Injection Of Pure Spin Current Research Articles

Colossal anisotropic absorption of spin currents induced by chirality.

The chiral induced spin selectivity (CISS) effect, in which the structural chirality of a material determines the preference for the transmission of electrons with one spin orientation over that of the other, is emerging as a design principle for creating next-generation spintronic devices. CISS implies that the spin preference of chiral structures persists upon injection of pure spin currents and can act as a spin analyzer without the need for a ferromagnet. Here, we report an anomalous spin current absorption in chiral metal oxides that manifests a colossal anisotropic nonlocal Gilbert damping with a maximum-to-minimum ratio of up to 1000%. A twofold symmetry of the damping is shown to result from differential spin transmission and backscattering that arise from chirality-induced spin splitting along the chiral axis. These studies reveal the rich interplay of chirality and spin dynamics and identify how chiral materials can be implemented to direct the transport of spin current.

Chemical Approach Towards Broadband Spintronics on Nanoscale Pyrene Films

AbstractThe injection of pure spin current into the non‐magnetic layer plays a crucial role in transmitting, processing, and storing data information in the realm of spintronics. To understand broadband molecular spintronics, pyrene oligomer film (≈20 nm thickness) was prepared using an electrochemical method forming indium tin oxide (ITO) electrode/pyrene covalent interfaces. Permalloy (Ni80Fe20) films with different nanoscale thicknesses were used as top contact over ITO/pyrene layers to estimate the spin pumping efficiency across the interfaces using broadband ferromagnetic resonance spectra. The spintronic devices are composed of permalloy/pyrene/ITO orthogonal configuration, showing remarkable spin pumping from permalloy to pyrene film. The large spin pumping is evident from the linewidth broadening of 5.4 mT at 9 GHz, which is direct proof of spin angular momentum transfer across the interface. A striking observation is made with the high spin‐mixing conductance of ≈1.02×1018 m−2, a value comparable to the conventional heavy metals. Large spin angular moment transfer was observed at the permalloy‐pyrene interfaces, especially at the lower thickness of permalloy, indicating a strong spinterface effect. Pure spin current injection from ferromagnetic into electrochemically grown pyrene films ensures efficient broadband spin transport, which opens a new area in molecular broadband spintronics.

Chemical Approach Towards Broadband Spintronics on Nanoscale Pyrene Films.

The injection of pure spin current into the non-magnetic layer plays a crucial role in transmitting, processing, and storing data information in the realm of spintronics. To understand broadband molecular spintronics, pyrene oligomer film (~20 nm thickness) was prepared using an electrochemical method forming indium tin oxide (ITO) electrode/pyrene covalent interfaces. Permalloy (Ni80Fe20) films with different nanoscale thicknesses were used as top contact over ITO/pyrene layers to estimate the spin pumping efficiency across the interfaces using broadband ferromagnetic resonance spectra. The spintronic devices are composed of permalloy/pyrene/ITO orthogonal configuration, showing remarkable spin pumping from permalloy to pyrene film. The large spin pumping is evident from the linewidth broadening of 5.4 mT at 9 GHz, which is direct proof of spin angular momentum transfer across the interface. A striking observation is made with the high spin-mixing conductance of ~1.02 1018 m-2, a value comparable to the conventional heavy metals. Large spin angular moment transfer was observed at the permalloy-pyrene interfaces, especially at the lower thickness of permalloy, indicating a strong spinterface effect. Pure spin current injection from ferromagnetic into electrochemically grown pyrene films ensures efficient broadband spin transport, which opens a new area in molecular broadband spintronics.

Pure Spin Current Injection into a Helimagnet

Pure Spin Current Injection into a Helimagnet

Pure spin current injection of single-layer monochalcogenides

We compute the spectrum of pure spin current injection in ferroelectric single-layer SnS, SnSe, GeS, and GeSe. The formalism takes into account the coherent spin dynamics of optically excited conduction states split in energy by spin–orbit coupling. The velocity of the electron’s spins is calculated as a function of incoming photon energy and angle of linearly polarized light within a full electronic band structure scheme using density functional theory. We find peak speeds of 520, 360, 270 and 370 Km s−1 for SnS, SnSe, GeS and GeSe, respectively which are an order of magnitude larger than those found in bulk semiconductors, e.g., GaAs and CdSe. Interestingly, the spin velocity is almost independent of the direction of polarization of light in a range of photon energies. Our results demonstrate that single-layer SnS, SnSe, GeS and GeSe are candidates to produce on demand spin-current in spintronics applications.

Magnetization switching of a perpendicular nanomagnet induced by vertical nonlocal injection of pure spin current

Injection of pure spin current using a nonlocal geometry is a promising method for controlling magnetization in spintronic devices from the viewpoints of increasing freedom in device structure and avoiding problems related to charge current. Here, we report an experimental demonstration of magnetization switching of a perpendicular magnetic nanodot induced by vertical injection of pure spin current from a spin polarizer with perpendicular magnetization. In comparison with direct spin injection, the current amplitude required for magnetization switching is of the same order and shows smaller asymmetry between parallel-to-antiparallel and antiparallel-to-parallel switching. Simulation of spin accumulation reveals that, in the case of nonlocal spin injection, the spin torque is symmetric between the parallel and antiparallel configuration because current flows through only the spin polarizer, not the magnetic nanodot. This characteristic of nonlocal spin injection is the origin of the smaller asymmetry of the switching current and can be advantageous in spintronic applications.

Large enhancement of the spin Hall effect in Mn metal by Sn doping

The noncollinear kagome lattice antiferromagnets Mn${}_{3}X$ ($X$=Sn, Ge, Ga) have attracted much attention due to their large anomalous Hall effect (AHE). As the spin Hall effect (SHE) and AHE share the same origin, the injection of pure spin current into these alloys would also engender large SHE. The authors surprisingly find that even a Mn-Sn amorphous/nanocrystalline alloy exhibits large SHE. These results not only serve as an essential reference in studying the pure spin current phenomena in Mn-based alloys but also offer a promising method in exploring energy efficient spin Hall materials.

Pure spin current injection in hydrogenated graphene structures

We present a theoretical study of spin-velocity injection (SVI) of a pure spin current (PSC) induced by a linearly polarized light that impinges normally on the surface of two 50\% hydrogenated noncentrosymmetric two-dimensional (2D) graphene structures. The first structure, hydrogenated at only one side, labeled Up, also known as graphone, and the second, labelled Alt, is 25\% hydrogenated at both sides. The hydrogenation opens an energy gap in both structures. We analyze two possibilities: in the first, the spin is fixed along a chosen direction, and the resulting SVI is calculated; in the second, we choose the SVI direction along the surface plane, and calculate the resulting spin orientation. This is done by changing the energy $\hbar\omega$ and polarization angle $\alpha$ of the incoming light. The results are calculated within a full electronic band structure scheme using the Density Functional Theory (DFT) in the Local Density Approximation (LDA). The maxima of the spin-velocities are reached when $\hbar\omega=0.084$\,eV and $\alpha=35^\circ$ for the Up structure, and $\hbar\omega=0.720$\,eV and $\alpha=150^\circ$ for the Alt geometry. We find a speed of 668\,Km/s and 645\,Km/s for the Up and the Alt structures, respectively, when the spin points perpendicularly to the surface. Also, the response is maximized by fixing the spin-velocity direction along a high symmetry axis, obtaining a speed of 688Km/s with the spin pointing at $13^\circ$ from the surface normal, for the Up, and 906 Km/s and the spin pointing at $60^\circ$ from the surface normal, for the Alt system. These speed values are of order of magnitude larger than those of bulk semiconductors, such as CdSe and GaAs, thus making the hydrogenated graphene structures excellent candidates for spintronics applications.

Non-local electrical spin injection and detection in germanium at room temperature

Non-local carrier injection/detection schemes lie at the very foundation of information manipulation in integrated systems. This paradigm consists in controlling with an external signal the channel where charge carriers flow between a “source” and a well separated “drain.” The next generation electronics may operate on the spin of carriers in addition to their charge and germanium appears as the best hosting material to develop such a platform for its compatibility with mainstream silicon technology and the predicted long electron spin lifetime at room temperature. In this letter, we demonstrate injection of pure spin currents (i.e., with no associated transport of electric charges) in germanium, combined with non-local spin detection at 10 K and room temperature. For this purpose, we used a lateral spin valve with epitaxially grown magnetic tunnel junctions as spin injector and spin detector. The non-local magnetoresistance signal is clearly visible and reaches ≈15 mΩ at room temperature. The electron spin lifetime and diffusion length are 500 ps and 1 μm, respectively, the spin injection efficiency being as high as 27%. This result paves the way for the realization of full germanium spintronic devices at room temperature.

Spin caloritronic nano-oscillator

Energy loss due to ohmic heating is a major bottleneck limiting down-scaling and speed of nano-electronic devices, and harvesting ohmic heat for signal processing is a major challenge in modern electronics. Here, we demonstrate that thermal gradients arising from ohmic heating can be utilized for excitation of coherent auto-oscillations of magnetization and for generation of tunable microwave signals. The heat-driven dynamics is observed in Y3Fe5O12/Pt bilayer nanowires where ohmic heating of the Pt layer results in injection of pure spin current into the Y3Fe5O12 layer. This leads to excitation of auto-oscillations of the Y3Fe5O12 magnetization and generation of coherent microwave radiation. Our work paves the way towards spin caloritronic devices for microwave and magnonic applications.

Local injection of pure spin current generates electric current vortices

We show that local injection of pure spin current into an electrically disconnected ferromagnetic–normal-metal sandwich induces electric currents that run along the closed loops inside the device and are powered by the source of the spin injection. Such electric currents may significantly modify voltage distribution in spin-injection devices and induce long-range tails of spin accumulation.

Spin Hall voltages from a.c. and d.c. spin currents.

In spin electronics, the spin degree of freedom is used to transmit and store information. To this end the ability to create pure spin currents—that is, without net charge transfer—is essential. When the magnetization vector in a ferromagnet–normal metal junction is excited, the spin pumping effect leads to the injection of pure spin currents into the normal metal. The polarization of this spin current is time-dependent and contains a very small d.c. component. Here we show that the large a.c. component of the spin currents can be detected efficiently using the inverse spin Hall effect. The observed a.c.-inverse spin Hall voltages are one order of magnitude larger than the conventional d.c.-inverse spin Hall voltages measured on the same device. Our results demonstrate that ferromagnet–normal metal junctions are efficient sources of pure spin currents in the gigahertz frequency range.

Magnetic-texture-controlled transverse spin injection

We propose an effect whereby an electric current along the interface between a ferromagnetic and normal metal leads to injection of pure spin current into the normal metal, if the magnetization direction in the ferromagnet varies spatially along the direction of current. For the specific example of a ferromagnetic spin spiral, we compute the inverse spin Hall effect voltage this spin current gives rise to when injected into a Pt layer. Furthermore, we show that this pure spin current leads to modification of the parameters that govern spin transfer and current-driven domain-wall motion, which can be used to tune the latter in layered magnetic systems. This effect in principle enables in situ control over the location, magnitude, and polarization of spin-current injection in devices.

純スピン流注入磁化反転に対するスピン偏極方向の影響

The dynamics of the magnetization reversal process in permalloy films by the pure spin current injection has been analyzed. We have performed micromagnetics simulations based on the Landau-Lifshitz equation taking into account both the relaxation term and spin transfer torque term caused by the pure spin current which is injected into the small permalloy thin films laterally. We found that the magnetization switching time is decreased and the switching probability is increased by optimizing the spin polarization direction of the injecting pure spin current.

Large pure spin current generation in metallic nanostructures

Pure spin current corresponds to the flow of spin angular momentum without associating any net charge current, and possesses potential to be incorporated in special functional and high-performance devices based on nonlocal spin injection. To utilize pure spin current in practical devices, it is imperative to increase the spin generation efficiency. In this article we discuss two special configurations of nonlocal devices, known as multi-terminal injector and nanopillar devices, which possess immense potential to overcome the Joule heating problem, the key bottleneck to enhance the pure spin current generation. We also demonstrate magnetization switching of a nanosized ferromagnet due to pure spin current injection in a nanopillar-based nonlocal device.

Micromagnetics Simulation for Magnetization Switching of Permalloy Films with Pure Spin Current Injection

Pure spin current injection has recently been developed as an effective method of controlling magnetization in spintronic devices. In order to investigate the dynamics of the magnetization reversal process in magnetic films with pure spin current injection, we performed micromagnetics simulations based on the Landau-Lifshitz equation and taking into account a term representing spin transfer torque caused by the pure spin current. We studied the time evolution of magnetization in permalloy films and estimated the probability of magnetization switching to show how the probability depends on the width of the film and the intensity of the injected spin current. We found that, in narrow films, a transverse domain wall is formed in the film and that this wall moves outside the film when magnetization switching occurs. The switching time becomes shorter as the intensity of the injected spin current is increased. In wide films, on the other hand, magnetization switching does not occur even if the intensity of injected spin current is increased. In such cases, either a magnetic vortex core is formed or the configuration of the moments hardly changes from the initial magnetic state.

Spin current injection by spin Seebeck and spin pumping effects in yttrium iron garnet/Pt structures

It is reported an investigation of pure spin current injection in Pt strips deposited on yttrium iron garnet (YIG) films by means of the spin pumping (SPE) and spin Seebeck (SSE) effects. Both effects were characterized by measuring the DC voltage created along the Pt strips by means of the inverse spin Hall effect (VISHE). SPE and SSE are simultaneously activated by exciting the ferromagnetic resonance (FMR) of the YIG film at the same time that a temperature gradient is created along the sample length. While the FMR signal is little affected by the temperature gradient, the voltage measured at the Pt strip placed at the lower temperature end exhibits a very challenging behavior. The voltage excited by the FMR uniform mode increases by six times as the temperature difference (ΔT) between the two ends of the YIG slab changes from 0 to 12 K. In contrast the VISHE generated by other magnetostatic spin-wave modes decreases to zero as ΔT varies from 0 to 12 K.

純スピン流注入によるパーマロイ薄膜の磁化反転シミュレーション

純スピン流注入によるパーマロイ薄膜の磁化反転シミュレーション

Nonlocal injection of spin current into a superconducting Nb wire

Pure spin current injection into a superconductive Nb was demonstrated experimentally by employing a nonlocal spin absorption technique in a multiterminal lateral structure. From the reduction in spin signals, we found the absorption efficiency remained almost unchanged even after superconducting transition. The Nb/Cu interface properties were also complementarily examined by using a Nb/Cu/Nb junction incorporated in the same device, revealing superconductivity below the transition temperature. These experimental results support that the pure spin current can be absorbed into a superconductive Nb wire with almost the same efficiency as in a normal state.

Power dependence of pure spin current injection by quantum interference

We investigate the power dependence of pure spin current injection in GaAs bulk and quantum-well samples by a quantum interference and control technique. Spin separation is measured as a function of the relative strength of the two transition pathways driven by two laser pulses. By keeping the relaxation time of the current unchanged, we are able to relate the spin separation to the injected average velocity. We find that the average velocity is determined by the relative strength of the two transitions in the same way as in classical interference. Based on this, we conclude that the density of injected pure spin current increases monotonically with the excitation laser intensities. The experimental results are consistent with theoretical calculations based on Fermi's golden rule.