Rashba-Edelstein Effect Research Articles

Enhanced Spin Seebeck Thermopower in Pt/Holey MoS2/Y3Fe5O12 Hybrid Structure.

We first observed the spin-to-charge conversion due to both the inverse Rashba-Edelstein effect (IREE) and inverse spin-Hall effect in a holey multilayer molybdenum disulfide (MoS2) intermediate layer in a Pt/YIG structure via LSSE measurements under nonequilibrium magnetization. We found an enhancement of approximately 238%, 307%, and 290% in the longitudinal spin Seebeck effect (LSSE) voltage, spin-to-charge current, and thermoelectric (TE) power factor, respectively, compared with the monolayer MoS2 interlayer in a Pt/YIG structure. Such an enhancement in the LSSE performance of Pt/holey MoS2/YIG can be explained by the improvement of spin accumulation in the Pt layer by induced spin fluctuation as well as increased additional spin-to-charge conversion due to in-plane IREE. Our findings represent a significant achievement in the understanding of spin transport in atomically thin MoS2 interlayers and pave the way toward large-area TE energy-harvesting devices in two-dimensional transition metal dichalcogenide materials.

Strain Tunable Berry Curvature Dipole, Orbital Magnetization and Nonlinear Hall Effect in WSe2 Monolayer* *Supported by the National Key Research and Development Program of China (Grant Nos. 2018YFA0703703 and 2016YFA0300802), and the National Natural Science Foundation of China (Grant Nos. 91964201, 61825401, and 11774004).

The electronic topology is generally related to the Berry curvature, which can induce the anomalous Hall effect in time-reversal symmetry breaking systems. Intrinsic monolayer transition metal dichalcogenides possesses two nonequivalent K and K′ valleys, having Berry curvatures with opposite signs, and thus vanishing anomalous Hall effect in this system. Here we report the experimental realization of asymmetrical distribution of Berry curvature in a single valley in monolayer WSe2 via applying uniaxial strain to break C 3v symmetry. As a result, although the Berry curvature itself is still opposite in K and K′ valleys, the two valleys would contribute equally to nonzero Berry curvature dipole. Upon applying electric field E , the emergent Berry curvature dipole D would lead to an out-of-plane orbital magnetization M ∝ D ⋅ E , which further induces an anomalous Hall effect with a linear response to E 2, known as nonlinear Hall effect. We show the strain modulated transport properties of nonlinear Hall effect in monolayer WSe2 with moderate hole-doping by gating. The second-harmonic Hall signals show quadratic dependence on electric field, and the corresponding orbital magnetization per current density M/J can reach as large as 60. In contrast to the conventional Rashba–Edelstein effect with in-plane spin polarization, such current-induced orbital magnetization is along the out-of-plane direction, thus promising for high-efficient electrical switching of perpendicular magnetization.

Intrinsic spin Hall effect in topological insulators: A first-principles study

The intrinsic spin Hall conductivity of typical topological insulators Sb$_2$Se$_3$, Sb$_2$Te$_3$, Bi$_2$Se$_3$, and Bi$_2$Te$_3$ in the bulk form, is calculated from first-principles by using density functional theory and the linear response theory in a maximally localized Wannier basis. The results show that there is a finite spin Hall conductivity of 100--200 ($\hbar$/2e)(S/cm) in the vicinity of the Fermi energy. Although the resulting values are an order of magnitude smaller than that of heavy metals, they show a comparable spin Hall angle due to their relatively lower longitudinal conductivity. The spin Hall angle for different compounds are then compared to that of recent experiments on topological-insulator/ferromagnet heterostructures. The comparison suggests that the role of the bulk in generating a spin current and consequently a spin torque in magnetization switching applications is comparable to that of the surface including the spin-momentum locked surface states and the Rashba-Edelstein effect at the interface.

Fingerprint of the inverse Rashba-Edelstein effect at heavy-metal/Cu interfaces

We report the direct observation of the fingerprint of the inverse Rashba-Edelstein effect (IREE) via investigations of the spin-to-charge conversion in Ta(Pt)/Cu and Cu/Ta(Pt) as a function of the heavy-metal thickness. The converted charge voltages have opposite signs for samples with reversed stacking orders in the ultrathin regime and have the same sign at higher thickness. The effect is demonstrated with two independent experimental approaches and supported by first-principles calculations. Our observations unambiguously demonstrate the existence of the IREE at heavy-metal/Cu interfaces and provide a framework for manipulating the spin-charge conversion via interface engineering.

Strong interface-induced spin-charge conversion in YIG/Cr heterostructures

Here, we have investigated the spin pumping effect of Y3Fe5O12 (YIG)/Cu (tCu nm)/Cr heterostructures at room temperature with the thickness of the Cu interlayer varying from 0.4 nm to 5.0 nm. A huge charge signal Ic = 0.239 μA is observed in a YIG/Cr bilayer with direct contact, whereas Ic drops dramatically by two orders of magnitude when thin Cu interlayers down to 0.4 nm are inserted between YIG and Cr. Meanwhile, the injected spin current Js stays almost invariant for all the heterostructures. The effective spin Hall angle “θSH” of the YIG/Cr interface is found to be three orders of magnitude larger than the spin Hall angle θSH of the bulk Cr layer in YIG/Cu/Cr. The huge spin-charge conversion efficiency at the YIG/Cr interface is attributed to the inverse Rashba–Edelstein effect. Our experimental results demonstrate the dominant role of the interfacial effect in the spin-charge conversion process of the YIG/Cr heterostructures.

Enhancement of the Spin Hall Angle by Interdiffusion of Atoms in Co2FeAl0.5Si0.5 / n - Ge Heterostructures

Realizing efficient spin-charge conversion is an important issue in spin-based electronics, especially for the development of low-power-consumption magnetization switching. A spin-charge conversion is generally induced in a nonmagnetic (NM) layer by the inverse spin Hall effect in the bulk or by the inverse Rashba-Edelstein effect at surfaces or interfaces. In the bulk, the spin-charge conversion efficiency is called the spin Hall angle, ${\ensuremath{\theta}}_{\mathrm{SHE}}$, and it is limited by the spin-orbit interaction of the bulk layer. Thus, artificially enhancing ${\ensuremath{\theta}}_{\mathrm{SHE}}$ to greater than its intrinsic value is difficult. Here, using spin-pumping measurements on a well-controlled interface between a ferromagnetic (FM) ${\mathrm{Co}}_{2}{\mathrm{Fe}\mathrm{Al}}_{0.5}{\mathrm{Si}}_{0.5}$ (CFAS) layer and a NM n-type $\mathrm{Ge}$ layer, we demonstrate that ${\ensuremath{\theta}}_{\mathrm{SHE}}$ can be strongly enhanced by annealing the sample and the resulting interdiffusion of atoms within only about 3.7 nm around the CFAS/n-$\mathrm{Ge}$ interface. Because of this annealing process, ${\ensuremath{\theta}}_{\mathrm{SHE}}$ is increased from 0.0058--0.0079 to 0.015--0.019, which is much larger than the intrinsic value of ${\ensuremath{\theta}}_{\mathrm{SHE}}$ reported for $\mathrm{Ge}$ (0.00096--0.002) and is comparable to ${\ensuremath{\theta}}_{\mathrm{SHE}}$ values reported for $\mathrm{Pt}$. This enhancement is attributed to strong scattering of the spin current in the intermixed layer formed at the interface by annealing. Our results indicate that ${\ensuremath{\theta}}_{\mathrm{SHE}}$ is strongly influenced by the atomic structure of the FM/NM interface, which suggests an efficient method to control spin-charge conversion by precise control of the FM/NM interface.

Unconventional Charge-Spin Conversion in Weyl-Semimetal WTe2.

An outstanding feature of topological quantum materials is their novel spin topology in the electronic band structures with an expected large charge-to-spin conversion efficiency. Here, a charge-current-induced spin polarization in the type-II Weyl semimetal candidate WTe2 and efficient spin injection and detection in a graphene channel up to room temperature are reported. Contrary to the conventional spin Hall and Rashba-Edelstein effects, the measurements indicate an unconventional charge-to-spin conversion in WTe2 , which is primarily forbidden by the crystal symmetry of the system. Such a large spin polarization can be possible in WTe2 due to a reduced crystal symmetry combined with its large spin Berry curvature, spin-orbit interaction with a novel spin-texture of the Fermi states. A robust and practical method is demonstrated for electrical creation and detection of such a spin polarization using both charge-to-spin conversion and its inverse phenomenon and utilized it for efficient spin injection and detection in the graphene channel up to room temperature. These findings open opportunities for utilizing topological Weyl materials as nonmagnetic spin sources in all-electrical van der Waals spintronic circuits and for low-power and high-performance nonvolatile spintronic technologies.

Gate tunability of highly efficient spin-to-charge conversion by spin Hall effect in graphene proximitized with WSe2

The proximity effect opens ways to transfer properties from one material into another and is especially important in two-dimensional (2D) materials. In van der Waals heterostructures, transition metal dichalcogenides (TMDs) can be used to enhance the spin–orbit coupling of graphene leading to the prediction of gate controllable spin-to-charge conversion (SCC). Here, we report for the first time and quantify the spin Hall effect (SHE) in graphene proximitized with WSe2 up to room temperature. Unlike in other graphene/TMD devices, the sole SCC mechanism is the SHE and no Rashba–Edelstein effect is observed. Importantly, we are able to control the SCC by applying a gate voltage. The SCC shows a high efficiency, measured with an unprecedented SCC length larger than 20 nm. These results show the capability of 2D materials to advance toward the implementation of novel spin-based devices and future applications.

Current direction dependent spin Hall magnetoresistance in epitaxial Pt/Co bilayers on MgO(110)

We demonstrate that the spin Hall magnetoresistance (SMR) of epitaxial Pt/Co bilayers on MgO(110) single crystal substrates is strongly anisotropic and depends on the applied current direction with respect to the two primary in-plane crystal directions [001] and $[1\overline{1}0]$, in the Pt layer. SMR results at different temperatures qualitatively suggest the D'yakonov-Perel spin relaxation mechanism through the invariance of the spin diffusion length ${\ensuremath{\lambda}}_{sf}$ for a given current direction, while also suggesting an anisotropic Rashba-Edelstein effect from the ratio of ${\ensuremath{\lambda}}_{sf}^{[001]}$ to ${\ensuremath{\lambda}}_{sf}^{[1\overline{1}0]}$. Finally, deviation from the standard SMR model spurs the need for new theory that properly characterizes and quantifies the anisotropies in such epitaxial systems.

Disentanglement of Spin-Orbit Torques in Pt / Co Bilayers with the Presence of Spin Hall Effect and Rashba-Edelstein Effect

We demonstrate a quantitative disentanglement of current-induced spin-orbit torques (SOTs) in $\mathrm{Pt}$/$\mathrm{Co}$ bilayers, where both the spin Hall effect (SHE) and the Rashba-Edelstein effect (REE) are present. The SOTs originating from the SHE in bulk $\mathrm{Pt}$ and from the REE at substrate (sub.)/$\mathrm{Pt}$ and $\mathrm{Pt}$/$\mathrm{Co}$ interfaces are successfully disentangled and quantified utilizing harmonic Hall measurements, by taking advantage of different characteristic lengths between the two effects during spin-current transport. The fieldlike (FL) torque in our samples originates from the REE, while both SHE and REE contribute to the dampinglike (DL) torque. The experimentally extracted transport characteristic lengths in bulk $\mathrm{Pt}$ (an effective spin-diffusion length) are 1.8--3.0 nm, while those at $\mathrm{Pt}$ interfaces (an effective REE thickness) are 0.2--0.6 nm. The extracted REE-induced FL torque (and DL torque) efficiencies at sub./$\mathrm{Pt}$ and $\mathrm{Pt}$/$\mathrm{Co}$ interfaces are of the same order of magnitude, but of opposite signs, which is consistent with the REE scenario. The origin of the observed temperature-dependent SOT sign reversal is clarified with our disentanglement analysis, demonstrating the significant role of the REE at both sub./$\mathrm{Pt}$ and $\mathrm{Pt}$/$\mathrm{Co}$ interfaces, in contributing to the overall SOTs, as the thickness of the heavy-metal layer is thinner than its spin-diffusion length or comparable to a REE thickness.

Magnetic asymmetry induced anomalous spin-orbit torque in IrMn

We demonstrate an anomalous spin-orbit torque induced by the broken magnetic symmetry in the antiferromagnet IrMn. We study the magnetic structure of three phases of IrMn thin films using neutron diffraction technique. The magnetic mirror symmetry M' is broken laterally in both L10-IrMn and L12-IrMn3 but not {\gamma}-IrMn3. We observe an out-of-plane damping-like spin-orbit torque in both L10-IrMn/permalloy and L12-IrMn3/permalloy bilayers but not in {\gamma}-IrMn3/permalloy. This is consistent with both the symmetry analysis on the effects of a broken M' on spin-orbit torque and the theoretical predictions of the spin Hall effect and the Rashba-Edelstein effect. In addition, the measured spin-orbit torque efficiencies are 0.61+-0.01, 1.01+-0.03 and 0.80+-0.01 for the L10, L12 and {\gamma} phases, respectively. Our work highlights the critical roles of the magnetic asymmetry in spin-orbit torque generation.

Surface Rashba-Edelstein Spin-Orbit Torque Revealed by Molecular Self-Assembly

We report the observation of a spin-orbit torque (SOT) originating from the surface Rashba-Edelstein effect. We found that the SOT in a prototypical spin-orbitronic system, a Pt/Co bilayer, can be manipulated by molecular self-assembly on the Pt surface. This evidences that the Rashba spin-orbit coupling at the Pt surface generates a sizable SOT, which has been hidden by the strong bulk and interface spin-orbit coupling. We show that the molecular tuning of the surface Rashba-Edelstein SOT is consistent with density functional theory calculations. These results illustrate the crucial role of the surface spin-orbit coupling in the SOT generation, which alters the landscape of metallic spin-orbitronic devices.

Spin-Orbit Torque in a Perpendicularly Magnetized Ferrimagnetic Tb - Co Single Layer

Spin-orbit torque (SOT) has been studied extensively in a heavy-metal (HM)/ferromagnet (FM) bilayer structure, where a HM is an essential ingredient because a spin current is generated via the spin Hall effect within the HM layer and/or the Rashba-Edelstein effect from the HM/FM interface. Here, we report the observation of SOT in a ferrimagnetic $\mathrm{Tb}\text{\ensuremath{-}}\mathrm{Co}$ single layer with perpendicular magnetic anisotropy without a HM layer. Using harmonic Hall voltage measurements, we investigate the SOT-induced dampinglike effective field $({B}_{\mathrm{DL}})$ in a $\mathrm{Tb}\text{\ensuremath{-}}\mathrm{Co}$ layer; the sign of ${B}_{\mathrm{DL}}$ is opposite from that of a $\mathrm{Pt}/\mathrm{Tb}\text{\ensuremath{-}}\mathrm{Co}$ bilayer, and the magnitude of ${B}_{\mathrm{DL}}$ increases as the $\mathrm{Tb}\text{\ensuremath{-}}\mathrm{Co}$ composition approaches its magnetization compensation point. Moreover, we analyze the elemental composition of $\mathrm{Tb}\text{\ensuremath{-}}\mathrm{Co}$ as a function of film thickness using scanning transmission electron microscopy and electron energy-loss spectroscopy, indicating that the sign and magnitude of the SOT are virtually insensitive to the vertical composition gradient within the $\mathrm{Tb}\text{\ensuremath{-}}\mathrm{Co}$ layer. Our results demonstrate that the bulk spin-orbit interaction within the $\mathrm{Tb}\text{\ensuremath{-}}\mathrm{Co}$ layer itself plays a major role in generating SOT in a $\mathrm{Tb}\text{\ensuremath{-}}\mathrm{Co}$ single layer.

Gate-Tunable Reversible Rashba-Edelstein Effect in a Few-Layer Graphene/2H-TaS2 Heterostructure at Room Temperature.

We report the observation of current-induced spin polarization, the Rashba-Edelstein effect (REE), and its Onsager reciprocal phenomenon, the spin galvanic effect (SGE), in a few-layer graphene/2H-TaS2 heterostructure at room temperature. Spin-sensitive electrical measurements unveil full spin-polarization reversal by an applied gate voltage. The observed gate-tunable charge-to-spin conversion is explained by the ideal work function mismatch between 2H-TaS2 and graphene, which allows for a strong interface-induced Bychkov-Rashba interaction with a spin-gap reaching 70 meV, while keeping the Dirac nature of the spectrum intact across electron and hole sectors. The reversible electrical generation and control of the nonequilibrium spin polarization vector, not previously observed in a nonmagnetic material, are elegant manifestations of emergent two-dimensional Dirac Fermions with robust spin-helical structure. Our experimental findings, supported by first-principles relativistic electronic structure and transport calculations, demonstrate a route to design low-power spin-logic circuits from layered materials.

Anomalous Spin-Orbit Field via the Rashba-Edelstein Effect at theW/PtInterface

We have studied spin-orbit (SO) field in Ni$_{80}$Fe$_{20}$(Py)/W/Pt trilayer by means of spin-torque ferromagnetic resonance, and demonstrated that the W/Pt interface generates an extra SO field acting on the Py layer. This unprecedented field originates from the following three processes, 1) spin accumulation at W/Pt interface via the Rashba-Edelstein effect, 2) diffusive spin transport in the W layer, and 3) spin absorption into the Py layer through accumulation at the Py/W interface. Our result means that we can create extra SO field away from the ferromagnet/ metal interface and control its strength by a combination of two different metals.

Current-induced spin–orbit field in permalloy interfaced with ultrathin Ti and Cu

How spin–orbit torques emerge from materials with weak spin–orbit coupling (e.g., light metals) is an open question in spintronics. Here, we report on a field-like spin–orbit torque (i.e., in-plane spin–orbit field transverse to the current axis) in SiO2-sandwiched Permalloy (Py), with the top Py-SiO2 interface incorporating ultrathin Ti or Cu. In both SiO2/Py/Ti/SiO2 and SiO2/Py/Cu/SiO2, this spin–orbit field opposes the classical Oersted field. While the magnitude of the spin–orbit field is at least a factor of 3 greater than the Oersted field, we do not observe evidence for a significant damping-like torque in SiO2/Py/Ti/SiO2 or SiO2/Py/Cu/SiO2. Our findings point to contributions from a Rashba-Edelstein effect or spin–orbit precession at the (Ti, Cu)-inserted interface.

Observation of long spin lifetime in MAPbBr3 single crystals at room temperature

The emergence of hybrid metal halides (HMH) materials, such as the archetypal CH3NH3PbBr3, provides an appealing material platform for solution-processed spintronic applications due to properties such as unprecedented large Rashba spin-splitting states and highly efficient spin-to-charge (StC) conversion efficiencies. Here we report the first study of StC conversion and spin relaxation time in MAPbBr3 single crystals at room temperature using a spin pumping approach. Microwave frequency and power dependence of StC responses are both consistent with the spin pumping model, from which an inverse Rashba–Edelstein effect coherence length of up to ∼30 picometer is obtained, highlighting a good StC conversion efficiency. The magnetic field angular dependence of StC is investigated and can be well-explained by the spin precession model under oblique magnetic field. A long spin relaxation time of up to ∼190 picoseconds is obtained, which can be attributed to the surface Rashba state formed at the MAPbBr3 interface. Our oblique Hanle effect by FMR-driven spin pumping technique provides a reliable and sensitive tool for measuring the spin relaxation time in various solution processed HMH single crystals.

Orbitally dominated Rashba-Edelstein effect in noncentrosymmetric antiferromagnets

Efficient manipulation of magnetic order with electric current pulses is desirable for achieving fast spintronic devices. The Rashba-Edelstein effect, wherein spin polarization is electrically induced in noncentrosymmetric systems, provides a mean to achieve staggered spin-orbit torques. Initially predicted for spin, its orbital counterpart has been disregarded up to now. Here we report a generalized Rashba-Edelstein effect, which generates not only spin polarization but also orbital polarization, which we find to be far from being negligible. We show that the orbital Rashba-Edelstein effect does not require spin-orbit coupling to exist. We present first-principles calculations of the frequency-dependent spin and orbital Rashba-Edelstein tensors for the noncentrosymmetric antiferromagnets CuMnAs and Mn{}_{2}Au. We show that the electrically induced local magnetization can exhibit Rashba-like or Dresselhaus-like symmetries, depending on the magnetic configuration. We compute sizable induced magnetizations at optical frequencies, which suggest that electric-field driven switching could be achieved at much higher frequencies.

In-situ study of oxygen exposure effect on spin-orbit torque in Pt/Co bilayers in ultrahigh vacuum

Oxygen incorporation has been reported to increase the current-induced spin-orbit torque in ferromagnetic heterostructures, but the underlying mechanism is still under active debate. Here, we report on an in-situ study of the oxygen exposure effect on spin-orbit torque in Pt/Co bilayers via controlled oxygen exposure, Co and Mg deposition, and electrical measurements in ultrahigh vacuum. We show that the oxygen exposure on Pt/Co indeed leads to an increase of spin-orbit torque, but the enhancement is not as large as those reported previously. Similar enhancement of spin-orbit torque is also observed after the deposition of an MgO capping layer. The results of ab initio calculations on the Rashba splitting of Pt/Co and Pt/Co/O suggest that the enhancement is due to enhanced Rashba-Edelstein effect by surface-adsorbed oxygen. Our findings shed some light on the varying roles of oxygen in modifying the spin torque efficiency reported previously.

Surface-enhanced spin current to charge current conversion efficiency in CH3NH3PbBr3-based devices.

Hybrid organic-inorganic perovskites have shown great promise for spintronic applications due to their large spin-orbit coupling induced by the Pb and halogen atoms. Particularly, the large observed surface-induced Rashba splitting in CH3NH3PbBr3 indicates efficient spin-current-to-charge-current (StC) conversion, which, however, has not been demonstrated yet. In this work, the StC conversion efficiency in ferromagnet/CH3NH3PbBr3-based devices is studied using the pulsed spin-pumping technique measured by the inverse spin Hall effect. We found that the StC conversion efficiency is anomalous in that it increases at small perovskite layer thickness. This indicates the existence of a surface-dominated StC mechanism such as the inverse Rashba-Edelstein effect. By inserting a thin LiF layer between the ferromagnet and the perovskite film, the StC conversion efficiency is greatly suppressed, validating the existence of a Rashba surface in the CH3NH3PbBr3 film.