Spin Pumping Technique Research Articles

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.

Surface-state mediated spin-to-charge conversion in Sb films via bilateral spin current injection

The spin-to-charge conversion phenomenon is investigated in a trilayer structure consisting of Co(12 nm)/Sb(t)/Py(12 nm), where the thickness t of the antimony layer is varied. Using the spin-pumping technique, a pure spin current is injected from both FM layers into the middle layer, the DC voltage is then measured. We observe a spin-to-charge mechanism in the Sb layer that exhibits striking similarities to the inverse Rashba–Edelstein effect (IREE), driven by surface states.

Unraveling the spin current flow in Bi layers

Although heavy metal materials, due to their large spin-orbit interaction, would be expected to act as an efficient converter of spin current to charge current, Bismuth does not obey this rule. Heterostructures of yttrium iron garnet (YIG)/Bi are investigated by means of the spin pumping technique and the results show that Bi exhibits negligible spin-charge conversion. In addition to not converting spin to charge, Bi exhibits an exotic spin relaxation mechanism, which is driven by its large diamagnetic response. By considering a nonlinear stochastic model of diffusing spin particles, the results show that the Bi layer acts as a binary classifier device that categorizes the pumped spins into relaxed spins and those that flow upwards and eventually reach the top layer of Pt. This shows that the investigation reported here opens different perspectives involving fundamental aspects in the fields of machine learning and condensed matter physics.

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.

Study of interfacial spin transport with an antiferromagnetic Cr2O3 interlayer

Interfacial spin transport has been studied in a $\mathrm{YIG}/{\mathrm{Cr}}_{2}{\mathrm{O}}_{3}/\mathrm{Pt}$ trilayer device via the spin-pumping technique. It is found that the linewidths and the magnitudes of the spin-pumping voltage signals suddenly decrease at just below the N\'eel temperature of the ${\mathrm{Cr}}_{2}{\mathrm{O}}_{3}$ interlayer. A detailed analysis shows that the inner decay in the ${\mathrm{Cr}}_{2}{\mathrm{O}}_{3}$ interlayer dominates the interfacial spin transport in a wide temperature range, while the interfacial suppression of pumped spin currents occurs at a temperature slightly lower than the N\'eel temperature, both of which contribute to the sudden changes of spin-pumping voltage signals around the N\'eel temperature. This work carries forward the understanding of the spin transport in an antiferromagnetic system, which would serve to inspire, as well as facilitate, further breakthroughs of the limits of spin-based devices.

Rashba-Edelstein magnetoresistance in two-dimensional materials at room temperature

We report the observation of the Rashba-Edelstein magnetoresistance (REMR) at room temperature in two two-dimensional (2D) materials, single layer graphene (SLG) and 2D semiconductor $\mathrm{Mo}{\mathrm{S}}_{2}$. The measurements were performed in heterostructures with yttrium iron garnet (YIG), namely, YIG/SLG and ${\text{YIG/MoS}}_{2}$, by means of the modulated magnetoresistance technique. We show that the action of the direct and inverse Rashba-Edelstein effects (REEs) in a YIG/2DM structure couples current-induced spin accumulation to the electric resistance, producing the REMR. The resistance change is similar to the one observed in spin Hall magnetoresistance experiments, but the origin of the change here lies in the 2D Rashba-Edelstein effects and not in the 3D spin Hall effects. The measured REE lengths for the two materials are in good agreement with the values obtained with the electric spin pumping technique.

Large spin to charge conversion in the topological superconductor β−PdBi2 at room temperature

\b{eta}-PdBi2 has attracted much attention for its prospective ability to possess simultaneously topological surface and superconducting states due to its unprecedented spin-orbit interaction (SOC). Whereas most works have focused solely on investigating its topological surface states, the coupling between spin and charge degrees of freedom in this class of quantum material remains unexplored. Here we first report a study of spin-to-charge conversion in a \b{eta}-PdBi2 ultrathin film grown by molecular beam epitaxy, utilizing a spin pumping technique to perform inverse spin Hall effect measurements. We find that the room temperature spin Hall angle of Fe/\b{eta}-PdBi2, {\theta}_SH=0.037. This value is one order of magnitude larger than that of reported conventional superconductors, and is comparable to that of the best SOC metals and topological insulators. Our results provide an avenue for developing superconductor-based spintronic applications.

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.

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.

Impact of oxygen interdiffusion on spin-to-charge conversion at nonmagnetic metal/Bi oxide interfaces

Spin-to-charge conversion at metal/oxide interfaces with stacked structures of permalloy/(Cu or Ag)/${\mathrm{Bi}}_{2}{\mathrm{O}}_{3}$ were systematically investigated by using the spin-pumping technique and cross-sectional transmission electron microscopy. Although the transport measurement reproduced the results in previous studies, the interfacial structure of (Cu or Ag)/${\mathrm{Bi}}_{2}{\mathrm{O}}_{3}$ was found to change depending on the crystalline orientation of the Cu or Ag underlayer. While the Ag/${\mathrm{Bi}}_{2}{\mathrm{O}}_{3}$ stacks with Ag(111) had steep interfaces, the formation of a nanometer-scale Cu-O layer can be found in the Cu/${\mathrm{Bi}}_{2}{\mathrm{O}}_{3}$ interface, which should be the main origin of the sign inversion of the conversion coefficients between Cu/${\mathrm{Bi}}_{2}{\mathrm{O}}_{3}$ and Ag/${\mathrm{Bi}}_{2}{\mathrm{O}}_{3}$. This study stresses the importance of nanostructure identification for discussing spin-to-charge conversions at metal/oxide interfaces.

Clear variation of spin splitting by changing electron distribution at non-magnetic metal/Bi2O3 interfaces

Large spin splitting at Rashba interface, giving rise to strong spin-momentum locking, is essential for efficient spin-to-charge conversion. Recently, a Cu/Bismuth oxide (Bi2O3) interface has been found to exhibit an efficient spin-to-charge conversion similar to a Ag/Bi interface with large Rashba spin splitting. However, the guiding principle of designing the metal/oxide interface for the efficient conversion has not been clarified yet. Here we report strong non-magnetic (NM) material dependence of spin splitting at NM/Bi2O3 interfaces. We employed spin pumping technique to inject spin current into the interface and evaluated the magnitude of interfacial spin-to-charge conversion. We observed large modulation and sign change in conversion coefficient which corresponds to the variation of spin splitting. Our experimental results together with first-principles calculations indicate that such large variation is caused by material dependent electron distribution near the interface. The results suggest that control of interfacial electron distribution by tuning the difference in work function across the interface may be an effective way to tune the magnitude and sign of spin-to-charge conversion and Rashba parameter at interface.

The ferromagnetic resonance in WxMo1−x/YIG heterostructures

The effective spin mixing conductance Gmixeff in WxMo1−x (WMo)/Y3Fe5O12 (YIG) heterostructure has been studied by the measurements of electron spin resonance and spin pumping technique which are based on the ferromagnetic resonance (FMR). It is interesting to find that the FMR linewidth for both YIG single layer and WMo/YIG heterostructures shows twofold and fourfold symmetry contributions although Gmixeff exhibits isotropic feature. The Gmixeff and resistivity ρ present a maximum and minimum value at x = 0.42 for WxMo1−x/YIG heterostructures, respectively, which could be ascribed to the complex phase constituents of the alloy films. This work emphasized that the consistent measurement orientation of YIG single layer and WMo/YIG heterostructures should be strictly obeyed to get the accurate Gmixeff owing to the anisotropic feature of in-plane FMR linewidth, which has always been ignored. Proper alloyed Mo could adjust the spin current related phenomenon of W based alloys, to achieve the high efficiency of spin current production from the large effective spin mixing conductance.

Magnetic Proximity Effect Free Spin Hall Magnetoresistance in YIG∖Pd

Magnetic Proximity Effect (MPE) at the interface of Y3Fe5O[Formula: see text] (YIG)[Formula: see text]Pd is investigated using X-ray Magnetic Circular Dichroism (XMCD) at the Pd-edges. No evidence of MPE is found even for Pd film thicknesses down to 0.5[Formula: see text]nm. The same bilayers have been thereafter used for inverse spin Hall Effect (ISHE) measurements using the spin-pumping technique. A large signal can be observed at the ferromagnetic resonance. The angular dependence of the Pd dc resistance has been measured as a function of the YIG magnetization direction. It shows the expected dependences for the spin Hall magnetoresistance (SMR). Our methodology allows to state that ISHE and SMR are intrinsic phenomena whose underlying physics is uncorrelated to MPE.

Spin–Charge Conversion Phenomena in Germanium

The spin–orbit coupling relating the electron spin and momentum allows for spin generation, detection and manipulation. It thus fulfils the three basic functions of the spin field-effect-transistor made of semiconductors. In this paper, we review our recent results on spin–charge conversion in bulk germanium and at the Ge(111) surface. We used the spin pumping technique to generate pure spin currents to be injected into bulk germanium and at the Fe/Ge(111) interface. The mechanism for spin–charge conversion in bulk germanium is the spin Hall effect and we could experimentally determine the spin Hall angle θSHE, i.e., the spin–charge conversion efficiency, in heavily doped n-type and p-type germanium. We found very small values at room temperature: θSHE ≈ (1–2) × 10−3 in n-Ge and θSHE ≈ (6–7) × 10−4 in p-Ge. Moreover, we pointed out the essential role of spin dependent scattering on ionized impurities in the spin Hall effect mechanism. We concluded that the spin Hall effect in bulk germanium is too weak to...

Universal scaling for the spin-electricity conversion on surface states of topological insulators

We have investigated spin-electricity conversion on surface states of bulk-insulating topological insulator (TI) materials using a spin pumping technique. The sample structure is Ni-Fe|Cu|TI trilayers, in which magnetic proximity effects on the TI surfaces are negligibly small owing to the inserted Cu layer. Voltage signals produced by the spin-electricity conversion are clearly observed, and enhanced with decreasing temperature in line with the dominated surface transport at lower temperatures. The efficiency of the spin-electricity conversion is greater for TI samples with higher resistivity of bulk states and longer mean free path of surface states, consistent with the surface spin-electricity conversion.

Temperature evolution of electromotive force from Pt on yttrium-iron-garnet under ferromagnetic resonance

Temperature evolution of electromotive force from Pt due to the inverse spin Hall effect is studied. Pure spin current is injected from yttrium-iron-garnet by using spin pumping technique. The electromotive force from the Pt monotonically decreases with decreasing temperature, and it is showed that there is a deviation between the measured and the calculated electromotive forces.

Spin-electricity conversion induced by spin injection into topological insulators.

We report successful spin injection into the surface states of topological insulators by using a spin pumping technique. By measuring the voltage that shows up across the samples as a result of spin pumping, we demonstrate that a spin-electricity conversion effect takes place in the surface states of bulk-insulating topological insulators Bi(1.5)Sb(0.5)Te(1.7)Se(1.3) and Sn-doped Bi(2)Te(2)Se. In this process, the injected spins are converted into a charge current along the Hall direction due to the spin-momentum locking on the surface state.

Conversion of pure spin current to charge current in amorphous bismuth

Spin Hall angle and spin diffusion length in amorphous bismuth (Bi) are investigated by using conversion of a pure spin current to a charge current in a spin pumping technique. In Bi/Ni80Fe20/Si(100) sample, a clear direct current (DC) electromotive force due to the inverse spin Hall effect of the Bi layer is observed at room temperature under a ferromagnetic resonance condition of the Ni80Fe20 layer. From the Bi thickness dependence of the DC electromotive force, the spin Hall angle and the spin diffusion length of the amorphous Bi film are estimated to be 0.02 and 8 nm, respectively.

Spin pumping technique and its observation

Ferromagnetic Resonance (FMR) induced electron transport change was observed for the first time. Spin inversion caused by FMR affected magnetization vector, which generated a variation in planar Hall effect. In perpendicular Hall effect measurements, no sign of change was observed even at low temperatures. The mechanism of spin pumping technique and the reason of observation are explained in terms of Anisotropic Magnetoresistance (AMR) theory. This change of electron transport property can be applied to specific frequency sensing sensor, and direct conversion filter, since the resonance occurs only when resonance condition meets.