS2 and graphene films. The results show that the presence of WS2 flakes improves the current conversion efficiency. These experimental results are corroborated by density functional theory (DFT) calculations, which reveal (i) Rashba spin-orbit splitting of graphene orbitals and (ii) electronic coupling between graphene and WS2 orbitals. This study provides valuable insights for optimizing the design and performance of spintronic devices.

Transport through a monolayer-tube junction: Sheet-to-tube spin current

We develop a method to calculate the electron flow between an arbitrary atomic monolayer sheet and an arbitrary tube by expressing the corresponding sheet-tube tunneling matrix elements with those between sheets. We use this method to calculate the spin current from a monolayer silicene sheet with sublattice-staggered current-induced spin polarization to a silicene tube. The calculated sheet-to-tube spin current exhibits an oscillation as a function of the tube circumferential length because the Fermi points in the tube cross the Fermi circle in the sheet. Furthermore, the spin current with spin in the out-of-plane direction, which is absent in the sheet-sheet junction (including twisted sheets) with C3 rotational symmetry, appears in an oscillating form in the tube-sheet junction due to the broken C3 rotational symmetry. This is an example of the symmetry manipulation which realizes switching a particular component of the spin current.

Chiral edge states and direct edge-to-edge transport in a bosonic magnetic ladder

Abstract Three-legged magnetic ladder reproduces the main features of two-dimensional Hofstadter butterfly spectrum and the related Chern insulating phases with typical topological edge states, providing new opportunity to study novel quantum states and chiral physics. Here, we propose an efficient scheme to detect various edge states and realize edge-to-edge transport in a three-legged bosonic magnetic ladder. Under the mean field approximation, the eigenstates, chiral edge-state dynamics, and edge-to-edge transport in the system are studied. The energy spectrum and the eigenstates of the system are presented, and both bulk and edge states are obtained, depending on the energy spectrum of the system. The existence of rich edge states (including symmetric and unsymmetric edge states) provides the evidence for realizing the topological transport in the system. Furthermore, chiral edge-state dynamics is excited by applying a weak linear external force, reproducing the underlying eigenstates dynamically, offering a robust way to detect the edge states of the system. Particularly, direct edge-to-edge transport is observed which can be identified by the observables (spin polarization, spin tensor, and chiral currents). The reversal of chiral currents induces the edge-to-edge transport. The edge-to-edge transport time can be manipulated by adjusting the external force and magnetic field. We provide a robust and efficient atomic transport scheme with potential applications in manipulating topological quantum transport and storing quantum states in the ultracold atomic system.

Spin-polarized thermoelectrical currents induced in Aharonov-Bohm rings

We propose an interferometric device that generates spin-polarized electric currents based on an Aharonov-Bohm ring geometry. The device is driven by a temperature gradient and a magnetic flux without any voltage bias. Remarkably, no ferromagnetic contact is necessary once the two-path interference effects are present combined with a spin-orbit interaction. A large amplitude of spin current signal occurs where the charge current is small or even vanishing, thus easily creating pure spin currents by applying a magnetic field. Our proposal provides an ingredient of information processing devices in a generic caloritronic setup. Published by the American Physical Society 2024

An Ultrasensitive Molecular Detector for Direct Sensing of Spin Currents at Room Temperature.

The experimental analysis of pure spin currents at interfaces is one major goal in the field of magnonics and spintronics. Complementary to the established Spin-Hall effect using the spin-to-charge conversion in heavy metals for information processing, we present a novel approach based on spin pumping detection by an interfacial, resonantly excited molecular paramagnet adsorbed to the surface of the spin current generating magnet. Here, we show that the sensitivity of this electron paramagnetic resonance (EPR) detector can be enhanced by orders of magnitude through intramolecular transfer of spin polarization at room temperature. Our proof-of-principle sample consists of octahedral-shaped ferrimagnetic Fe3O4 nanoparticles covered by oleic acid (OA) which has two paramagnetic centers, S1 and S2. S1 arises from the chemisorption of OA and is located directly at the interface to Fe3O4. S2 originates from radical formation at the center of the molecule close to the double bond of oleic acid and is not influenced by chemisorption. Using ferromagnetic resonance (FMR) excitation of the Fe3O4 nanoparticles to pump spins into S1, a population inversion of the spin-split levels of S2 is formed, vastly enhancing the detection sensitivity on the atomic scale.

Improving spin manipulation in HgTe/CdTe quantum wells via electrical means

We present a theoretical investigation of electron transport properties through a two-dimensional electron gas (2DEG) HgTe/CdTe quantum wells (QWs) heterostructure that incorporate two Rashba-modulated regions. Our study demonstrates that the transmission can be effectively tuned by varying the Rashba spin-orbit coupling (RSOC) strength, Fermi energy, and electrical voltage. Efficient spin-conserving and spin-flip transmission can be achieved by adjusting the RSOC modulation strength. Furthermore, fully spin-polarized spin or current transmission with PZ=±1, as well as complete spin-flip conversion with a rate of Tsf=1 are attained by fine-tuning the amplitudes of the two Rashba modulations using gate voltage or adjusting the RSOC strength. This electrical mechanism provides a convenient approach to electrically manipulate both spin and charge currents, which holds promise for low-power device applications.

Electric polarization evolution equation for antiferromagnetic multiferroics with the polarization proportional to the scalar product of the spins

The spin current model of electric polarization in multiferroics is justified via the quantum hydrodynamic method and the mean-field part of the spin-orbit interaction. The spin current model is applied to derive the electric polarization proportional to the scalar product of the spins of the nearby ions, which appears to be caused by the Dzylaoshinskii-Moriya interaction. The symmetric tensor spin structure of the polarization is discussed as well. We start our derivations for the ferromagnetic multiferroic materials and present further generalizations for the antiferromagnetic multiferroic materials. We rederive the operator of the electric dipole moment, which provides the macroscopic polarization obtained via the spin current model. Finally, we use the quantum average of the found electric dipole moment operator to derive the polarization evolution equation for the antiferromagnetic multiferroic materials. The possibility of spiral spin structures is analyzed.