Spin-charge Conversion Research Articles

Large damping-like spin–orbit torque and perpendicular magnetization switching in sputtered WTex films

We report on the study of charge-spin conversion in sputter-deposited WTex films using harmonic Hall measurements. Strong damping-like (DL) spin–orbit torque (SOT) is observed in WTex thin films with the largest SOT efficiency of 0.67 × 105 Ω−1 m−1 at a WTex thickness of 5 nm, which quickly decreases to around zero as the thickness increases to 10 nm. In comparison, the field-like SOT is around one order smaller than the DL SOT. Making use of the large DL SOT, we realize efficient current-induced perpendicular magnetization switching in WTex/Ti/CoFeB multilayers. Our findings suggest the great potential of sputter-deposited WTex in spintronics applications.

Opposite current-induced spin polarizations in bulk-metallic Bi2Se3 and bulk-insulating Bi2Te2Se topological insulator thin flakes

One of the most fundamental and exotic properties of three-dimensional (3D) topological insulators (TIs) is spin-momentum locking (SML) of their topological surface states (TSSs), promising for potential applications in future spintronics. However, other possible conduction channels, such as a trivial two-dimensional electron gas (2DEG) with strong Rashba-type spin-orbit interaction (SOI) and bulk-conducting states that may possess a spin Hall effect (SHE), can coexist in 3D TIs, making determining the origin of the current-induced spin polarization (CISP) difficult. In this work, we directly compared the CISP between bulk-insulating ${\mathrm{Bi}}_{2}{\mathrm{Te}}_{2}\mathrm{Se}$ (BTS221) and bulk-metallic ${\mathrm{Bi}}_{2}{\mathrm{Se}}_{3}$ thin flakes using spin potentiometry. In the bulk-insulating BTS221, the observed CISP has a sign consistent with the expected helicity of the SML of the TSS, but an opposite sign to its calculated bulk spin Hall conductivity. However, compared to BTS221, an opposite CISP is observed in the bulk-metallic ${\mathrm{Bi}}_{2}{\mathrm{Se}}_{3}$, consistent with both the expectations of its Rashba-Edelstein effect of the band-bending induced 2DEG and bulk intrinsic spin Hall Effect (SHE). If one assumes a representative occupation of the Rashba band of $3\ifmmode\times\else\texttimes\fi{}{10}^{13}\phantom{\rule{0.16em}{0ex}}\mathrm{c}{\mathrm{m}}^{\ensuremath{-}2}$ in ${\mathrm{Bi}}_{2}{\mathrm{Se}}_{3}$ with a relevant relaxation time of 100 fs, the contribution to the CISP could be more dominated by the bulk intrinsic SHE. Our results provide an electrical way to distinguish the TSS from other possible conducting channels in spin transport measurements on 3D TIs, and open ways for the potential applications in charge-spin conversion devices.

Quantitative estimation of thermoelectric contributions in spin pumping signals through microwave photoresistance measurements

Spin pumping is a technique widely used to generate pure spin current and characterize the spin-charge conversion efficiency of heavy metals. Upon microwave excitation, the sample may also be heated, and the parasitic thermoelectric signals could contaminate the spin pumping results. Owing to their identical angular dependences with respect to the magnetic field, it is difficult to isolate one from the other. In this paper, we present a quantitative method to separate thermoelectric contributions from spin pumping signals in both $\mathrm{Py}({\mathrm{Ni}}_{80}{\mathrm{Fe}}_{20})/\mathrm{Pt}$ and $\mathrm{YIG}({\mathrm{Y}}_{3}{\mathrm{Fe}}_{5}{\mathrm{O}}_{12})/\mathrm{Pt}$ bilayers through microwave photoresistance measurements. We find that the microwave absorption indeed can raise the temperature of samples, resulting a field-dependent thermoelectric hysteresis loop. However, the additional heat dissipation due to the resonant precession of the magnetization in the ferromagnet is negligibly small compared with the measured spin pumping signal. Thus, we conclude that the spin pumping signal is free of any detectable thermoelectric contributions.

Resilient and Secure Hardware Devices Using ASL

Due to the globalization of Integrated Circuit (IC) design in the semiconductor industry and the outsourcing of chip manufacturing, Third-Party Intellectual Properties (3PIPs) become vulnerable to IP piracy, reverse engineering, counterfeit IC, and hardware trojans. To thwart such attacks, ICs can be protected using logic encryption techniques. However, strong resilient techniques incur significant overheads. Side-channel attacks (SCAs) further complicate matters by introducing potential attacks post fabrication. One of the most severe SCAs is power analysis (PA) attacks, in which an attacker can observe the power variations of the device and analyze them to extract the secret key. PA attacks can be mitigated via adding large extra hardware; however, the overheads of such solutions can render them impractical, especially when there are power and area constraints. All Spin Logic Device (ASLD) is one of the most promising spintronic devices due to its unique properties: small area, no spin-charge signal conversion, zero leakage current, non-volatile memory, high density, low operating voltage, and its compatibility with conventional CMOS technology. In this article, we extend the work in Reference [1] on the usage of ASLD to produce secure and resilient circuits that withstand IC attacks (during the fabrication) and PA attacks (after fabrication), including reverse engineering attacks. First, we show that ASLD has another unique feature: identical power dissipation through the switching operations, where such properties can be effectively used to prevent PA and IC attacks. We then evaluate the proposed ASLD-based on performance overheads and security guarantees.

Skew-scattering-induced giant antidamping spin-orbit torques: Collinear and out-of-plane Edelstein effects at two-dimensional material/ferromagnet interfaces

Heavy metal/ferromagnet interfaces feature emergent spin-orbit effects absent in the bulk materials. Because of their inherent strong coupling between spin, charge, and orbital degrees of freedom, such systems provide a platform for technologically sought-after spin-orbit torques (SOTs). However, the microscopic origin of purely interfacial antidamping SOT, especially in the ultimate atomically thin limit, has proven elusive. Here, using two-dimensional (2D) van der Waals materials as a test bed for interfacial phenomena, we address this problem by means of a microscopic framework accounting for band structure effects and impurity scattering on equal footing and nonperturbatively. A number of unconventional and measurable effects are predicted, the most remarkable of which is a giant enhancement of antidamping SOT in the dilute disorder limit induced by a robust skew scattering mechanism, which is operative in realistic interfaces and does not require magnetic impurities. The newly unveiled skew scattering mechanism activates rich semiclassical spin-charge conversion effects that have gone unnoticed in the literature, including a collinear Edelstein effect with nonequilibrium spin polarization aligned with the direction of the applied current.Received 14 May 2020Revised 19 August 2020Accepted 25 November 2020DOI:https://doi.org/10.1103/PhysRevResearch.2.043401Published 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 AreasMagnetismMagnetoelectric effectRashba couplingSpin torqueSpin-orbit couplingSpintronicsPhysical Systems2-dimensional systemsBilayer filmsGrapheneTransition metal dichalcogenidesTechniquesBoltzmann theoryDiagrammatic methodsGreen's function methodsCondensed Matter, Materials & Applied Physics

Charge-spin conversion signal in WTe2 van der Waals hybrid devices with a geometrical design

The efficient generation and control of spin polarization via charge-spin conversion in topological semimetals are desirable for future spintronic and quantum technologies. Here, we report the charge-spin conversion (CSC) signals measured in a Weyl semimetal candidate WTe2-based hybrid graphene device with a geometrical design. Notably, the geometrical angle of WTe2 on the graphene spin-valve channel yields contributions to symmetric and anti-symmetric CSC signal components. The spin precession measurements of the CSC signal at different gate voltages and ferromagnet magnetization show the robustness of the CSC in WTe2 at room temperature. These results can be useful for the design of heterostructure devices and in the architectures of two-dimensional spintronic circuits.

Ultrafast spin-currents and charge conversion at 3d-5d interfaces probed by time-domain terahertz spectroscopy

Spintronic structures are extensively investigated for their spin–orbit torque properties, required for magnetic commutation functionalities. Current progress in these materials is dependent on the interface engineering for the optimization of spin transmission. Here, we advance the analysis of ultrafast spin-charge conversion phenomena at ferromagnetic-transition metal interfaces due to their inverse spin-Hall effect properties. In particular, the intrinsic inverse spin-Hall effect of Pt-based systems and extrinsic inverse spin-Hall effect of Au:W and Au:Ta in NiFe/Au:(W,Ta) bilayers are investigated. The spin-charge conversion is probed by complementary techniques—ultrafast THz time-domain spectroscopy in the dynamic regime for THz pulse emission and ferromagnetic resonance spin-pumping measurements in the GHz regime in the steady state—to determine the role played by the material properties, resistivities, spin transmission at metallic interfaces, and spin-flip rates. These measurements show the correspondence between the THz time-domain spectroscopy and ferromagnetic spin-pumping for the different set of samples in term of the spin mixing conductance. The latter quantity is a critical parameter, determining the strength of the THz emission from spintronic interfaces. This is further supported by ab initio calculations, simulations, and analysis of the spin-diffusion and spin-relaxation of carriers within the multilayers in the time domain, permitting one to determine the main trends and the role of spin transmission at interfaces. This work illustrates that time-domain spectroscopy for spin-based THz emission is a powerful technique to probe spin-dynamics at active spintronic interfaces and to extract key material properties for spin-charge conversion.

Highly Tunable Spin-Orbit Torque and Anisotropic Magnetoresistance in a Topological Insulator Thin Film Attached to Ferromagnetic Layer.

We investigate spin-charge conversion phenomena in hybrid structures of topological insulator thin films and magnetic insulators. We find an anisotropic inverse spin-galvanic effect that yields a highly tunable spin-orbit torque. Concentrating on the quasiballistic limit, we also predict a giant anisotropic magnetoresistance at low dopings. These effects, which have no counterparts in thick topological insulators, depend on the simultaneous presence of the hybridization between the surface states and the in-plane magnetization. Both the inverse spin-galvanic effect and anisotropic magnetoresistance exhibit a strong dependence on the magnetization and the Fermi level position and can be used for spintronics and spin-orbit-torque-based applications at the nanoscale.

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.

Quantification of interfacial spin-charge conversion in hybrid devices with a metal/insulator interface

We present and experimentally verify a universal theoretical framework for the description of spin-charge interconversion in non-magnetic metal/insulator structures with interfacial spin–orbit coupling (ISOC). Our formulation is based on drift-diffusion equations supplemented with generalized boundary conditions. The latter encode the effects of ISOC and relate the electronic transport in such systems to spin loss and spin-charge interconversion at the interface. We demonstrate that the conversion efficiency depends solely on these interfacial parameters. We apply our formalism to two typical spintronic devices that exploit ISOC: a lateral spin valve and a multilayer Hall bar, for which we calculate the non-local resistance and the spin Hall magnetoresistance, respectively. Finally, we perform measurements on these two devices with a BiOx/Cu interface and verify that transport properties related to the ISOC are quantified by the same set of interfacial parameters.

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.

Doping-induced spin Hall ratio enhancement in A15-phase, Ta-doped β-W thin films

As spintronic devices become more and more prevalent, the desire to find Pt-free materials with large spin Hall effects is increasing. Previously it was shown that β-W, the metastable A15 structured variant of pure W, has charge-spin conversion efficiencies on par with Pt, and it was predicted that β-W/Ta alloys should be even more efficient. Here we demonstrate the enhancement of the spin Hall ratio (SHR) in A15-phase β-W films doped with Ta (W4−xTax where x = 0.34 ± 0.06) deposited at room temperature using DC magnetron co-sputtering. In close agreement with theoretical predictions, we find that the SHR of the doped films was ∼9% larger than pure β-W films. We also found that the SHR’s in devices with Co2Fe6B2 were nearly twice as large as the SHR’s in devices with Co4Fe4B2. This work shows that by optimizing deposition parameters and substrates, the fabrication of the optimum W3Ta alloy should be feasible, opening the door to commercially viable, Pt-free, spintronic devices.

Charge-spin conversion in layered semimetal TaTe2 and spin injection in van der Waals heterostructures

A spin-polarized current source using nonmagnetic layered materials is promising for next-generation all-electrical spintronic science and technology. Here we electrically created spin polarization in a layered semimetal TaTe2 via the charge-spin conversion process. Using a hybrid device of TaTe2 in a van der Waals heterostructure with graphene, the spin polarization in TaTe2 is efficiently injected and detected by nonlocal spin-switch, Hanle spin precession, and inverse spin Hall effect measurements. Systematic experiments at different bias currents and gate voltages in a vertical geometry prove the TaTe2 as a nonmagnetic spin source at room temperature. These findings demonstrate the possibility of making an all-electrical spintronic device in a two-dimensional van der Waals heterostructure, which can be essential building blocks in energy-efficient spin-orbit technology.

Enhancement of spin-charge current interconversion by oxidation of rhenium

We have demonstrated enhancement of the spin-charge current interconversion by oxidation of rhenium (Re). Although amorphous rhenium oxide (ReOX) is electrically conductive, the spin–orbit torque efficiency measured by spin-torque ferromagnetic resonance is 0.074, and 37 times larger than the pure Re case whose amplitude is 0.002. Furthermore, we have also found that the values of the damping-like and field-like torque efficiencies are 0.030 and −0.052, respectively. These values are comparable with those for conventional spin–orbit materials such as Pt or Ta, suggesting that ReOX is a candidate material for spintronics devices. THz-time domain spectroscopy was performed to confirm the inverse spin-charge current conversion process in ReOX.

Large Damping-Like Spin-Orbit Torque in a 2D Conductive 1T-TaS2 Monolayer.

A damping-like spin-orbit torque (SOT) is a prerequisite for ultralow-power spin logic devices. Here, we report on the damping-like SOT in just one monolayer of the conducting transition-metal dichalcogenide (TMD) TaS2 interfaced with a NiFe (Py) ferromagnetic layer. The charge-spin conversion efficiency is found to be 0.25 ± 0.03 in TaS2(0.88)/Py(7), and the spin Hall conductivity is found to be superior to values reported for other TMDs. We also observed sizable field-like torque in this heterostructure. The origin of this large damping-like SOT can be found in the interfacial properties of the TaS2/Py heterostructure, and the experimental findings are complemented by the results from density functional theory calculations. It is envisioned that the interplay between interfacial spin–orbit coupling and crystal symmetry yielding large damping-like SOT. The dominance of damping-like torque demonstrated in our study provides a promising path for designing the next-generation conducting TMD-based low-powered quantum memory devices.

Gate-tunable spin-galvanic effect in graphene-topological insulator van der Waals heterostructures at room temperature

Unique electronic spin textures in topological states of matter are promising for emerging spin-orbit driven memory and logic technologies. However, there are several challenges related to the enhancement of their performance, electrical gate-tunability, interference from trivial bulk states, and heterostructure interfaces. We address these challenges by integrating two-dimensional graphene with a three-dimensional topological insulator (TI) in van der Waals heterostructures to take advantage of their remarkable spintronic properties and engineer proximity-induced spin-charge conversion phenomena. In these heterostructures, we experimentally demonstrate a gate-tunable spin-galvanic effect (SGE) at room temperature, allowing for efficient conversion of a non-equilibrium spin polarization into a transverse charge current. Systematic measurements of SGE in various device geometries via a spin switch, spin precession, and magnetization rotation experiments establish the robustness of spin-charge conversion in the Gr-TI heterostructures. Importantly, using a gate voltage, we reveal a strong electric field tunability of both amplitude and sign of the spin-galvanic signal. These findings provide an efficient route for realizing all-electrical and gate-tunable spin-orbit technology using TIs and graphene in heterostructures, which can enhance the performance and reduce power dissipation in spintronic circuits.

Giant momentum-dependent spin splitting in centrosymmetric low- Z antiferromagnets

The energy vs. crystal momentum E(k) diagram for a solid (band structure) constitutes the road map for navigating its optical, magnetic, and transport properties. By selecting crystals with specific atom types, composition and symmetries, one could design a target band structure and thus desired properties. A particularly attractive outcome would be to design energy bands that are split into spin components with a momentum-dependent splitting, as envisioned by Pekar and Rashba [Zh. Eksperim. i Teor. Fiz. 47 (1964)], enabling spintronic application. The current paper provides "design principles" for wavevector dependent spin splitting (SS) of energy bands that parallels the traditional Dresselhaus and Rashba spin-orbit coupling (SOC) - induce splitting, but originates from a fundamentally different source -- antiferromagnetism. We identify a few generic AFM prototypes with distinct SS patterns using magnetic symmetry design principles. These tools allow also the identification of specific AFM compounds with SS belonging to different prototypes. A specific compound -- centrosymmetric tetragonal MnF2 -- is used via density functional band structure calculations to quantitatively illustrate one type of AFM SS. Unlike the traditional SOC-induced effects restricted to non-centrosymmetric crystals, we show that antiferromagnetic-induced spin splitting broadens the playing field to include even centrosymmetric compounds, and gives SS comparable in magnitude to the best known ('giant') SOC effects, even without SOC, and consequently does not rely on the often-unstable high atomic number elements required for high SOC. We envision that use of the current design principles to identify an optimal antiferromagnet with spin-split energy bands would be beneficial for efficient spin-charge conversion and spin orbit torque applications without the burden of requiring compounds containing heavy elements.

A computational study of spin Hall effect device based on 2D materials

Efficient spin-charge conversion is indispensable in spintronic computing and memory technologies for achieving low power. Spin Hall and quantum spin Hall effects have been demonstrated in certain atomically thin two-dimensional (2D) materials. In this work, we develop a multiscale simulation method from atomistic quantum transport simulation to the circuit model for the spin Hall effect (SHE) device based on 2D materials. Numerical implementation to speed up atomistic transport simulations with the non-equilibrium Green's function formalism is described for the crossbar SHE device. The multiscale method can treat atomistic scale features and compute the spintronic device performance metrics of the modeled device. As an example, the effect of edge roughness on the SHE devices based on monolayer 2D materials is investigated. The results illustrate that aggressively scaled monolayer SHE devices can efficiently transduce charge to spin in the presence of edge roughness.

Induced spin textures at 3d transition metal–topological insulator interfaces

While some of the most elegant applications of topological insulators, such as quantum anomalous Hall effect, require the preservation of Dirac surface states in the presence of time-reversal symmetry breaking, other phenomena such as spin-charge conversion rather rely on the ability for these surface states to imprint their spin texture on adjacent magnetic layers. In this work, we investigate the spin-momentum locking of the surface states of a wide range of monolayer transition metals (3$d$-TM) deposited on top of Bi$_{2}$Se$_{3}$ topological insulators using first principles calculations. We find an anticorrelation between the magnetic moment of the 3$d$-TM and the magnitude of the spin-momentum locking {\em induced} by the Dirac surface states. While the magnetic moment is large in the first half of the 3$d$ series, following Hund's rule, the spin-momentum locking is maximum in the second half of the series. We explain this trend as arising from a compromise between intra-atomic magnetic exchange and covalent bonding between the 3$d$-TM overlayer and the Dirac surface states. As a result, while Cr and Mn overlayers can be used successfully for the observation of quantum anomalous Hall effect or the realization of axion insulators, Co and Ni are substantially more efficient for spin-charge conversion effects, e.g. spin-orbit torque and charge pumping.