Spin Current Generation Research Articles

Robust spin–orbit torques in ferromagnetic multilayers with weak bulk spin Hall effect

The spin currents generated by spin–orbit coupling in nonmagnetic metals or at interfaces with broken inversion symmetry are particularly important for devices with extraordinary performance. We have investigated the spin–orbit torques (SOTs) in Ru/Fe/HfO2 heterostructures with weak perpendicular magnetic anisotropy. Although the bulk spin Hall effect (SHE) of the Ru layer is smaller than that of Pt, Ta, or W, robust SOT-induced magnetization switching can also be realized. Through nonadiabatic harmonic measurements, the effective spin Hall angle in Ru/Fe/HfO2 heterostructures is calculated to be −0.045. We have explored the mechanisms of spin current generation, which could stem from the Ru/Fe interfaces other than the weak bulk SHE of Ru. Our results suggest the spin current generation mechanisms in ferromagnet/nonmagnetic metal heterostructures remain to be further explored.

Spin and charge currents driven by the Higgs mode in high-field superconductors

The Higgs mode in superconducting materials describes slowly decaying oscillations of the order parameter amplitude. We demonstrate that in superconductors with a built-in spin-splitting field the Higgs mode is strongly coupled to the spin degrees of freedom, allowing for the generation of time-dependent spin currents. Converting such spin currents to electric signals by spin-filtering elements provides a tool for the second-harmonic generation and the electrical detection of the Higgs mode generated by the external irradiation. The nonadiabatic spin torques generated by these spin currents allow for the magnetic detection of the Higgs mode by measuring the precession of the magnetic moment in the adjacent ferromagnet. We discuss also the reciprocal effect, which is the generation of the Higgs mode by the magnetic precession. Coupling the collective modes in superconductors to light and magnetic dynamics provides an opportunity for the study of superconducting optospintronics.Received 1 July 2019Revised 23 August 2020Accepted 25 August 2020DOI:https://doi.org/10.1103/PhysRevResearch.2.033416Published by the American Physical Society under the terms of the Creative Commons Attribution 4.0 International license. Further distribution of this work must maintain attribution to the author(s) and the published article's title, journal citation, and DOI.Published by the American Physical SocietyPhysics Subject Headings (PhySH)Research AreasFFLOOptical conductivitySpin currentSuperconducting phase transitionPhysical SystemsFerromagnetic superconductorsHiggs bosonsMultilayer thin filmsTunnel junctionsTechniquesMethods in superconductivityCondensed Matter & Materials Physics

Generating pure spin current in zigzag graphene nanoribbons by a thermal gradient: the effect of edge doping with BN pairs

Based on density functional theory combined with non-equilibrium Green’s function method, we investigate the thermoelectric transport properties of zigzag-edged graphene nanoribbons (ZGNRs) co-doped by two BN pairs at one edge. By applying the co-doping scheme of BN pairs instead of single B and N atoms as reported in literature, the energy of the quasi-bound states gets very close to the Fermi level. Due to the back scattering from the quasi-bound states, transmission valleys appear around the Fermi level, which results in a large transmission slope and Seebeck thermopower for both spin channels. Interestingly, the signs of the Seebeck thermopower of the two spin channels are opposite and the magnitudes are nearly equal, which can be utilized for generating pure spin current with a thermal gradient. More importantly, the thermal spin current is robust against the change of the distance between the two BN pairs and the doping configuration. These findings suggest a fascinating strategy for thermal spin current generation using BN pair co-doping and will be significant for the design of spintronic devices based on graphene nanoribbons.

Spin current generation and detection in uniaxial antiferromagnetic insulators

Spin current represents a flow of spin angular momentum and does not require movement of charges. Magnetic insulators can therefore work as a source as well as a medium of spin currents, which has been established in ferrimagnetic insulators. Here, we report recent progress in the generation and electrical detection of spin currents in uniaxial antiferromagnetic insulators carried by antiferromagnetic magnons, both thermally and resonantly excited, respectively, via the spin Seebeck effect and antiferromagnetic spin pumping by subterahertz microwaves. These findings open up many exciting possibilities of potential antiferromagnetic applications.

Electrical control of coherent spin rotation of a single-spin qubit

Nitrogen vacancy (NV) centers, optically active atomic defects in diamond, have attracted tremendous interest for quantum sensing, network, and computing applications due to their excellent quantum coherence and remarkable versatility in a real, ambient environment. One of the critical challenges to develop NV-based quantum operation platforms results from the difficulty in locally addressing the quantum spin states of individual NV spins in a scalable, energy-efficient manner. Here, we report electrical control of the coherent spin rotation rate of a single-spin qubit in NV-magnet based hybrid quantum systems. By utilizing electrically generated spin currents, we are able to achieve efficient tuning of magnetic damping and the amplitude of the dipole fields generated by a micrometer-sized resonant magnet, enabling electrical control of the Rabi oscillation frequency of NV spins. Our results highlight the potential of NV centers in designing functional hybrid solid-state systems for next-generation quantum-information technologies. The demonstrated coupling between the NV centers and the propagating spin waves harbored by a magnetic insulator further points to the possibility to establish macroscale entanglement between distant spin qubits.

Electrical evaluation of the alternating spin current generated via spin-vorticity coupling

In many interacting-electron systems, the microscopic spin angular momentum of electrons is conserved along with the macroscopic angular momentum in electrical current flows, i.e., vorticity. Such spin-vorticity coupling expands the choice of materials for spintronics devices. In this paper, we evaluate the magnitude of an alternating spin current generated by the spin-vorticity coupling with a gigahertz-order surface acoustic wave in a Cu thin film. We measure the gigahertz alternating spin current by an electrical method based on the inverse spin-Hall effect. From the amplitude of the spin current, we can determine the conversion efficiency of the angular momentum between local lattice rotation and electron spin in the Cu film. The conversion efficiency is four orders of magnitude larger than the case of spin current generation via kilohertz-order vorticity in turbulent flow of liquid mercury [M. Matsuo et al., Phys. Rev. B 96, 020401(R) (2017)]. Such a huge conversion efficiency is attributable to a smaller inconsistency of energy scale between lattice rotation and electron spin than the case of liquid vorticity.

Edge passivation control of graphene nanoribbons for robust pure spin current generation with photogalvanic effect

Photogalvanic effect has been demonstrated to induce pure spin current in several spin polarized systems, but limited to specific photon energy or specific polarization/helicity angles of the incident light in any of the reported systems. The requirement of such a precise tuning to the specific photon energy or polarization/helicity angles makes it extremely difficult to achieve pure spin current practically. In this work, we propose a realization scheme with zigzag-edged graphene nanoribbons (ZGNRs) by edge passivation control. In detail, we divide a ZGNR into two halves, with one half monohydrogenated at the upper edge and dihydrogenerated at the lower edge, while the other half hydrogenated oppositely. By extensive first principles calculations, we show that pure spin current can always be generated, neither dependent of the photon energy and polarization/helicit angle, nor dependent of whether it is linearly, circularly or elliptically polarized. Such a robustness in pure spin current generation is found to originate from the inversion symmetry in the geometry structure and the spin semiconducting feature of the ZGNRs. This suggests a fantastic scheme for practically generating pure spin current with photogalvanic effect in graphene and will be of great significance in spintronics.

Giant spin Seebeck effect in two-dimensional ferromagnetic CrI3 monolayer

Spin Seebeck effect (SSE) is a key factor in the spin caloritronics field, and extensive studies have been performed for potential spin thermoelectric modulator device applications. However, the performance of spin current generation was not high enough, and this is due to the weak yield of the SSE. Despite the many studies for the SSE in bulk materials, no reports are available yet in the pure two-dimensional (2D) ferromagnetic material. Hereby, we investigated the SSE of two-dimensional ferromagnetic CrI3 using the Boltzmann transport approach allowing diffusive scattering. We obtained a giant effective spin Seebeck effect of 1450 μV K−1, and this value is at least 4–5 times larger than previously reported values in bulk systems. Therefore, our finding may suggest that 2D CrI3 can be a potential material to open another perspective for 2D materials in the spin caloritronics field.

Pure spin current generation via photogalvanic effect with spatial inversion symmetry

We propose a new idea to generate pure spin current with photogalvanic effect (PGE) by designing devices with spatial inversion symmetry based on two-dimensional spin semiconducting materials. Due to the preservation of spatial inversion symmetry, the electric current generated by the PGE must be zero. However, finite spin-dependent current ${I}_{\ensuremath{\uparrow}/\ensuremath{\downarrow}}$ may still be produced. Once an amount of spin-up electrons flow into the device region from lead $\ensuremath{\alpha}$ and flow out from lead $\ensuremath{\beta}$ under light irradiation, the same amount of spin-down electrons will flow into the device region from lead $\ensuremath{\beta}$ and flow out from lead $\ensuremath{\alpha}$, which results in ${I}_{c}^{\ensuremath{\alpha}/\ensuremath{\beta}}={I}_{\ensuremath{\uparrow}}^{\ensuremath{\alpha}/\ensuremath{\beta}}+{I}_{\ensuremath{\downarrow}}^{\ensuremath{\alpha}/\ensuremath{\beta}}=0$ and ${I}_{s}^{\ensuremath{\alpha}/\ensuremath{\beta}}={I}_{\ensuremath{\uparrow}}^{\ensuremath{\alpha}/\ensuremath{\beta}}\ensuremath{-}{I}_{\ensuremath{\downarrow}}^{\ensuremath{\alpha}/\ensuremath{\beta}}\ensuremath{\ne}0$ simultaneously for each lead and thus finite pure spin current arises. As a concrete example, this idea is demonstrated by calculating the PGE in a photoelectric device constructed with an armchair-edged graphene nanoribbon which is divided into two semi-infinite ribbons. The two semi-infinite ribbons are then periodically and symmetrically patterned with ferromagnetic triangle antidots in the form of ``$▹◃$'' to achieve spatial inversion symmetry of the device. We find that the pure spin current can be robustly generated, neither dependent on the photon energy and polarization/helicity angle, nor dependent on whether it is linearly, circularly, or elliptically polarized.

Deterministic all-optical magnetization writing facilitated by non-local transfer of spin angular momentum

Ever since its discovery around a decade ago, all-optical magnetization switching (AOS) using femtosecond laser pulses has shown potential for future data storage and logic devices. In particular, single pulse helicity independent AOS in certain ferrimagnetic alloys and multilayers is highly efficient and ultrafast. However, in most cases it is a toggle mechanism, which is not desirable for applications. Here we experimentally demonstrate conversion from toggle switching to a deterministic mechanism by biasing AOS in a Co/Gd bilayer with a spin polarized current which is optically generated in an adjacent ferromagnetic reference layer. We show deterministic writing of an ‘up’ and ‘down’ state using a sequence of one or two pulses, respectively, and demonstrate the non-local origin by varying the magnitude of the generated spin current. Our demonstration of deterministic magnetization writing could provide an essential step towards the implementation of future optically addressable spintronic memory devices.

Two-Dimensional Materials for Energy-Efficient Spin-Orbit Torque Devices.

Spin-orbit torques (SOTs), which rely on spin current generation from charge current in a nonmagnetic material, promise an energy-efficient scheme for manipulating magnetization in magnetic devices. A critical topic for spintronic devices using SOTs is to enhance the charge to spin conversion efficiency. Besides, the current-induced spin polarization is usually limited to in-plane, whereas out-of-plane spin polarization could be favored for efficient perpendicular magnetization switching. Recent advances in utilizing two important classes of two-dimensional materials-topological insulators and transition-metal dichalcogenides-as spin sources to generate SOT shed light on addressing these challenges. Topological insulators such as bismuth selenide have shown a giant SOT efficiency, which is larger than those from three-dimensional heavy metals by at least 1 order of magnitude. Transition-metal dichalcogenides such as tungsten telluride have shown a current-induced out-of-plane spin polarization, which is allowed by the reduced symmetry. In this review, we use symmetry arguments to predict and analyze SOTs in two-dimensional material-based heterostructures. We summarize the recent progress of SOT studies based on topological insulators and transition-metal dichalcogenides and show how these results are in line with the symmetry arguments. At last, we identify unsolved issues in the current studies and suggest three potential research directions in this field.

Modulation of Magnetization Precession Trajectories by Perpendicular Magnetic Anisotropy in CoFeB Thin Films

Precession trajectories during excitation of magnetization dynamics play an important role in determining spin wave propagation, emission power of spin torque oscillator, and spin current generated by spin pumping. However, comparatively little information has emerged on the effect of perpendicular magnetic anisotropy (PMA) on dynamical magnetization trajectories. The effect of PMA on magnetization dynamics has become particularly important since the discovery of voltage control of magnetic anisotropy in CoFeB/MgO junctions. This motivates us to investigate the effect of PMA on dynamical magnetization trajectories of CoFeB thin films using micromagnetic simulations. The ellipticity of the trajectories is found to increase monotonically with PMA. On the other hand, the area of such elliptical trajectories, which determine the spin current generation, shows a non-monotonous change with respect to PMA. This area can be expected to be maximum for the case where ellipticity is ~0.5.

Giant Spin Hall Effect in Cu-Tb Alloy Thin Films.

We report the giant spin current generation in CuTb alloys arising from the spin Hall effect. The maximum spin Hall angle from our CuTb-based magnetic heterostructures was found to be -0.35 ± 0.02 for Cu0.39Tb0.61. We find that the contribution of skew scattering is larger than the side jump for lower Tb concentrations (<14.9%), while the converse is true for higher Tb concentrations. Additionally, we also studied the Gilbert damping parameter, spin diffusion length, and spin-mixing conductance. Interfacial spin transparency was found to be 0.55 ± 0.03 for the CoFeB/Cu0.53Tb0.47 interface. The spin diffusion length and spin-mixing conductance of the Cu0.53Tb0.47 alloy are λsd = 2.5 ± 0.3 nm and G↓↑ = (24.2 ± 1.0) × 1015 cm-2, respectively. Our results pave a way for rare-earth metals to be used as a spin Hall material in highly efficient SOT devices.

Enhancement of acoustic spin pumping by acoustic distributed Bragg reflector cavity

Surface acoustic waves (SAWs) in the GHz frequency range can inject spin currents dynamically into adjacent non-magnetic layers via the spin pumping effect associated with ferromagnetic resonance. Here, we demonstrate an enhancement of acoustic ferromagnetic resonance and spin current generation by a pair of SAW reflector gratings, which form an acoustic analog of the distributed Bragg reflector cavity. In the experiment, we confirmed 2.04 ± 0.02 times larger SAW power absorption in a device with cavity than in the case of no acoustic cavity. We confirmed up to 2.96 ± 0.02 times larger spin current generation by measuring electric voltages generated by the inverse Edelstein effect at the interface between Cu and Bi2O3. The results suggest that acoustic cavities would be useful to enhance the conversion efficiency in SAW driven coupled magnon–phonon dynamics.

Thermal behavior of spin-current generation in PtxCu1-x devices characterized through spin-torque ferromagnetic resonance

High temperature studies of spin Hall effect have often been neglected despite its profound significance in real-world devices. In this work, high temperature spin torque ferromagnetic resonance measurement was performed to evaluate the effects of temperature on the Gilbert damping and spin Hall efficiency of PtxCu1−x. When the temperature was varied from 300 K to 407 K, the Gilbert damping was relatively stable with a change of 4% at composition x = 66%. Alloying Pt and Cu improved the spin Hall efficiency of Pt75Cu25/Co/Ta by 29% to a value of 0.31 ± 0.03 at 407 K. However, the critical switching current density is dependent on the ratio between the Gilbert damping and spin Hall efficiency and the smallest value was observed when x = 47%. It was found that at this concentration, the spin transparency was at its highest at 0.85 ± 0.09 hence indicating the importance of interfacial transparency for energy efficient devices at elevated temperature.

X-ray detection of ultrashort spin current pulses in synthetic antiferromagnets

We explore the ultrafast generation of spin currents in magnetic multilayer samples by applying fs laser pulses to one layer and measuring the magnetic response in the other layer by element-resolved x-ray spectroscopy. In Ni(5 nm)/Ru(2 nm)/Fe(4 nm), the Ni and Fe magnetization directions couple antiferromagnetically due to the Ruderman–Kittel–Kasuya–Yosida interaction but may be oriented parallel through an applied magnetic field. After exciting the top Ni layer with a fs laser pulse, we also find that the Fe layer underneath demagnetizes, with a 4.1±1.9% amplitude difference between parallel and antiparallel orientation of the Ni and Fe magnetizations. We attribute this difference to the influence of a spin current generated by the fs laser pulse that transfers angular momentum from the Ni into the Fe layer. Our results confirm that superdiffusive spin transport plays a role in determining the sub-ps demagnetization dynamics of synthetic antiferromagnetic layers, but also evidence large depolarization effects due to hot electron dynamics, which are independent of the relative alignment of the magnetization in Ni and Fe.

Control of damping in perpendicularly magnetized thin films using spin-orbit torques

Magnetic damping plays a crucial role in the dynamics of magnetic systems. Hence, control over the damping is highly desirable for the development of magnetic devices. One of the possible ways to manipulate magnetic damping in a ferromagnetic material is the injection of spin currents generated by the spin Hall effect. The generated spin currents can be used to manipulate magnetic moments as well as the effective damping of the ferromagnetic material. In this paper, we demonstrate that the influence of the spin current on the damping is highly sensitive to the relative direction of the spin polarization and the precessional axis of the magnetization. The observations are important for various applications in magnetic storage devices and in microwave signal processing.

Thermally driven pure spin current through an interacting quantum dot

We study spin-dependent electron transport properties of a thermally driven interacting quantum dot. When an external magnetic field is applied to the quantum dot, the effective transmissions of spin-up and spin-down electrons are separated from each other and have a perfect mirror symmetry with respect to the incident energy at a certain gate voltage. A pure spin current can be induced in the system and modulated by a magnetic field. Under certain magnetic field strengths, a larger pure spin current can be obtained at gate voltages with the values in a range, not just at a specific voltage. These results indicate that the system can be worked as a pure spin current generator.

Spin-Hall magnetoresistance in Ta/Cr/YIG trilayers with different Cr thicknesses

We report an investigation of spin-Hall magnetoresistance (SMR) in Ta/Cr/Y3Fe5O12 (Ta/Cr/YIG) trilayers with different thicknesses of the Cr inserted layer. The Ta/Cr/YIG film with a 1-nm-thick Cr layer shows a maximum SMR of 4.2×10-4 at 100 K, which is an order of magnitude larger than that previously reported in the Ta/YIG or Cr/YIG systems. The enhancement of SMR in the Ta/Cr/YIG is ascribed to the strong spin–orbit coupling (SOC) effect of both Ta and Cr and the anti-oxidation protection by the Ta coating. A negative SMR different from the conventional SMR is found for the Ta/Cr/YIG with the Cr thicknesses of 3 and 6 nm at the temperature below 100 and 260 K, respectively, which suggests the occurrence of the spin-flop coupling between the antiferromagnetic Cr and ferromagnetic YIG moments. Both the anisotropic magnetoresistance (AMR) and the anomalous-Hall resistance (AHR) indicate an induced ferromagnetism of Cr which is likely due to the spin-flop transition or the uncompensated magnetic moments. Our results suggest that the 3d transition metal Cr can be used as a spin current generator, and the antiferromagnetic structure of Cr can be employed to regulate the transport of spin current.

Acoustic ferromagnetic resonance and spin pumping induced by surface acoustic waves

Voltage induced magnetization dynamics of magnetic thin films is a valuable tool to study anisotropic fields, exchange couplings, magnetization damping and spin pumping mechanism. A particularly well established technique is the ferromagnetic resonance (FMR) generated by the coupling of microwave photons and magnetization eigenmodes in the GHz range. Here we review the basic concepts of the so-called acoustic-FMR (a-FMR) induced by the coupling of surface acoustic waves (SAW) and magnetization of thin films. Interestingly, additional to the benefits of the microwave excited FMR technique, the coupling between SAW and magnetization also offers fertile ground to study magnon-phonon and spin rotation couplings. We describe the in-plane magnetic field angle dependence of the a-FMR by measuring the absorption / transmission of SAW and the attenuation of SAW in the presence of rotational motion of the lattice, and show the consequent generation of spin current by acoustic spin pumping.