Rashba-Edelstein Effect Research Articles

Unidirectional orbital magnetoresistance in light-metal–ferromagnet bilayers

We report the observation of a unidirectional magnetoresistance (UMR) that originates from the nonequilibrium orbital momentum induced by an electric current in a naturally oxidized Cu/Co bilayer. The orbital-UMR scales with the torque efficiency due to the orbital Rashba-Edelstein effect upon changing the Co thickness and temperature, reflecting their common origin. We attribute the UMR to orbital-dependent electron scattering and orbital-to-spin conversion in the ferromagnetic layer. In contrast to the spin-current induced UMR, the magnon contribution to the orbital-UMR is absent in thin Co layers, which we ascribe to the lack of coupling between low energy magnons and orbital current. The magnon contribution to the UMR emerges in Co layers thicker than about 5 nm, which is comparable to the orbital-to-spin conversion length. Our results provide insight into orbital-to-spin momentum transfer processes relevant for the optimization of spintronic devices based on light metals and orbital transport.

Charge-to-spin conversion in Bi/Ag bilayer investigated by spin-torque ferromagnetic resonance

The charge-to-spin conversion in Bi/Ag bilayer is studied by spin-torque ferromagnetic resonance experimentally. Fe and permalloy layers sandwich the Bi/Ag bilayer in a multilayer structure, which allows the injection of spin current generated in the Bi/Ag bilayer into the two ferromagnetic layers from opposite directions. The spin Hall effect and the Rashba–Edelstein effect in Bi/Ag are expected to show qualitatively different spin-torque ferromagnetic resonance signals in such a setup. The results show the dominant contribution from the spin Hall effect rather than the Rashba–Edelstein effect, consistent with our recent finding of the spin-to-charge conversion in Bi/Ag bilayer (Shen et al., 2021).

Coherent Picture on the Pure Spin Transport between Ag/Bi and Ferromagnets.

In a joint effort of both experiments and first-principles calculations, we resolve a hotly debated controversy and provide a coherent picture on the pure spin transport between Ag/Bi and ferromagnets. We demonstrate a strong inverse Rashba-Edelstein effect (IREE) at the interface in between Ag/Bi with a ferromagnetic metal (FM) but not with a ferromagnetic insulator. This is in sharp contrast to the previously claimed IREE at Ag/Bi interface or inverse spin Hall effect dominated spin transport. A more than one order of magnitude modulation of IREE signal is realized for different Ag/Bi-FM interfaces, casting strong tunability and a new direction for searching efficient spintronics materials.

Phase Transition and Oxidation Dependence of Charge–Spin Conversion in Rare‐Earth Gadolinium

The rare‐earth metal gadolinium (Gd) is characterized by strong spin‐orbit coupling and a paramagnetic‐to‐ferromagnetic phase transition that enables modulation of a charge–spin conversion via temperature. Charge–spin conversion is experimentally observed and the conversion efficiency is ≈0.26, which is slightly larger than that for Pt and Ta and smaller than that for W. The temperature dependence of charge–spin conversion initially decreases, followed by increasing, because of the paramagnetic‐to‐ferromagnetic phase transition. The charge–spin conversion efficiencies arising from the bulk‐spin Hall effect and the interfacial Rashba–Edelstein effect, respectively, via MgO/Gd/Py and MgO/Py/GdOx systems, are distinguished. The latter has a more significant enhancement because of ambient oxidation that generates the Gd/GdOx interface. The possibility of modulating the conversion efficiency via temperature and oxidation enables applications in spintronic and oxide electronic devices.

Enhancement of spin-orbit torque efficiency by tailoring interfacial spin-orbit coupling in Pt-based magnetic multilayers

We study inserting Co layer thickness-dependent spin transport and spin–orbit torques (SOTs) in the Pt/Co/Py trilayers by spin-torque ferromagnetic resonance. The interfacial perpendicular magnetic anisotropy (IPMA) energy density (K s = 2.7 erg/cm2, 1 erg = 10−7 J), which is dominated by interfacial spin–orbit coupling (ISOC) in the Pt/Co interface, total effective spin-mixing conductance ) and two-magnon scattering (β TMS = 0.46 nm2) are first characterized, and the damping-like torque (ξ DL = 0.103) and field-like torque (ξ FL = –0.017) efficiencies are also calculated quantitatively by varying the thickness of the inserting Co layer. The significant enhancement of ξ DL and ξ FL in Pt/Co/Py than Pt/Py bilayer system originates from the interfacial Rashba–Edelstein effect due to the strong ISOC between Co-3d and Pt-5d orbitals at the Pt/Co interface. Additionally, we find a considerable out-of-plane spin polarization SOT, which is ascribed to the spin anomalous Hall effect and possible spin precession effect due to IPMA-induced perpendicular magnetization at the Pt/Co interface. Our results demonstrate that the ISOC of the Pt/Co interface plays a vital role in spin transport and SOTs-generation. Our finds offer an alternative approach to improve the conventional SOTs efficiencies and generate unconventional SOTs with out-of-plane spin polarization to develop low power Pt-based spintronic via tailoring the Pt/FM interface.

Tailoring Neuromorphic Switching by CuNx -Mediated Orbital Currents

Current-induced spin-orbit torque (SOT) is regarded as a promising mechanism for driving neuromorphic behavior in spin-orbitronic devices. In principle, the strong SOT in a heavy-metal-based magnetic heterostructure is attributed to the spin-orbit coupling (SOC)-induced spin Hall effect and/or the spin Rashba-Edelstein effect. Recently, SOC-free mechanisms such as the orbital-angular-momentum-induced orbital Hall effect and/or the orbital Rashba-Edelstein effect have been proposed to generate sizable torques comparable to those from the conventional spin Hall mechanism. In this work, we show that the orbital current can be effectively generated by the nitrided light metal $\mathrm{Cu}$. The overall dampinglike SOT efficiency, which consists of both the spin and the orbital current contributions, can be tailored from ${\ensuremath{\xi}}_{\mathrm{DL}}\ensuremath{\approx}0.06\text{ to }0.4$ in a $\mathrm{Pt}/\mathrm{Co}/{\mathrm{Cu}\mathrm{N}}_{x}$ magnetic heterostructure by tuning the nitrogen doping concentration. Current-induced magnetization switching further verifies the efficacy of such orbital current with a critical switching current density as low as ${J}_{c}$ \ensuremath{\sim} 5 \ifmmode\times\else\texttimes\fi{} ${10}^{10}\phantom{\rule{0.25em}{0ex}}{\mathrm{A}/\mathrm{m}}^{2}$. Most importantly, orbital-current-mediated memristive switching behavior can be observed in such heterostructures, which reveals that gigantic SOT and efficient magnetization switching are the trade-offs for the applicable window of memristive switching. Our work provides insights into the role that orbital current might play in SOT neuromorphic devices for making energy-efficient spin-orbitronic devices.

Ag growth on the Ag2Bi Rashba alloy

Multilayer structures comprising Bi/Ag bilayers display spin-charge conversion, a phenomenon of interest in spintronics whose origin (inverse Rashba-Edelstein vs. inverse spin Hall effects) is still debated. The Bi/Ag interfaces are assumed to be stable in model calculations, but the experimental determination of their structural properties is missing. Here, we explore by photoemission spectroscopy the stability and electronic structure of a Bi/Ag interface with strong Rashba interaction. We saturate the surface of a Ag(111) crystal with the Ag2Bi alloy, which presents surface-embedded Bi atoms and characteristic Rashba-split surface states, and deposit Ag layers on top of it at room temperature. The intensity of the Rashba-split bands weakens sizably after the deposition of few Ag monolayers, although most of the Bi atoms segregate at the surface and retain the original local coordination. These findings are consistent with the surfactant behavior of Bi atoms, which favors a layer-by-layer Ag growth without preserving the long-range order of the Ag2Bi alloy. They also suggest the need of detailed chemical and structural analysis for an accurate description of Bi/Ag bilayers.

Current-induced switching of antiferromagnetic order in Mn2Au from first principles

It is well established that it is possible to switch certain antiferromagnets electrically, yet the interplay of N\'eel-spin-orbit torques and thermal activation is only poorly understood. Combining ab initio calculations and atomistic spin dynamics simulations we develop a multiscale model to study the current-induced switching in ${\mathrm{Mn}}_{2}\mathrm{Au}$. We compute from first principles the strength and direction of the electrically induced magnetic moments, caused by the Rashba-Edelstein effect, and take these into account in atomistic spin dynamics simulations. Our simulations reveal the switching paths as well as the timescales for switching. The size of the induced moments, however, turns out to be insufficient to lead to fully deterministic switching. Instead, we find that a certain degree of thermal activation is required to help overcome the relevant energy barrier.

A room temperature polar magnetic metal

The emergence of long-range magnetic order in noncentrosymmetric compounds has stimulated interest in the possibility of exotic spin transport phenomena and topologically protected spin textures for applications in next-generation spintronics. Polar magnets, with broken symmetries of spatial inversion and time reversal, usually host chiral spin textures. This work reports on a wurtzite-structure polar magnetic metal, identified as AA′-stacked (Fe0.5Co0.5)5GeTe2, which exhibits a Néel-type skyrmion lattice as well as a Rashba-Edelstein effect at room temperature. Atomic resolution imaging of the structure reveals a structural transition as a function of Co-substitution, leading to the emergence of the polar phase at 50% Co. This discovery reveals an unprecedented layered polar magnetic system for investigating intriguing spin topologies, and it ushers in a promising new framework for spintronics.Received 16 October 2021Accepted 8 March 2022DOI:https://doi.org/10.1103/PhysRevMaterials.6.044403©2022 American Physical SocietyPhysics Subject Headings (PhySH)Research AreasMagnetotransportSkyrmionsSpintronicsPhysical SystemsTransition metal dichalcogenidesCondensed Matter, Materials & Applied Physics

Unconventional Spin Currents Generated by the Spin-Orbit Precession Effect in Perpendicularly Magnetized Co−Tb Ferrimagnetic System

Charge-spin interconversion via spin-orbit coupling (SOC) provides an effective way to generate spin currents for electrical control of nanomagnets for data storage and processing. Except for the widely studied giant spin Hall effect (SHE) and Rashba-Edelstein effect (REE) that can generate a spin current with in-plane transverse spin polarization, theoretically proposed spin swapping or spin-orbit precession effect (SOPE) also can substantially generate spin currents with different spin polarizations in magnetic multilayers. Here, we experimentally demonstrate that except the SHE-induced large effective SOT ${\ensuremath{\theta}}_{y}^{\mathrm{eff}}\ensuremath{\sim}0.10\ensuremath{-}0.14$ comparable to the reported giant SHE \ensuremath{\beta}-phase $\mathrm{Ta}$, SOPE generated two considerable unconventional $\mathrm{SOTs}$ with in-plane longitudinal ${\ensuremath{\theta}}_{x}^{\mathrm{eff}}\phantom{\rule{0.25em}{0ex}}$ and out-of-plane spin polarizations ${\ensuremath{\theta}}_{z}^{\mathrm{eff}}$, which are also verified in a $\mathrm{Co}\text{\ensuremath{-}}\mathrm{Tb}/\mathrm{Cu}/\mathrm{Py}$ trilayer structure. The former ${\ensuremath{\theta}}_{x}^{\mathrm{eff}}$ changes its sign with the unit magnetization reversal of the perpendicularly magnetized $\mathrm{Co}\text{\ensuremath{-}}\mathrm{Tb}$ layer, while the latter ${\ensuremath{\theta}}_{z}^{\mathrm{eff}}$ does not change its sign, consistent with the theoretical model of the SOPE mechanism. Our results pave an alternative way to design current-induced high-efficient SOT-based devices using abundant ferrimagnetic alloys.

Interface-driven spin pumping and inverse Rashba-Edelstein effect in FeGaB/Ag/BiSb multilayers

Spin-orbit-coupling (SOC) plays a critical role in spin-to-charge conversion (SCC) mechanism. The SCC process is usually investigated by injecting a spin current from ferromagnet (FM) into a large SOC normal metal (SOC-NM) and then measuring the voltage generated by inverse spin Hall effect (ISHE). Recently, a new mechanism of SCC at inversion symmetry breaking interfaces between topological insulators (TIs) and magnetically ordered materials has been introduced, which is driven by the inverse Rashba-Edelstein effect (IREE). Motivated by this result, we have investigated the IREE effect of a simple binary three-dimensional TI Bi85Sb15 (BiSb). The model system used here is a trilayer of FeGaB/Ag/BiSb with different Ag thickness made insitu with a combination of dc magnetron sputtering and pulsed laser ablation. The SCC efficiency has been evaluated by measuring ferromagnetic resonance (FMR) driven ISHE voltage and extraction of the Gilbert damping (α) and interfacial spin mixing conductance (g↑↓) from the FMR line shape analysis. It is interesting to note that the charge current (Ic) increases with Ag thickness. This observation indicated that the IREE process at the Ag/BiSb interface dominates over ISHE in the bulk of BiSb film. The conversion efficiency of IREE is determined by calculating the IREE length (λIREE), which increases with the thickness of the Ag layer. These findings offer a method to generate and detect the IREE in a room temperature deposited heterostructure using simple coplanar waveguide setup.

Dissipationless Spin-Charge Conversion in Excitonic Pseudospin Superfluid.

Spin-charge conversion by the inverse spin Hall effect or inverse Rashba-Edelstein effect is prevalent in spintronics but dissipative. We propose a dissipationless spin-charge conversion mechanism by an excitonic pseudospin superfluid in an electron-hole double-layer system. Magnetic exchange fields lift singlet-triplet degeneracy of interlayer exciton levels in the double-layer system. Condensation of the singlet-triplet hybridized excitons breaks both a U(1) gauge symmetry and a pseudospin rotational symmetry around the fields, leading to spin-charge coupled superflow in the system. We demonstrate the mechanism by deriving spin-charge coupled Josephson equations for the excitonic superflow from a coupled quantum-dot model.

Terahertz emission from LaAlO<sub>3</sub>/SrTiO<sub>3</sub> heterostructures pumped with femtosecond laser

Since the discovery of the ultrafast demagnetization of the ferromagnetic metal, the spin degree of electrons is gradually used to generate terahertz radiation. The terahertz radiation generated by the inverse Rashba-Edelstein effect was confirmed first at the interface of Ag/Bi. However, the spin-to-charge conversion efficiency of the LaAlO<sub>3</sub>/SrTiO<sub>3</sub> interface is one order of magnitude lager than that of the Ag/Bi interface under equilibrium or quasi-equilibrium condition. Whether the LaAlO<sub>3</sub>/SrTiO<sub>3</sub> heterostructures can be used to convert spin current to generate terahertz radiation remains to be systemically studied. In this work, we fabricate the NiFe/LaAlO<sub>3</sub>//SrTiO<sub>3</sub> heterostructures and investigate the generation of terahertz radiation by femtosecond laser pumping and its dependence of the magnetic field direction. We change the thickness of the LaAlO<sub>3</sub> to show the applicability of the superdiffusive spin transport model and optical transmission model. We find the multireflections at the LaAlO<sub>3</sub>/SrTiO<sub>3</sub> interface weaken the terahertz radiation intensity. This work provides experimental and theoretical support for further optimizing the generation of terahertz electromagnetic waves.

Charge-spin interconversion in graphene-based systems from density functional theory

We present a methodology to address, from first principles, charge-spin interconversion in two-dimensional materials with spin-orbit coupling. Our study relies on an implementation of density functional theory based quantum transport formalism adapted to such purpose. We show how an analysis of the $k$-resolved spin polarization gives the necessary insight to understand the different charge-spin interconversion mechanisms. We have tested it in the simplest scenario of isolated graphene in a perpendicular electric field where effective tight-binding models are available to compare with. Our results show that the flow of an unpolarized current across a single layer of graphene produces, as expected, a spin separation perpendicular to the current for two of the three spin components (out-of-plane and longitudinal), which is the signature of the spin Hall effect. Additionally, it also yields an overall spin accumulation for the third spin component (perpendicular to the current), which is the signature of the Rashba-Edelstein effect. Even in this simple example, our results reveal an unexpected competition between the Rashba and the intrinsic spin-orbit coupling. Remarkably, the sign of the accumulated spin density does not depend on the electron or hole nature of the injected current for realistic values of the Rashba coupling.

Reliability of spin-to-charge conversion measurements in graphene-based lateral spin valves

Understanding spin physics in graphene is crucial for developing future two-dimensional spintronic devices. Recent studies show that efficient spin-to-charge conversions (SCCs) via either the inverse spin Hall effect or the inverse Rashba–Edelstein effect (IREE) can be achieved in graphene by proximity with an adjacent spin–orbit coupling (SOC) material. Lateral spin valve devices, made up of a graphene Hall bar and ferromagnets, are best suited for such studies. Here, we report that signals mimicking the IREE can be measured in pristine graphene possessing negligible SOC, confirming that these signals are unrelated to SCC. We identify either the anomalous Hall effect in the ferromagnet or the ordinary Hall effect in graphene induced by stray fields as the possible sources of this artefact. By quantitatively comparing these options with finite-element-method simulations, we conclude the latter better explains our results. Our study deepens the understanding of SCC measurement schemes in graphene, which should be taken into account when designing future experiments.

Externally controlled and switchable two-dimensional electron gas at the Rashba interface between ferroelectrics and heavy d metals

Strong spin-orbit coupling in noncentrosymmetric materials and interfaces results in remarkable physical phenomena, such as nontrivial spin textures, which may exhibit Rashba, Dresselhaus, and other intricated configurations. This provides a promising basis for nonvolatile spintronic devices and further implications. Here, we simulate from first principles a two-dimensional electron gas in ultrathin platinum and palladium layers grown on ferroelectric PbTiO$_3$(001). The latter allows, in principle, to switch and control the spin-to-charge conversion by the polarization reversal. We show how the band structure and its Rashba splitting differ in the Pt and Pd overlayers and how these electronic features change with increasing the overlayer thickness and upon reversal of polarization. Besides, for both overlayers, we simulated their current-voltage ($I-V$) characteristics, the resistance of which upon the polarization reversal changes between 20% and several hundred percent. The reported findings can be used to model directly the Rashba-Edelstein effect.

Emergent quasi-two-dimensional electron gas betweenLi1±xNbO3andLaAlO3and its prospectively switchable magnetism

Oxide interfaces between insulating perovskites can provide a two-dimensional electron gas (2DEG), which was observed initially beneath polar $\mathrm{La}\mathrm{Al}{\mathrm{O}}_{3}$ (LAO) grown on $\mathrm{Ti}{\mathrm{O}}_{2}$-terminated $\mathrm{Sr}\mathrm{Ti}{\mathrm{O}}_{3}$(001). Here, we suggest to use a solid electrolyte $\mathrm{Li}\mathrm{Nb}{\mathrm{O}}_{3}$ (LiNO) instead of $\mathrm{Sr}\mathrm{Ti}{\mathrm{O}}_{3}$ to create in LAO/LiNO a robustly switchable and magnetic quasi 2DEG (q2DEG). This prospective phenomenon is achieved by charging and discharging lithium niobate while its $n\ensuremath{\rightarrow}p$ doping induces the alteration of magnetic order in LAO/LiNO and, thus, the spin degrees of freedom in q2DEG. On the basis of ab initio calculations and starting from the defectless and insulating LAO/LiNO superlattice, we demonstrate that either a ferromagnetic q2DEG or weakly magnetic quasi-2D hole gas emerge there due to the presence of excessive electrons or holes, respectively. These defects were simulated by one extra Li and a lithium vacancy in LiNO. By varying the $\mathrm{Li}\mathrm{Nb}{\mathrm{O}}_{3}$ thicknesses up to 18 formula units, we found that (i) the spatial extent of q2DEG is 1.5 nm along the stacking direction and that (ii) the dopant position at the Al-terminated interface is energetically preferable by more than 0.5 eV compared to the deep layers. In the context of the advanced design of q2DEG, the out-of-plane electric polarization, which intrinsically appears in multiferroic LAO/LiNO, allows to switch externally the electric-field dependence of the Rashba effect. Moreover, the in-plane charge current via q2DEG may generate there a transverse spin density due to the Rashba-Edelstein effect. We discuss this scenario using the relativistic electronic structures and locally projected density of states which differ remarkably in $\mathrm{L}\mathrm{A}\mathrm{O}/{\mathrm{Li}}_{1+x}\mathrm{N}\mathrm{O}$ and $\mathrm{L}\mathrm{A}\mathrm{O}/{\mathrm{Li}}_{1\ensuremath{-}x}\mathrm{N}\mathrm{O}$.

Direct observation of spin accumulation and spin-orbit torque driven by Rashba-Edelstein effect in an InAs quantum-well layer

For semiconductor spintronics, efficient spin generation in semiconductor and spin transfer to ferromagnetic metal (FM) are essentially required. Two-dimensional electron gas (2DEG) of semiconductor quantum wells is a promising system for generating spin via the Rashba-Edelstein effect (REE) because of its strong inversion symmetry breaking. In this study, we investigate spin accumulation through REE and spin Hall effect (SHE) in the 2DEG of an InAs quantum well. We use spatial- and polarization-resolved measurements of spin, which reveals that REE dominates SHE in 2DEG. Furthermore, REE in 2DEG induces a spin-orbit torque on FM in a 2DEG/insulator/FM heterostructure. Using direction- and time-resolved measurements of FM magnetization, we determine a sizeable fieldlike torque, which is attributed to the phonon-mediated spin transport from 2DEG to FM.

Growth optimization of TaN for superconducting spintronics

We have optimized the growth of superconducting TaN thin films on SiO2 substrates via dc magnetron sputtering and extract a maximum superconducting transition temperature of T c = 5 K as well as a maximum critical field μ 0 H c2 = (13.8 ± 0.1) T. This material is of interest for both different fields of quantum technology and superconducting spintronics as it represents a magnetic field-robust superconductor with strong spin–orbit interaction (SOI). After presenting the results of the growth optimization, we investigate in the second part the impact of the strong SOI in TaN on superconductor/ferromagnet heterostructures. To this end, we analyze the magnetization dynamics of both normal state and superconducting TaN/Ni80Fe20 (permalloy, Py)-bilayers as a function of temperature using broadband ferromagnetic resonance spectroscopy. In particular, we quantify the inverse current-induced torques of the bilayers and compare these results to NbN/Py-bilayers. In the normal state of TaN, we detect a positive damping-like current-induced torque σ d from the inverse spin Hall effect and a small field-like torque σ f attributed to the inverse Rashba–Edelstein effect at the TaN/Py-interface. In the superconducting state of TaN, we detect a negative σ d attributed to the quasiparticle mediated inverse spin Hall effect (QMiSHE) and the unexpected manifestation of a large positive field-like σ f of unknown origin matching our previous results for NbN/Py-bilayers. The QMiSHE can be used to probe spin currents in emergent quantum materials.

Giant inverse Rashba-Edelstein effect: Application to monolayer OsBi2

We propose that the hybridization between two sets of Rashba bands can lead to an unconventional topology where the two Fermi circles from different bands own in-plane helical spin textures with the same chiralities, and possess group velocities with the same directions. Under the weak spin injection, the two Fermi circles both give the positive contributions to the spin-to-charge conversion and thus induce the giant inverse Rashba-Edelstein Effect with large conversion efficiency, which is very different from the conventional Rashba-Edelstein Effect. More importantly, through the first-principles calculations, we predict that monolayer OsBi2 could be a good candidate to realize the giant inverse Rashba-Edelstein Effect. Our studies not only demonstrate a new mechanism to achieve highly efficient spin-to-charge conversion in spintronics, but also provide a promising material to realize it.