Manipulation Of Spin Currents Research Articles

Spin transport characteristics modulated by the GeBi interlayer in Y3Fe5O12/GeBi/Pt heterostructures

For the past few years, germanium-based semiconductor spintronics has attracted considerable interest due to its potential for integration into mainstream semiconductor technology. The main challenges in the development of modern semiconductor spintronics are the generation, detection, and manipulation of spin currents. Here, the transport characteristics of a spin current generated by spin pumping through a GeBi semiconductor barrier in Y3Fe5O12/GeBi/Pt heterostructures were investigated systematically. The effective spin-mixing conductance and inverse spin Hall voltage to quantitatively describe the spin transport characteristics were extracted. The spin-injection efficiency in the Y3Fe5O12/GeBi/Pt heterostructures is comparable to that of the Y3Fe5O12/Pt bilayer, and the inverse spin Hall voltage exponential decays with the increase in the barrier thickness. Furthermore, the band gap of the GeBi layer was tuned by changing the Bi content. The spin-injection efficiency at the YIG/semiconductor interface and the spin transportation within the semiconductor barrier are related to the band gap of the GeBi layer. Our results may be used as guidelines for the fabrication of efficient spin transmission structures and may lead to further studies on the impacts of different kinds of barrier materials.

Enhanced Spin Current in Ni81 Fe19 /Cu-CuOx Bilayer with Top and Sideways Oxidization.

Generation and manipulation of spin current are the cores of spintronic devices, which are intensely pursued. Heavy metals with strong spin-orbit coupling are commonly used for the generation of spin current, but areincompatible with the mass production of devices, and the polarization of spin current is limited to be in-plane. Here, it is shown that the spin current with strong out-of-plane polarization component can be generated and transmitted in Ni81 Fe19 /Cu-CuOx bilayer with sideways and top oxidizations. The charge-to-spin current conversion efficiency can be enhanced through the spin currents consisting of both out-of-plane polarization (σz ) and in-plane polarization (σy ) induced by spin-vorticity coupling. Such a spin current is demonstrated to be closely related to the lateral oxidization gradient and can be controlled by changing the temperatures and times of annealing. The finding here provides a novel degree of freedom to produce and control the spin current in spintronic devices.

Non-local architecture for spin current manipulation in silicon platforms

We have developed a non-local architecture for spin current injection, manipulation, and detection in n-doped bulk Si at room temperature. Spins are locally generated at the indirect gap of bulk Si by means of circularly polarized light and then detected by exploiting the inverse spin-Hall effect (ISHE) occurring inside a thin Pt pad deposited at the top of the Si substrate. We demonstrate that it is possible to modulate the transport properties of the optically injected spin current by applying a bias voltage along the direction of motion of the particles. In this case, we are able to explore both the spin diffusion regime, characterized by a spin diffusion length Ls ≈ 12 μm, and the spin drift regime with applied electric fields up to E = 35 V/cm. We demonstrate that the spin transport length of the electrons can be increased (or decreased) by more than 100% for electric fields antiparallel (or parallel) to the diffusion direction. As a consequence, the ISHE signal can be electrically controlled to have high or low output voltages from the non-local device.

Automated mechanical exfoliation technique: a spin pumping study in YIG/TMD heterostructures.

Spintronics devices rely on the generation and manipulation of spin currents. Two-dimensional transition-metal dichalcogenides (TMDs) are among the most promising materials for a spin current generation due to a lack of inversion symmetry at the interface with the magnetic material. Here, we report on the fabrication of Yttrium Iron Garnet(YIG)/TMD heterostructures by means of a crude and fast method. While the magnetic insulator single-crystalline YIG thin films were grown by magnetron sputtering, the TMDs, namely MoS2 and MoSe2, were directly deposited onto YIG films using an automated mechanical abrasion method. Despite the brute force aspect of the method, it produces high-quality interfaces, which are suitable for spintronic device applications. The spin current density and the effective spin mixing conductance were measured by ferromagnetic resonance, whose values found are among the highest reported in the literature. Our method can be scaled to produce ferromagnetic materials/TMD heterostructures on a large scale, further advancing their potential for practical applications.

Spin-Orbit Coupling in 2D Semiconductors: A Theoretical Perspective.

This theoretical Perspective reviews spin-orbit coupling (SOC), including the Rashba effect and Dresselhaus effect, in two-dimensional (2D) semiconductors. We first introduce the origin of the Rashba effect and Dresselhaus effect using the Hamiltonian models; we then summarize 2D Rashba semiconductors predicted by first-principles density functional theory (DFT) calculations, including AB binary monolayers, Janus monolayers, 2D perovskites, and so on. We also review various manipulating techniques of the Rashba effect on 2D semiconductors, such as external electric field, strain engineering, charge doping, interlayer interactions, proximity effect of substrates, and external magnetic field. We then briefly summarize the applications of SOC, including the generation, detection, and manipulation of spin currents in spin Hall effect transistors and spin field effect transistors. Finally, we conclude this Perspective and propose three promising research fields of SOC in low-dimensional semiconductors, including the nonlinear SOC Hamiltonian model, 2D ferroelectric SOC semiconductors, and 1D Rashba model and semiconductors. This theoretical Perspective enriches the fundamental understanding of SOC in 2D semiconductors and will help in the design of new types of spintronic devices in future experiments.

Electrical detection of spin-microwave coupling in a cavity

Via exploring the spin dynamics and transport properties at interfaces of magnetic material/non-magnetic metal bilayers, spintronics advanced many techniques on generation, detection, and manipulation of spin currents, which laid down a foundation for designing spintronic devices. New ideas and methods for manipulating spin are required in the spintronics community. Merging with cavity quantum electrodynamics, magnetic resonance in ferromagnets inside of a microwave cavity, realizes a strong magnon-microwave coupling at room temperature, opening an avenue to manipulate spin and spin current for the spintronics community. Spin-microwave coupling strength was significantly improved by replacing a few spins in a paramagnetic spin system with magnetization in ferromagnetic materials. Thus, a strongly coupled magnon-microwave system offers a hybrid quantum platform with many measures to tune the coupled system, such as an exchange magnetic field, an anisotropy field, controlled coupling strength, controlled damping parameters, and so on. Many devices were proposed and demonstrated with potential applications. In this review, we briefly introduce the concept and mechanism of magnon-microwave coupling, a classical electrodynamical coupling model for the coupling, spin currents produced by the coupled magnon, and manipulation of the spin current via the strong coupling. In ferromagnets, magnons can couple to a microwave mode as a magnon-polariton propagating in the materials. The strongly coupled magnon-microwave system is a magnon-polariton in a cavity where the microwave was confined as a resonance mode with a high quality factor. Using the Landau-Lifshitz-Gilbert equation and Maxwell equations, we introduce the classical electrodynamical coupling model and reveal that the magnon is driven by a torque produced by the microwave magnetic field on the magnetization and the microwave is feedback via Faraday’s law. Therefore, the coupling strength of the magnon-microwave system can be controlled by changing the torque. Magnon-microwave coupling is experimentally studied using many techniques, such as microwave transmission, Brillouin light scattering, Faraday rotation measurement, and spin pumping electrical detection. Spin pumping is well explored in the spintronics community as a spin current produced by magnon and detected as a voltage via inverse spin Hall effect in a novel metal. Combining with microwave transmission of the coupled microwave subsystem, spin pumping electrical detection by directly detecting the spin current produced by the magnon subsystem in the coupled system, illustrates a more complete profile of the coupled magnon-microwave system. In addition, the strongly coupled magnon-microwave system offers a new technique to manipulate spin currents. We experimentally demonstrate that spin currents produced by the strongly coupled system are correlated with each other mediated by the microwave cavity, and can be manipulated remotely. Two almost identical magnet samples are used to couple to a cavity and the coupling strengths between each magnet and the cavity are tunable individually. By using electrical detection to locally detect each magnet sample respectively, we find that the spin currents produced by two magnet samples are coherently correlated. By tuning the coupling strength of one magnet sample to the cavity, the spin current produced by the second magnet sample is manipulated distantly over a few centimeters, which is far longer than the distance of spin-orbit interaction or exchange interaction. This distant control is only limited by the coherence length of the microwave and the dimension of the cavity. This flexibility opens the door to improve spin current generation and manipulation for cavity spintronic devices.

Generation of DC, AC, and Second-Harmonic Spin Currents by Electromagnetic Fields in an Inversion-Asymmetric Antiferromagnet

Manipulating spin currents in magnetic insulators is a key technology in spintronics. We theoretically study a simple inversion-asymmetric model of quantum antiferromagnets, where both the exchange interaction and the magnetic field are staggered. We calculate spin currents generated by external electric and magnetic fields by using a quantum master equation. We show that an ac electric field with amplitude E 0 leads, through exchange-interaction modulation, to the dc and second-order harmonic spin currents proportional to E 0 2 . We also show that dc and ac staggered magnetic fields B 0 generate the dc and ac spin currents proportional to B 0 , respectively. We elucidate the mechanism by an exactly solvable model, and thereby propose the ways of spin current manipulation by electromagnetic fields.

Spin supercurrent in two-dimensional superconductors with Rashba spin-orbit interaction

Spin current is a central theme in spintronics, and its generation is a keen issue. The spin-polarized current injection from the ferromagnet, spin battery, and spin Hall effect have been used to generate spin current, but Ohmic currents in the normal state are involved in all of these methods. On the other hand, the spin and spin current manipulation by the supercurrent in superconductors is a promising route for dissipationless spintronics. Here we show theoretically that, in two-dimensional superconductors with Rashba spin-orbit interaction, the generation of dissipationless bulk spin current by charge supercurrent becomes highly efficient, exceeding that in normal states in the dilute limit, i.e. when the chemical potential is close to the band edge, although the spin density becomes small there. This result manifests the possibility of creating new spintronic devices with long-range coherence.

Rashba-like spin splitting along three momentum directions in trigonal layered PtBi2

Spin-orbit coupling (SOC) has gained much attention for its rich physical phenomena and highly promising applications in spintronic devices. The Rashba-type SOC in systems with inversion symmetry breaking is particularly attractive for spintronics applications since it allows for flexible manipulation of spin current by external electric fields. Here, we report the discovery of a giant anisotropic Rashba-like spin splitting along three momentum directions (3D Rashba-like spin splitting) with a helical spin polarization around the M points in the Brillouin zone of trigonal layered PtBi2. Due to its inversion asymmetry and reduced symmetry at the M point, Rashba-type as well as Dresselhaus-type SOC cooperatively yield a 3D spin splitting with αR ≈ 4.36 eV Å in PtBi2. The experimental realization of 3D Rashba-like spin splitting not only has fundamental interests but also paves the way to the future exploration of a new class of material with unprecedented functionalities for spintronics applications.

Pure spin current manipulation in antiferromagnetically exchange coupled heterostructures

We present a model to describe the spin currents generated by ferromagnet/spacer/ferromagnet exchange coupled trilayer systems and heavy metal layers with strong spin-orbit coupling. By exploiting the magnitude of the exchange coupling (oscillatory RKKY-like coupling) and the spin-flop transition in the magnetization process, it has been possible to produce spin currents polarized in arbitrary directions. The spin-flop transition of the trilayer system originates pure spin currents whose polarization vector depends on the exchange field and the magnetization equilibrium angles. We also discuss a protocol to control the polarization sign of the pure spin current injected into the metallic layer by changing the initial conditions of magnetization of the ferromagnetic layers previously to the spin pumping and inverse spin Hall effect experiments. The small differences in the ferromagnetic layers lead to a change in the magnetization vector rotation that permits the control of the sign of the induced voltage components due to the inverse spin Hall effect. Our results can lead to important advances in hybrid spintronic devices with new functionalities, particularly, the ability to control microscopic parameters such as the polarization direction and the sign of the pure spin current through the variation of macroscopic parameters, such as the external magnetic field or the thickness of the spacer in antiferromagnetic exchange coupled systems.

Spin Seebeck effect and thermal spin galvanic effect in Ni80Fe20/p-Si bilayers

The development of spintronics and spin-caloritronics devices needs efficient generation, detection, and manipulation of spin current. The thermal spin current from the spin-Seebeck effect has been reported to be more energy efficient than the electrical spin injection methods. However, spin detection has been the one of the bottlenecks since metals with large spin-orbit coupling is an essential requirement. In this work, we report an efficient thermal generation and interfacial detection of spin current. We measured a spin-Seebeck effect in Ni80Fe20 (25 nm)/p-Si (50 nm) (polycrystalline) bilayers without a heavy metal spin detector. p-Si, having a centrosymmetric crystal structure, has insignificant intrinsic spin-orbit coupling, leading to negligible spin-charge conversion. We report a giant inverse spin-Hall effect, essential for the detection of spin-Seebeck effects, in the Ni80Fe20/p-Si bilayer structure, which originates from Rashba spin orbit coupling due to structure inversion asymmetry at the interface. In addition, the thermal spin pumping in p-Si leads to spin current from p-Si to the Ni80Fe20 layer due to the thermal spin galvanic effect and the spin-Hall effect, causing spin-orbit torques. The thermal spin-orbit torques lead to collapse of magnetic hysteresis of the 25 nm thick Ni80Fe20 layer. The thermal spin-orbit torques can be used for efficient magnetic switching for memory applications. These scientific breakthroughs may give impetus to the silicon spintronics and spin-caloritronics devices.

Spin-Current Manipulation of Photo-Induced Magnetization Dynamics in Heavy Metal/Ferromagnet Double Layer-Based Nanostructures

Spin currents offer a way to control static and dynamic magnetic properties, and therefore they are crucial for next-generation MRAM devices or spin-torque oscillators. Manipulating the dynamics is especially interesting within the context of photo-magnonics. In typical $3d$ transition metal ferromagnets like CoFeB, the lifetime of light-induced magnetization dynamics is restricted to about 1 ns, which e.g. strongly limits the opportunities to exploit the wave nature in a magnonic crystal filtering device. Here, we investigate the potential of spin-currents to increase the spin wave lifetime in a functional bilayer system, consisting of a heavy metal (8 nm of $\beta$-Tantalum (Platinum)) and 5 nm CoFeB. Due to the spin Hall effect, the heavy metal layer generates a transverse spin current when a lateral charge current passes through the strip. Using time-resolved all-optical pump-probe spectroscopy, we investigate how this spin current affects the magnetization dynamics in the adjacent CoFeB layer. We observed a linear spin current manipulation of the effective Gilbert damping parameter for the Kittel mode from which we were able to determine the system's spin Hall angles. Furthermore, we measured a strong influence of the spin current on a high-frequency mode. We interpret this mode an an exchange dominated higher order spin-wave resonance. Thus we infer a strong dependence of the exchange constant on the spin current.

Cavity Mediated Manipulation of Distant Spin Currents Using a Cavity-Magnon-Polariton.

Using electrical detection of a strongly coupled spin-photon system comprised of a microwave cavity mode and two magnetic samples, we demonstrate the long distance manipulation of spin currents. This distant control is not limited by the spin diffusion length, instead depending on the interplay between the local and global properties of the coupled system, enabling systematic spin current control over large distance scales (several centimeters in this work). This flexibility opens the door to improved spin current generation and manipulation for cavity spintronic devices.

Proposed manipulation of spin currents in GaAs crystal using the Pockels effect

A spin current could induce linear electro-optical electric susceptibility, thus the incident light could interact with the spin current via Pockels effect. In this letter, we find by appropriately selecting the directions of the applied electric field, the propagation direction of the incident light and its polarization, there will exist spin angular momentum transfer between the incident light and the spin polarized electrons resulting in the change of the spin current. Basing on this, a method for manipulating spin current is put forward. In our method, we could adjust the magnitude of the applied electric field or the intensity of incident light to manipulate the spin currents. The numerical results show that, for the condition of the applied electric field being 1×103 Vm-1 and the intensity of incident light being 1×105 Wm-2, the ratio of the changed pure spin current versus initial pure spin current will drop to 0.1%, and the spin polarization of the generated spin-polarized current will reach to 99.9%.

Nonreciprocal magnon propagation in a noncentrosymmetric ferromagnetLiFe5O8

Relativistic spin-orbit interaction drastically modifies electronic band and endows emergent functionalities. One of the example is the Rashba effect. In noncentrosymmetric systems such as interface and polar materials, the electronic band is spin-splitted depending on the momentum direction owing to the spin-orbit interaction, which is useful for the electric manipulation of spin current. Similar relativistic band-modification is also emergent for spin wave (magnon) in magnetic materials. The asymmetric magnon band dispersion induced by the Dzyaloshinskii-Moriya interaction, which is antisymmetric exchange interaction originating from the spin-orbit interaction, is theoretically expected, and experimentally observed recently in noncentrosymmetric ferromagnets. Here, we demonstrate that the nonreciprocal microwave response can be induced by the asymmetric magnon band in a noncentrosymmetric ferrimagnet LiFe$_5$O$_8$. This result may pave a new path to designing magnonic device based on the relativistic band engineering.

Quantum enhancement of spin drag in a Bose gas

In spintronics the active control and manipulation of spin currents is studied in solid-state systems. Opposed to charge currents, spin currents are strongly damped due to collisions between different spin carriers in addition to relaxation due to impurities and lattice vibrations. The phenomenon of relaxation of spin currents is called spin drag. Here we study spin drag in ultra-cold bosonic atoms deep in the hydrodynamic regime and show that spin drag is the dominant damping mechanism for spin currents in this system. By increasing the phase space density we find that spin drag is enhanced in the quantum regime by more than a factor of two due to Bose stimulation, which is in agreement with recent theoretical predictions and, surprisingly, already occurs considerably above the phase transition.

Polariton spin whirls

We report on the observation of spin whirls in a radially expanding polariton condensate formed under non-resonant optical excitation. Real space imaging of polarization- and time-resolved photoluminescence reveal a spiralling polarization pattern in the plane of the microcavity. Simulations of the spatiotemporal dynamics of a spinor condensate reveal the crucial role of polariton interactions with a spinor exciton reservoir. Harnessing spin dependent interactions between the exciton reservoir and polariton condensates allows for the manipulation of spin currents and the realization of dynamic collective spin effects in solid state systems.

Electrically tunable localized tunneling channels in silicene nanoribbons

The topological phase of a silicene nanoribbon holding edge states in the bulk energy gap can be easily broken by an external electric field. Here, we show through low-energy Green's function calculations that it is possible to localize conducting channels anywhere in a silicene nanoribbon by applying an inhomogeneous electric field. The spin degeneracy of these channels can also be broken in the same manner, allowing conduction of spin as well as charge. On this basis, we suggest design of a ternary logic device, which could be used in low-power circuits. Our study demonstrates that silicene and related group IV elements with honeycomb structure could provide a platform for efficient manipulation of spin currents via external electric fields, without the need to switch magnetic fields for spintronics applications.

Thermoelectric and thermospin switch realized by a three-terminal nanojunction

Motivated by the result in the recent work [A. Slachter, F. L. Bakker, J.-P. Adam, and B. J. van Wees, Nature Phys. 6, 879 (2010)], we research the thermally driven electron transport through a quantum dot (QD) coupled to two ferromagnetic leads and one normal-metallic lead. By applying temperature bias between the two ferromagnetic leads, we calculate the charge and spin currents in the normal-metallic lead and the spin accumulation in the QD. As a result, we observe that the charge and pure spin currents in the normal-metallic lead, as well as the spin accumulation in the QD, can be independently driven by the temperature bias. According to these results, we propose this system to be a promising candidate of the nanoswitch for the charge and spin current manipulation.

Spin current manipulation through a Rashba dot by tunable nonequilibrium Fano-Kondo effect

Though the Rashba-type spin-orbit interaction offers a possibility of manipulating electron's spin totally by electric field, it has been argued that the mere existence of spin-orbit interaction is insufficient to produce such spin-dependent transport. In the present work, we investigate how spin transport through a single-level dot is possible: Finite spin current appears as a result of spin-orbit interaction, strong electronic correlation and finite bias voltage. We show, by applying finite interaction slave-boson mean field theory, that Kondo physics is responsible for generating such spin-dependent transport.