Spin Valve Signal Research Articles

Room-Temperature and Tunable Tunneling Magnetoresistance in Fe3GaTe2-Based 2D van der Waals Heterojunctions.

Magnetic tunnel junctions (MTJs) based on van der Waals (vdW) heterostructures with sharp and clean interfaces on the atomic scale are essential for the application of next-generation spintronics. However, the lack of room-temperature intrinsic ferromagnetic crystals with perpendicular magnetic anisotropy has greatly hindered the development of vertical MTJs. The discovery of room-temperature intrinsic ferromagnetic two-dimensional (2D) crystal Fe3GaTe2 has solved the problem and greatly facilitated the realization of practical spintronic devices. Here, we demonstrate a room-temperature MTJ based on a Fe3GaTe2/WS2/Fe3GaTe2 heterostructure for the first time. The tunneling magnetoresistance (TMR) ratio is up to 213% with a high spin polarization of 72% at 10 K, the highest ever reported in Fe3GaTe2-based MTJs up to now. A tunneling spin-valve signal robustly persists at room temperature (300 K) with a bias current down to 10 nA. Moreover, the spin polarization can be modulated by bias current and the TMR shows a sign reversal at a large bias current. Our work sheds light on the potential application of low-energy consumption in all-2D vdW spintronics and offers alternative routes for the electronic control of spintronic devices.

High current limits in chemical vapor deposited graphene spintronic devices

Understanding the stability and current-carrying capacity of graphene spintronic devices is key to their applications in graphene channel-based spin current sensors, spin-torque oscillators, and potential spin-integrated circuits. However, despite the demonstrated high current densities in exfoliated graphene, the current-carrying capacity of large-scale chemical vapor deposited (CVD) graphene is not established. Particularly, the grainy nature of chemical vapor deposited graphene and the presence of a tunnel barrier in CVD graphene spin devices pose questions about the stability of high current electrical spin injection. In this work, we observe that despite structural imperfections, CVD graphene sustains remarkably highest currents of 5.2 × 108 A/cm2, up to two orders higher than previously reported values in multilayer CVD graphene, with the capacity primarily dependent upon the sheet resistance of graphene. Furthermore, we notice a reversible regime, up to which CVD graphene can be operated without degradation with operating currents as high as 108 A/cm2, significantly high and durable over long time of operation with spin valve signals observed up to such high current densities. At the same time, the tunnel barrier resistance can be modified by the application of high currents. Our results demonstrate the robustness of large-scale CVD graphene and bring fresh insights for engineering and harnessing pure spin currents for innovative device applications.

Effective modulation of spin accumulation using a ferromagnetic/nonmagnetic bilayer spin channel

A lateral spin valve consisting of highly spin-polarized CoFeAl electrodes with a CoFeAl/Cu bilayer spin channel has been developed. Despite a large spin absorption into the CoFeAl capping channel layer, an efficient spin injection and detection using the CoFeAl electrodes enable us to observe a clear spin valve signal. We demonstrate that the nonlocal spin accumulation signal is significantly modulated depending on the relative angle of the magnetizations between the spin injector and absorber. The observed modulation phenomena is explained by the longitudinal and transverse spin absorption effects into the CoFeAl channel layer with the spin resistance model.

Electronic and magnetic properties of α−FeGe2 films embedded in vertical spin valve devices

We studied metastable $\ensuremath{\alpha}\text{\ensuremath{-}}\mathrm{Fe}{\mathrm{Ge}}_{2}$, a layered tetragonal material, embedded as a spacer layer in spin valve structures with ferromagnetic ${\mathrm{Fe}}_{3}\mathrm{Si}$ and ${\mathrm{Co}}_{2}\mathrm{Fe}\mathrm{Si}$ electrodes. For both types of electrodes, spin valve operation is demonstrated with A metallic transport behavior of the $\ensuremath{\alpha}\text{\ensuremath{-}}\mathrm{Fe}{\mathrm{Ge}}_{2}$ spacer layer. The spin valve signals are found to increase with both temperature and spacer thickness, which is discussed in terms of a decreasing magnetic coupling strength between the ferromagnetic bottom and top electrodes. The temperature-dependent resistances of the spin valve structures exhibit characteristic features, which are explained by ferromagnetic phase transitions between 55 and 110 K. The metallic transport characteristics as well as the low-temperature ferromagnetism are found to be consistent with the results of first-principles calculations.

Large spin signals in n+-Si/MgO/Co2Fe0.4Mn0.6Si lateral spin-valve devices

The spin polarization factor was investigated using electrical spin injection at low temperatures in n+-Si(100)/MgO/ferromagnet lateral spin-valve devices with Co2Fe0.4Mn0.6Si (CFMS) and CoFe electrodes. CFMS films were annealed at different post-annealing temperatures (Ta). Although atomic diffusion of CFMS into the silicon channel was observed at high annealing temperatures, the CFMS device annealed at a Ta of 350 °C, clearly showing a narrow Hanle signal measured using the three-terminal Hanle effect; a consistent spin relaxation time of 7.1 ± 0.4 ns and spin diffusion length of 1.6 ± 0.2 μm were obtained at 10 K. A local three-terminal spin-valve (L-3TSV) signal from the CFMS lateral spin-valve device was obtained at about 370 μV, three times larger than that of the CoFe device. The tunnel spin polarization factor was evaluated from the L-3TSV signals by an analytical equation that considered the spin drift effect. The estimated tunnel spin polarization factor for CFMS was 45% at a Vbias of about 600 mV, while that for CoFe was 18%. This result indicates that the high spin polarization of CFMS is responsible for the large intensity of the L-3TSV signal and that CFMS is a promising FM material for electrical spin injection into silicon.

Spin transport in antimony-doped germanium detected using vertical spin-valve structures

Using a vertical spin-valve structure, we observe spin-dependent transport through antimony (Sb)-doped n-type germanium (n-Ge). Epitaxial intermediate Sb-doped Ge layers can be formed between ferromagnetic CoFe and Fe3Si epilayers on Si(111), where the doping concentration of Sb is estimated to be ∼3 × 1019 cm−3. From the measurement of the current perpendicular to the plane magnetoresistance, we can evidently detect the spin-valve signals from low temperatures up to room temperature. The room-temperature spin diffusion length in the Sb-doped Ge layer can roughly be estimated to be 150–290 nm. By comparing this result and the previous ones, we can clarify the influence of the dopant species on the spin transport even at room temperature in Ge.

Enhancement of Room-Temperature Effective Spin Diffusion Length in a Si-Based Spin MOSFET With an Inversion Channel

We have demonstrated a Si-based bottom-gate-type spin metal-oxide-semiconductor field-effect transistor (spin MOSFET) with an electron inversion channel at room temperature and showed the enhancement of the room-temperature effective spin diffusion length by “spin drift”. Our spin MOSFET was fabricated on a (001)-oriented silicon-on-insulator (SOI) substrate with a p -type ultrathin (8 nm) Si layer, in which the channel length was $1.0~\mu \text{m}$ , the spin injector/detector electrodes were ferromagnetic multilayer (from top to bottom) Fe(4nm)/Mg(1nm)/MgO(1nm) tunnel junctions, and a 200-nm-thick buried oxide layer was used for the gate dielectric. Using various gate electric fields and source-drain electron currents, two types of spin-dependent transport signals were measured: spin-valve signals with an in-plane magnetic field and Hanle signals with an out-of-plane magnetic field. Clear Hanle signals and spin-valve signals were obtained for various bias conditions. We analyzed the Hanle signals and the change of the spin-valve signals based on our original formulas that take into account the distribution of the lateral electric field along the electron transport, and revealed that the spin drift can enhance the effective spin diffusion length by the lateral electric field parallel to the electron transport in the inversion channel. The effective spin diffusion length becomes 3–13 times larger than the intrinsic spin diffusion length ${\lambda }_{\mathrm{ S}}= 0.89\mu \text{m}$ owing to the increase in the lateral electric field. It was confirmed that the spin drift is very useful to achieve larger spin-valve signals in spin MOSFETs with an electron inversion channel.

Asymmetric nonlocal signal induced by thermoelectric effects in a lateral spin valve

Abstract We demonstrate that the heat flow arising from the Joule heating produces not only the conventional thermal spin injection but also additional thermo-electric effects both in current and voltage probes. The excellent thermal spin injection property for the CoFeAl enables us to distinguish the ordinal thermal spin valve signal and the unconventional asymmetric field dependence in the nonlocal 2nd harmonic signal. We show that the observed unique field dependence can be well explained by the anomalous Nenrst effect in the ferromagnetic voltage probe and the anomalous thermal Hall effect in the ferromagnetic current probe. The validity of the proposed mechanism was confirmed in various devices with different probe configurations.

Spin generation in completely MBE-grown Co2FeSi/MgO/GaAs lateral spin valves

We demonstrate first measurements of successful spin generation in crystalline ${\mathrm{Co}}_{2}\mathrm{FeSi}$/MgO/GaAs hybrid structures grown by molecular-beam epitaxy (MBE) with different MgO interlayer thicknesses. Using nonlocal spin valve and nonlocal Hanle measurement configurations, we determine spin lifetimes of $\ensuremath{\tau}\ensuremath{\approx}100$ ns and spin diffusion lengths of $\ensuremath{\lambda}\ensuremath{\approx}5.6\phantom{\rule{4pt}{0ex}}\ensuremath{\mu}\mathrm{m}$ for different MgO layer thicknesses, proving the high quality of the GaAs transport channel. For an optimized MgO layer thickness, the bias dependence of the spin valve signals indicates the verification of the half-metallic gap (upper edge) of ${\mathrm{Co}}_{2}\mathrm{FeSi}$ in accordance with first-principles calculations. In addition to that, spin generation efficiencies of up to $18%$ reveal the high potential of MgO interlayers at the ${\mathrm{Co}}_{2}\mathrm{FeSi}$/GaAs interface for further device applications.

Tunnel spin polarization of Fe/MgO/Si contacts reaching 90% with increasing MgO thickness

Using nonlocal spin-transport devices, the tunnel spin polarization (TSP) of Fe/MgO tunnel contacts on Si is determined as a function of the thickness of the MgO. The TSP, extracted from the magnitude of the nonlocal spin-valve and Hanle signals, increases with MgO thickness and is shown to reach values of around 90% at low temperatures. Such a near-perfect spin polarization of the tunnel current indicates that symmetry-based spin filtering due to coherent tunneling occurs in Fe/MgO tunnel contacts on Si, despite the significant lattice mismatch. For MgO thicknesses below 1 nm, the TSP drops unexpectedly rapidly (to values below 25%), which is attributed to the lower crystalline quality of thin MgO layers. In fact, the data suggest that the first 0.7 nm of MgO on the Si contributes very little to the spin filtering.

Hanle spin precession in a two-terminal lateral spin valve

Spin voltages due to Hanle spin precession in a two-terminal (2T) device, composed of a silicon channel and two ferromagnetic tunnel contacts (Fe/MgO), are observed and explained. The 2T Hanle signals contain not only contributions due to spin transport from one electrode to the other but also local contributions from the spin accumulation in the Si created and detected at each of the two contacts separately. Consequently, the salient features of the 2T Hanle signals differ from those in nonlocal devices and the familiar correspondence between Hanle and spin-valve signals is absent. The local and transport contributions to the 2T Hanle signal can be disentangled and described by the same set of device parameters as the nonlocal Hanle signals.

Electrical spin injection into AlGaAs/GaAs-based two-dimensional electron gas systems with Co2MnSi spin source up to room temperature

We demonstrated electrical spin injection into an AlGaAs/GaAs-based high-mobility two-dimensional electron gas (2DEG) system using Co2MnSi as a spin source. A non-local spin-valve signal was observed from 4.2 K to room temperature. Interestingly, the spin-valve signal does not show a monotonic decrease with increasing temperature and reaches a peak at about 80 K. This contrasts with the result observed in bulk GaAs, in which a monotonic decrease in spin-valve signals with increasing temperature was observed. Moreover, the spin-valve signal decreases by a factor of about 5.6 with increasing temperature from 4.2 K to 294 K, and this factor is smaller than those values reported in bulk GaAs devices. This result suggests that the spin-valve signal in a 2DEG device is less sensitive to temperature than that in a bulk device, which is promising for realizing future spin transistors that can operate at room temperature.

Detection of the interfacial exchange field at a ferromagnetic insulator-nonmagnetic metal interface with pure spin currents

At the interface between a nonmagnetic metal (NM) and a ferromagnetic insulator (FI) spin current can interact with the magnetization, leading to a modulation of the spin current. The interfacial exchange field at these FI-NM interfaces can be probed by placing the interface in contact with the spin transport channel of a lateral spin valve (LSV) device and observing additional spin relaxation processes. We study interfacial exchange field in lateral spin valve devices where Cu spin transport channel is in proximity with ferromagnetic insulator EuS (EuS-LSV) and yttrium iron garnet Y$_3$Fe$_5$O$_{12}$ (YIG-LSV). The spin signals were compared with reference lateral spin valve devices fabricated on nonmagnetic Si/SiO$_2$ substrate with MgO or AlO$_x$ capping. The nonlocal spin valve signal is about 4 and 6 times lower in the EuS-LSV and YIG-LSV, respectively. The suppression in the spin signal has been attributed to enhanced surface spin-flip probability at the Cu-EuS (or Cu-YIG) interface due to interfacial spin-orbit field. Besides spin signal suppression we also found widely observed low temperature peak in the spin signal at $T \sim$30 K is shifted to higher temperature in the case of devices in contact with EuS or YIG. Temperature dependence of spin signal for different injector-detector distances reveal fluctuating exchange field at these interfaces cause additional spin decoherence which limit spin relaxation time in addition to conventional sources of spin relaxation. Our results show that temperature dependent measurement with pure spin current can be used to probe interfacial exchange field at the ferromagnetic insulator-nonmagnetic metal interface.

Electrical Spin Injection and Detection in Silicon Nanowires with Axial Doping Gradient.

The interest in spin transport in nanoscopic semiconductor channels is driven by both the inevitable miniaturization of spintronics devices toward nanoscale and the rich spin-dependent physics the quantum confinement engenders. For such studies, the all-important issue of the ferromagnet/semiconductor (FM/SC) interface becomes even more critical at nanoscale. Here we elucidate the effects of the FM/SC interface on electrical spin injection and detection at nanoscale dimensions, utilizing a unique type of Si nanowires (NWs) with an inherent axial doping gradient. Two-terminal and nonlocal four-terminal lateral spin-valve measurements were performed using different combinations from a series of FM contacts positioned along the same NW. The data are analyzed with a general model of spin accumulation in a normal channel under electrical spin injection from a FM, which reveals a distinct correlation of decreasing spin-valve signal with increasing injector junction resistance. The observation is attributed to the diminishing contribution of the d-electrons in the FM to the injected current spin polarization with increasing Schottky barrier width. The results demonstrate that there is a window of interface parameters for optimal spin injection efficiency and current spin polarization, which provides important design guidelines for nanospintronic devices with quasi-one-dimensional semiconductor channels.

Pure spin current transport in a SiGe alloy

Using four-terminal nonlocal magnetoresistance measurements in lateral spin-valve devices with Si0.1Ge0.9, we study pure spin current transport in a degenerate SiGe alloy (n ∼ 5.0 × 1018 cm−3). Clear nonlocal spin-valve signals and Hanle effect curves, indicating generation, transport, and detection of pure spin currents, are observed. The spin diffusion length and spin lifetime of the Si0.1Ge0.9 layer at low temperatures are reliably estimated to be ∼0.5 µm and ∼0.2 ns, respectively. This study demonstrates the possibility of exploring physics and developing spintronic applications using SiGe alloys.

Possibility of Cooper-pair formation controlled by multi-terminal spin injection

A multi-terminal lateral spin valve consisting of three ferromagnetic nanopillars on a Cu/Nb bilayer has been fabricated. We investigated the influence of the spin injection on the superconducting properties at the Cu/Nb interface. The non-local spin valve signal exhibits a clear spin insulation signature due to the superconducting gap of the Nb. The magnitude of the spin signal is found to show the probe configuration dependence. From the careful analysis of the bias current dependence, we found the suppression of the superconductivity due to the exchange interaction between the Cooper pair and accumulated spin plays an important role in the multi-terminal spin injections. We also discuss about the possibility of the Cooper-pair formation due to the spin injection from the two injectors with the anti-parallel alignment.

Influence of DC-biasing on the performance of graphene spin valve

Generating and controlling the spin valve signal are key factors in ‘spintronics’, which aims to utilize the spin degree of electrons. For this purpose, spintronic devices are constructed that can detect the spin signal. Here we investigate the effect of direct current (DC) on the magnetoresistance (MR) of graphene spin valve. The DC input not only decreases the magnitude of MR but also distorts the spin valve signal at higher DC inputs. Also, low temperature measurements revealed higher MR for the device, while the magnitude is noticed to decrease at higher temperatures. Furthermore, the spin polarization associated with NiFe electrodes is continuously increased at low DC bias and low temperatures. We also demonstrate the ohmic behavior of graphene spin valve by showing linear current-voltage (I-V) characteristics of the junction. Our findings may contribute significantly in modulating and controlling the spin transport properties of vertical spin valve structures.

Spin injection into silicon in three-terminal vertical and four-terminal lateral devices with Fe/Mg/MgO/Si tunnel junctions having an ultrathin Mg insertion layer

We demonstrate that the spin injection/extraction efficiency is enhanced by an ultrathin Mg insertion layer (\ensuremath{\leqslant}2 nm) in $\mathrm{Fe}\text{/}\mathrm{Mg}\text{/}\mathrm{MgO}\text{/}{n}^{+}\text{\ensuremath{-}}\mathrm{Si}$ tunnel junctions. In diode-type vertical three-terminal devices fabricated on a Si substrate, we observe the narrower three-terminal Hanle (N-3TH) signals indicating true spin injection into Si and estimate the spin polarization in Si to be 16% when the thickness of the Mg insertion layer is 1 nm, whereas no N-3TH signal is observed without the Mg insertion. This means that the spin injection/extraction efficiency is enhanced by suppressing the formation of a magnetically dead layer at the Fe/MgO interface. We also observe clear spin transport signals, such as nonlocal Hanle signals and spin-valve signals, in a lateral four-terminal device with the same $\mathrm{Fe}\text{/}\mathrm{Mg}\text{/}\mathrm{MgO}\text{/}{n}^{+}\text{\ensuremath{-}}\mathrm{Si}$ tunnel junctions fabricated on a Si-on-insulator substrate. It is found that both the intensity and linewidth of the spin signals are affected by the geometrical effects (device geometry and size). We have derived analytical functions taking into account the device structures, including channel thickness and electrode size, and estimated important parameters: spin lifetime and spin polarization. Our analytical functions explain the experimental results very well. Our study shows the importance of suppressing a magnetically dead layer and provides a unified understanding of spin injection/detection signals in different device geometries.

Systematic investigations of transient response of nuclear spins in the presence of polarized electrons

We electrically probed the transient response of nuclear spins in an $n$-GaAs channel by performing Hanle signal and spin-valve signal measurements on an all-electrical spin-injection device having a half-metallic spin source of $\mathrm{C}{\mathrm{o}}_{2}\mathrm{MnSi}$. Furthermore, we simulated the Hanle and spin-valve signals by using the time evolution of nuclear-spin polarization under the presence of polarized electron spins by taking both ${T}_{1\mathrm{e}}$ and ${T}_{1}$ into consideration, where ${T}_{1\mathrm{e}}^{\ensuremath{-}1}$ is the polarization rate of nuclear spins through the transfer of angular momentum from polarized electron spins and ${T}_{1}^{\ensuremath{-}1}$ is the depolarization rate of nuclear spins through the interaction with the lattice. The simulation results reproduced our experimental results on all the nuclear-spin-related phenomena appearing in the Hanle and spin-valve signals at different measurement conditions, providing quantitative explanation for the transient response of nuclear spins in GaAs to a change in magnetic fields and an estimate of the time scales of ${T}_{1\mathrm{e}}$ and ${T}_{1}$. These experimental and simulated results will deepen the understanding of nuclear-spin dynamics in semiconductors.

Bias induced up to 100% spin-injection and detection polarizations in ferromagnet/bilayer-hBN/graphene/hBN heterostructures

We study spin transport in a fully hBN encapsulated monolayer-graphene van der Waals heterostructure at room temperature. A top-layer of bilayer-hBN is used as a tunnel barrier for spin-injection and detection in graphene with ferromagnetic cobalt electrodes. We report surprisingly large and bias-induced (differential) spin-injection (detection) polarizations up to 50% (135%) at a positive voltage bias of + 0.6 V, as well as sign inverted polarizations up to −70% (−60%) at a reverse bias of −0.4 V. This demonstrates the potential of bilayer-hBN tunnel barriers for practical graphene spintronics applications. With such enhanced spin-injection and detection polarizations, we report a record two-terminal (inverted) spin-valve signals up to 800 Ω with a magnetoresistance ratio of 2.7%, and achieve spin accumulations up to 4.1 meV. We propose how these numbers can be increased further, for future technologically relevant graphene based spintronic devices.