Inverse Edelstein Effect Research Articles

Non-volatile Fermi level tuning for the control of spin-charge conversion at room temperature

Current silicon-based CMOS devices face physical limitations in downscaling size and power loss, restricting their capability to meet the demands for data storage and information processing of emerging technologies. One possible alternative is to encode the information in a non-volatile magnetic state and manipulate this spin state electronically, as in spintronics. However, current spintronic devices rely on the current-driven control of magnetization, which involves Joule heating and power dissipation. This limitation has motivated intense research into the voltage-driven manipulation of spin signals to achieve energy-efficient device operation. Here, we show non-volatile control of spin-charge conversion at room temperature in graphene-based heterostructures through Fermi level tuning. We use a polymeric ferroelectric film to induce non-volatile charging in graphene. To demonstrate the switching of spin-to-charge conversion we perform ferromagnetic resonance and inverse Edelstein effect experiments. The sign change of output voltage is derived by the change of carrier type, which can be achieved solely by a voltage pulse. Our results provide an alternative approach for the electric-field control of spin-charge conversion, which constitutes a building block for the next generation of spin-orbitronic memory and logic devices.

Giant Spin‐Charge Conversion in Ultrathin Films of the MnPtSb Half‐Heusler Compound

AbstractHalf‐metallic half‐Heusler compounds with strong spin‐orbit‐coupling and broken inversion symmetry in their crystal structure are promising materials for generating and absorbing spin‐currents, thus enabling the electric manipulation of magnetization in energy‐efficient spintronic devices. In this work, the spin‐to‐charge conversion in the sputtered half‐Heusler MnPtSb within thickness (t) range from 1 to 6 nm is reported. A combination of X‐ray and transmission electron microscopy measurements evidence the epitaxial nature of these ultrathin non‐centrosymmetric layers, with a clear (111)‐orientation on top of (0001) single‐crystal sapphire. By broadband ferromagnetic resonance (FMR), a four orders of magnitude tunable spin accumulation in the MnPtSb‐based heterostructures, within t = 1–6 nm range, is observed. By using spin pumping FMR, a remarkable t‐dependent spin‐charge conversion in the MnPtSb layers is measured, which clearly demonstrates the interfacial origin of the conversion. When interpreted within the inverse Edelstein effect (IEE), the spin‐charge conversion efficiency extracted at room temperature for the thinnest MnPtSb layer reaches λIEE≈3 nm, representing an extremely high conversion. The still never explored ultrathin regime of the MnPtSb films studied in this work and the discovery of their outstanding functionality are two ingredients that demonstrate the potentiality of such materials for future applications in spintronics.

Large Tunable Spin-to-Charge Conversion in Ni80Fe20/Molybdenum Disulfide by Cu Insertion.

Spin-to-charge conversion at the interface between magnetic materials and transition metal dichalcogenides has drawn great interest in the research efforts to develop fast and ultralow power consumption devices for spintronic applications. Here, we report room temperature observations of spin-to-charge conversion arising from the interface of Ni80Fe20 (Py) and molybdenum disulfide (MoS2). This phenomenon can be characterized by the inverse Edelstein effect length (λIEE), which is enhanced with decreasing MoS2 thicknesses, demonstrating the dominant role of spin-orbital coupling (SOC) in MoS2. The spin-to-charge conversion can be significantly improved by inserting a Cu interlayer between Py and MoS2, suggesting that the Cu interlayer can prevent magnetic proximity effect from the Py layer and protect the SOC on the MoS2 surface from exchange interactions with Py. Furthermore, the Cu-MoS2 interface can enhance the spin current and improve electronic transport. Our results suggest that tailoring the interface of magnetic heterostructures provides an alternative strategy for the development of spintronic devices to achieve higher spin-to-charge conversion efficiencies.

Temperature-Dependent Spin-to-Charge Conversion and Efficient Manipulation of Elliptical THz Waves in Bi2Te3/TbFeCo Heterostructures.

Topological insulators (TIs) with spin-momentum-locked surface states and considerable spin-to-charge conversion (SCC) efficiency are ideal substitutes for the nonmagnetic layer in the traditional ferromagnetic/nonmagnetic (FM/NM) spintronic terahertz (THz) emitters. Here, the TI/ferrimagnetic structure as an effective polarization tunable THz source is verified by terahertz emission spectroscopy. The emitted THz electric field can be separated into two THz components utilizing their opposite symmetry on pump polarization and the magnetic field. TI not only emits a THz electric field via the linear photogalvanic effect (LPGE) but also serves as the medium of SCC via the inverse Edelstein effect (IEE) in the heterostructure. In addition, the amplitude and polarity of the SCC component can be efficiently manipulated by temperature in our ferrimagnetic TbFeCo layer compared with Co or Fe. Once these two THz components are delicately set orthogonally, an elliptical THz wave is generated by the intrinsic phase difference at the THz frequency range. The feasible control of its polarization and chirality is demonstrated by three means: pump polarization, magnetic field, and temperature. These appealing observations may pave the way for the development of elliptical THz wave emitters and polarization-sensitive THz spectroscopy.

Enhancement of spin to charge conversion efficiency at the topological surface state by inserting normal metal spacer layer in the topological insulator based heterostructure

In this study, we report efficient spin to charge conversion (SCC) in the topological insulator (TI) based heterostructure (BiSbTe1.5Se1.5/Cu/Ni80Fe20) by using spin-pumping technique, where BiSbTe1.5Se1.5 is the TI and Ni80Fe20 is the ferromagnetic (FM) layer. The SCC, characterized by inverse Edelstein effect length (λIEE) in the TI material, gets altered with an intervening Copper (Cu) layer, and it depends on the interlayer thickness. The introduction of Cu layer at the interface of TI and FM metal provides a new degree of freedom for tuning the SCC efficiency of the topological surface states. The significant enhancement of the measured spin-pumping voltage and the increased linewidth of ferromagnetic resonance absorption spectra due to the insertion of Cu layer at the interface indicate a reduction in spin memory loss at the interface that resulted from the presence of exchange coupling between the surface states of TI and the local moments of FM metal. The temperature dependence (from 8 to 300 K) of the evaluated λIEE data for all the trilayer systems, TI/Cu/FM with different Cu thicknesses, confirms the effect of exchange coupling between the TI and FM layer on the SCC efficiency of the topological surface state. This study offers promising ways for designing more efficient spin-charge conversion devices of the TI-based heterostructure by controlling the interface effects.

Exploiting the Close-to-Dirac Point Shift of the Fermi Level in the Sb2Te3/Bi2Te3 Topological Insulator Heterostructure for Spin-Charge Conversion.

Properly tuning the Fermi level position in topological insulators is of vital importance to tailor their spin-polarized electronic transport and to improve the efficiency of any functional device based on them. Here, we report the full in situ metal organic chemical vapor deposition (MOCVD) and study of a highly crystalline Bi2Te3/Sb2Te3 topological insulator heterostructure on top of large area (4″) Si(111) substrates. The bottom Sb2Te3 layer serves as an ideal seed layer for the growth of highly crystalline Bi2Te3 on top, also inducing a remarkable shift of the Fermi level to place it very close to the Dirac point, as visualized by angle-resolved photoemission spectroscopy. To exploit such ideal topologically protected surface states, we fabricate the simple spin-charge converter Si(111)/Sb2Te3/Bi2Te3/Au/Co/Au and probe the spin-charge conversion (SCC) by spin pumping ferromagnetic resonance. A large SCC is measured at room temperature and is interpreted within the inverse Edelstein effect, thus resulting in a conversion efficiency of λIEEE ∼ 0.44 nm. Our results demonstrate the successful tuning of the surface Fermi level of Bi2Te3 when grown on top of Sb2Te3 with a full in situ MOCVD process, which is highly interesting in view of its future technology transfer.

Spin-to-charge conversion origin in graphene on ferromagnetic substrate revealed: Rashba effects at Dirac point

Spin-to-charge current conversion (SCC) by inverse Rashba Edelstein effect (IREE) has been recently observed by spin-pumping experiment in single layer graphene (SLG) placed on ferromagnetic substrate composed mostly of Ni. However, a prior angle-resolved photoemission spectroscopy (ARPES) shows a negligible Rashba splitting at Γ−M, unsupportive of SCC by IREE. In this work, using density functional theory with spin-orbit coupling (DFT + SOC) calculations, we obtained the Rashba-type spin-resolved bandstructure on relevant symmetric k-points of SLG on Ni(111). We found no Rashba splitting at Γ−M confirming ARPES, however, we discovered considerable Rashba splittings in the dispersions along M−K−M, that is, involving the Dirac point, paving the way for SCC by IREE. Thus, for the first time, this work has reconciled the above seemingly contradicting spin-pumping and ARPES experiments, revealed the physical and magnetic origin of SCC by IREE and proposed means to increase the SCC coefficient without the use of heavy precious metals.

All‐Electrical Detection of the Spin‐Charge Conversion in Nanodevices Based on SrTiO3 2‐D Electron Gases

AbstractThe Magnetoelectric Spin‐Orbit (MESO) technology aims to bring logic into memory by combining a ferromagnet with a magnetoelectric (ME) element for information writing, and a spin‐orbit (SO) element for information read‐out through spin‐charge conversion. Among candidate SO materials to achieve a large MESO output signal, oxide Rashba two‐dimensional electron gases (2DEGs) have shown very large spin‐charge conversion efficiencies, albeit mostly in spin‐pumping experiments. Here, all‐electrical spin‐injection and spin‐charge conversion experiments in nanoscale devices harnessing the inverse Edelstein effect of SrTiO3 2DEGs are reported. Nanodevices aredesigned, patterned, and fabricated in which a spin current injected from a cobalt layer into the 2DEG is converted into a charge current. The spin‐charge conversion signal is optimized by applying back‐gate voltages and studied its temperature evolution. It further disentangles the inverse Edelstein contribution from spurious effects such as the planar Hall effect, the anomalous Hall effect, or the anisotropic magnetoresistance. The combination of non‐volatility and high energy efficiency of these devices can potentially lead to new technology paradigms for beyond‐CMOS computing architectures.

Room Temperature Spin-to-Charge Conversion in Amorphous Topological Insulating Gd-Alloyed BixSe1-x/CoFeB Bilayers.

Disordered topological insulator (TI) films have gained intense interest by benefiting from both the TI's exotic transport properties and the advantage of mass production by sputtering. Here, we report on the clear evidence of spin-charge conversion (SCC) in amorphous Gd-alloyed BixSe1-x (BSG)/CoFeB bilayers fabricated by sputtering, which could be related to the amorphous TI surface states. Two methods have been employed to study SCC in BSG (tBSG = 6-16 nm)/CoFeB(5 nm) bilayers with different BSG thicknesses. First, spin pumping is used to generate a spin current in CoFeB and detect SCC by the inverse Edelstein effect (IEE). The maximum SCC efficiency (SCE) is measured to be as large as 0.035 nm (IEE length λIEE) in a 6 nm thick BSG sample, which shows a strong decay when tBSG increases due to the increase of BSG surface roughness. The second method is THz time-domain spectroscopy, which reveals a small tBSG dependence of SCE, validating the occurrence of a pure interface state-related SCC. Furthermore, our angle-resolved photoemission spectroscopy data show dispersive two-dimensional surface states that cross the bulk gap until the Fermi level, strengthening the possibility of SCC due to the amorphous TI states. Our studies provide a new experimental direction toward the search for topological systems in amorphous solids.

Pump wavelength-dependent terahertz spin-to-charge conversion in CoFeB/MgO Rashba interface

Spin/charge interconversion mechanisms provide an essential handle to generate and detect spin currents. Their applications at different timescales are critical in spintronics since they cover a technologically relevant broadband spectrum. While the inverse spin Hall effect is known to be robust from quasi-static to sub-picosecond timescales, the conversion efficiency evolution of the inverse Edelstein effect has not been addressed yet. In this work, we report that while the quasi-static response of the inverse Edelstein effect can be comparable to that of the most efficient inverse spin Hall systems, a drastic drop of efficiency is observed in the terahertz (THz) regime. This behavior at the sub-picosecond timescale is qualitatively understood from the dependence of the inverse Edelstein effect on the energy distribution of spin-carrier entities, which is different between thermalized carriers in the quasi-static regime and hot carriers generated by light pulses. This finding is supported by the pump-laser wavelength dependence in the THz regime for the inverse Edelstein effect, which offers a promising route for tunability of spintronic devices.

Effect of Cu Intercalation Layer on the Enhancement of Spin-to-Charge Conversion in Py/Cu/Bi2Se3.

The spin-to-charge conversion in Permalloy (Py)/Cu/Bi2Se3 is tunable by changing the Cu layer thickness. The conversion rate was studied using the spin pumping technique. The inverse Edelstein effect (IEE) length λIEE is found to increase up to ~2.7 nm when a 7 nm Cu layer is introduced. Interestingly, the maximized λIEE is obtained when the effective spin-mixing conductance (and thus Js) is decreased due to Cu insertion. The monotonic increase in λIEE with decreasing Js suggests that the IEE relaxation time (τ) is enhanced due to the additional tunnelling barrier (Cu layer) that limits the interfacial transmission rate. The results demonstrate the importance of interface engineering in the magnetic heterostructure of Py/topological insulators (TIs), the key factor in optimizing spin-to-charge conversion efficiency.

Gate modulation of the spin current in graphene/WSe2 van der Waals heterostructure at room temperature

Two-dimensional transition metal dichalcogenide semiconductors owning a large intrinsic spin-orbit coupling (SOC) are considered the best candidates to generate, detect and manipulate the spin currents. The SOC defines the interconversion of spin and charge currents via Rashba Edelstein effect (spin Hall effect) and its reciprocal as inverse Rashba Edelstein effect (inverse spin Hall effect). However, the spin signal originated in low dimensional materials because of Rashba Edelstein effect or spin Hall effect yet needed to be addressed with distinguishable measurement technique. Here, we demonstrate experimentally the room temperature interconversion of spin and charge currents in Graphene/WSe 2 van der Waals heterostructure which is induced by proximity effect. Remarkably, the spin currents induced by Rashba Edelstein effect and spin Hall effect are discriminated and extracted individually via external magnetic field, respectively. The magnitude of spin transport and their corresponding spin efficiencies ( α RE E = 1.47 ± 0.03 % ) and (θ SHE = = 4.5 ± 0.07 % ) are modulated via applied electric field and temperature. Such electric, and magnetic field tunability of the spin transport through the non-magnetic materials may provide a new approach to fabricate the fast and low-power spintronic devices for quantum scale applications. • Experimental demonstration of interconversion of spin and charge currents in Graphene/WSe 2 van der Waals heterostructure, induced by proximity of WSe 2 , at room temperature. • Remarkably, the spin currents induced by Rashba Edelstein effect and spin Hall effect are discriminated and their corresponding spin efficiencies are extracted individually via external magnetic field, respectively.

Voltage Signals Caused by Surface Acoustic Wave Driven Ferromagnetic Resonance Under Out‐of‐Plane External Fields

AbstractUnder an external magnetic field, surface acoustic waves (SAWs) propagating onto a ferromagnetic thin film can excite ferromagnetic resonance (FMR). The magnetization precessional motion in resonance pumps spin current into the Rashba interface across an adjacent non‐magnetic layer, which can be converted to charge current via the inverse Edelstein effect (IEE). Here, the SAW‐driven FMR and the IEE voltage signals under in‐ and out‐of‐plane external magnetic fields are reported. When the external magnetic field has only an in‐plane component, the SAW‐driven FMR for positive and negative resonant fields have the same sign (even), while the spin‐conversion due to IEE results in voltage signals with opposite signs for positive and negative fields (odd). However, when the out‐of‐plane component of the external magnetic field increases, the acoustic ferromagnetic resonance signal remains even. In contrast, the IEE voltage signal exhibits a sum of even and odd signal contributions. It is discussed that the appearance of the even contribution to the voltage signal does not correspond to well‐known spin rectification mechanisms. The result may constitute a novel resonant spin rectification mechanism under SAW excitations.

Elastic and Inelastic Spin Transport in SrTiO3‐based Magnetic Heterostructure

AbstractWith recent developments in the field of spin‐orbitronics, two‐dimensional electron gases (2DEGs) with Rashba‐type spin‐orbit coupling have emerged as an important component of spin‐based devices. One of the promising platforms is the 2DEG in SrTiO3‐based structures, which has been the fundamental building block of oxide electronics. The 2DEG formed due to the redox reactions at metal/SrTiO3 interfaces has been incorporated into magnetic heterostructures, demonstrating efficient spin‐to‐charge conversion through the inverse Edelstein effect due to the sizable Rashba spin‐orbit coupling. However, evidence for the charge‐to‐spin conversion, a technologically more important process, has been lacking, and furthermore, the spin transport mechanism in the magnetic heterostructure has been unclear. Here, the charge‐to‐spin conversion induced by the direct Edelstein effect in the 2DEG at an Al/SrTiO3 interface is demonstrated. The effective charge‐to‐spin conversion efficiency of the 2DEG exceeds 10% at room temperature, which is comparable to that of the archetypal spin‐to‐charge converter, Pt. The high effective charge‐to‐spin conversion efficiency is found to be suppressed by decreasing temperature, demonstrating a crossover of the dominant spin transport mechanism from the inelastic tunneling to elastic tunneling induced by decreasing temperature. These findings will provide a clue to unlock the full potential of oxide‐based spin‐orbitronic devices.

KTaO3 -The New Kid on the Spintronics Block.

Long after the heady days of high-temperature superconductivity, the oxides came back into the limelight in 2004 with the discovery of the 2D electron gas (2DEG) in SrTiO3 (STO) and several heterostructures based on it. Not only do these materials exhibit interesting physics, but they have also opened up new vistas in oxide electronics and spintronics. However, much of the attention has recently shifted to KTaO3 (KTO), a material with all the "good" properties of STO (simple cubic structure, high mobility, etc.) but with the additional advantage of a much larger spin-orbit coupling. In this state-of-the-art review of the fascinating world of KTO, it is attempted to cover the remarkable progress made, particularly in the last five years. Certain unsolved issues are also indicated, while suggesting future research directions as well as potential applications. The range of physical phenomena associated with the 2DEG trapped at the interfaces of KTO-based heterostructures include spin polarization, superconductivity, quantum oscillations in the magnetoresistance, spin-polarized electron transport, persistent photocurrent, Rashba effect, topological Hall effect, and inverse Edelstein Effect. It is aimed to discuss, on a single platform, the various fabrication techniques, the exciting physical properties and future application possibilities of this family of materials.

Generation and Control of Terahertz Spin Currents in Topology-Induced 2D Ferromagnetic Fe3 GeTe2 |Bi2 Te3 Heterostructures.

Future information technologies for low-dissipation quantum computation, high-speed storage, and on-chip communication applications require the development of atomically thin, ultracompact, and ultrafast spintronic devices in which information is encoded, stored, and processed using electron spin. Exploring low-dimensional magnetic materials, designing novel heterostructures, and generating and controlling ultrafast electron spin in 2D magnetism at room temperature, preferably in the unprecedented terahertz (THz) regime, is in high demand. Using THz emission spectroscopy driven by femtosecond laser pulses, optical THz spin-current bursts at room temperature in the 2D van der Waals ferromagnetic Fe3 GeTe2 (FGT) integrated with Bi2 Te3 as a topological insulator are successfully realized. The symmetry of the THz radiation is effectively controlled by the optical pumping incidence and external magnetic field directions, indicating that the THz generation mechanism is the inverse Edelstein effect contributed spin-to-charge conversion. Thickness-, temperature-, and structure-dependent nontrivial THz transients reveal that topology-enhanced interlayer exchange coupling increases the FGT Curie temperature to room temperature, which provides an effective approach for engineering THz spin-current pulses. These results contribute to the goal of all-optical generation, manipulation, and detection of ultrafast THz spin currents in room-temperature 2D magnetism, accelerating the development of atomically thin high-speed spintronic devices.

Theoretical analysis of the inverse Edelstein effect at the LaAlO3/SrTiO3 interface with an effective tight-binding model: important role of the second d xy subband

The two-dimensional electron gas formed at interfaces between SrTiO3 and other materials has attracted much attention since extremely efficient spin-to-charge current conversion has been recently observed at these interfaces. This has been attributed to their complicated quantized multi-orbital structures with a topological feature. However, there are few reports quantitatively comparing the conversion efficiency values between experiments and theoretical calculations at these interfaces. In this study, we theoretically explain the experimental temperature dependence of the spin-to-charge current conversion efficiency using an 8 × 8 effective tight-binding model considering the second d xy subband, revealing the vital role of the quantization of the multi-band structure.

Conversion between spin and charge currents in topological-insulator/nonmagnetic-metal systems

The charge current in a topological insulator (TI) will induce a spin accumulation (Edelstein effect or EE), from which the spin current will be generated. Inversely, the spin current injection into the TI will induce a charge current called the inverse Edelstein effect (IEE). Some experimental and theoretical works have been done for the understanding of either EE or IEE. However, little experimental work incorporating both processes in the same TI sample has been done. In this work, we propose a phenomenological model to understand EE and IEE in the TI-based system. Based on this model, efficiencies of EE and IEE can be directly derived, which is consistent with previous theoretical work based on Boltzmann transport theory and obeying the energy conservation law. We also measure EE and IEE efficiencies experimentally in a TI/Ru/CoFeB system by spin-torque ferromagnetic resonance and spin pumping, respectively. The experimental results are consistent with our model, which proves that the spin-charge conversion in TI can be understood in the framework of (I)EE instead of (inverse) spin Hall effect. By combining theories and experiments, we find that enhancing interfacial transparency is crucial for enhancing EE efficiency, and avoiding metallic contact is crucial for enhancing IEE efficiency.

Irreversible thermodynamics at surfaces: inverse Edelstein effect for topological insulator

We employ Onsager’s irreversible thermodynamics to study the inverse Edelstein effect (IEE) for a non-magnetic material (NM) adjacent to a topological insulator (TI) with a strong spin–orbit interaction. The TI surface state region is treated as quasi two-dimensional (2D). For the IEE, the source is a 3D spin flux incident from the NM that converts, at the NM–TI interface, to a quasi-2D charge current in the TI. For the Edelstein effect (EE), the source is a quasi-2D charge flux incident from the TI that converts, at the interface, to a three-dimensional (3D) spin flux in the NM. For strong spin–orbit coupling, as considered here, when the 3D spin flux crosses to the 2D TI, the quasi-2D charge current is produced along with a quasi-2D spin accumulation. (For weak spin–orbit coupling, production of charge current and of spin accumulation are distinct processes.) We compute the associated rates of heating.

Spin‐Charge Conversion in Fe/Au/Sb2Te3 Heterostructures as Probed By Spin Pumping Ferromagnetic Resonance

AbstractLarge‐area antimony telluride (Sb2Te3) thin films are grown by a metal organic chemical vapor deposition technique on 4” Si(111) substrates, and their topological character probed by magnetoconductance measurements. When interfaced with Fe thin films, broadband ferromagnetic resonance spectroscopy (BFMR) shows a clear increase of the damping parameter in Fe/Sb2Te3 when compared to a reference Fe layer, which may suggest the occurrence of spin pumping (SP) into Sb2Te3. Simultaneously, X‐ray reflectivity and conversion electron Mossbauer spectroscopy evidence the development of a chemically and magnetically pure Fe/Sb2Te3 interface. However, by conducting SP‐FMR, it is shown that no spin‐to‐charge conversion (S2C) occurs in Fe/Sb2Te3, while a clear SP signal develops by introducing a 5 nm Au interlayer between Fe and Sb2Te3, with a measured inverse Edelstein effect conversion efficiency of λIEE = 0.27 nm. The results shed some light on the correlation among the chemical‐structural‐magnetic properties of the Fe/Sb2Te3 interface, the broadening of the magnetic damping parameter as detected by BFMR, and the occurrence of S2C, as probed by SP‐FMR.