Nonlocal Spin Signal Research Articles

Half‐Metallic Heusler Alloy/GaN Heterostructure for Semiconductor Spintronics Devices

AbstractBecause spin‐orbit coupling in wurtzite semiconductors is relatively weak compared with that in zincblende ones, the III‐nitride semiconductor GaN is a promising material for high‐performance optical semiconductor spintronic devices such as spin lasers. For the purpose of reducing the operating power of spin lasers, it is necessary to demonstrate highly efficient electrical spin injection from a ferromagnetic material into GaN with a low‐resistance contact. Here, an epitaxial half‐metallic Heusler alloy Co2FeAlxSi1−x(CFAS)/GaN heterostructure is developed by inserting an ultrathin Co layer between the CFAS and GaN. The CFAS/n+‐GaN heterojunctions clearly show tunnel conduction with very small rectification and a low resistance‐area product of ≈3.8 kΩµm2, which is several orders of magnitude smaller than those reported in previous work, at room temperature. Nonlocal spin signals and a Hanle effect curve are observed at low temperatures using lateral spin‐valve devices with the CFAS/n+‐GaN contacts, suggesting pure spin current transport in bulk GaN. The spin transport is observed at temperatures as high as room temperature, with a high spin polarization of 0.2 at a low bias voltage less than 2.0 V. This study is expected to open a path to GaN‐based spintronic devices with highly spin‐polarized and low‐resistance contacts.

Temperature Dependence of Two-Terminal Local Magnetoresistance in Co-Based Heusler Alloy/Ge Lateral Spin-Valve Devices

We investigate temperature dependence of two-terminal local magnetoresistance (MR) effect for germanium (Ge)-based lateral spin-valve (LSV) devices with three different ferromagnet (FM)/Ge Schottky tunnel contacts. When we insert a 0.7-nm-thick Fe layer between Co-based Heusler alloy and Ge interfaces in LSV devices, the magnitude of the local MR signals is significantly improved from low temperatures to room temperature. However, the temperature dependence of the MR ratio is still large even for LSV devices with Fe insertion. From the analysis based on the standard theory, the temperature-dependent MR ratio can be interpreted in terms of thermally excited spin waves ( <inline-formula xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink"> <tex-math notation="LaTeX">$T^{3/{2}}$ </tex-math></inline-formula> law) in FM contacts. The physical origin of the decay of the two-terminal MR ratio in Ge-based LSV devices is the same as that of the four-terminal nonlocal spin signals because small temperature dependence of contact resistance is demonstrated in FM/Ge Schottky tunnel contacts which show almost linear <inline-formula xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink"> <tex-math notation="LaTeX">$|J|$ </tex-math></inline-formula> – <inline-formula xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink"> <tex-math notation="LaTeX">$V$ </tex-math></inline-formula> characteristics and low interface resistance.

Superimposed contributions to two-terminal and nonlocal spin signals in lateral spin-transport devices

The spin voltages produced by spin accumulation and Hanle spin precession in a lateral spin-transport device with a silicon channel and ferromagnetic tunnel contacts (Fe/MgO) are probed for a wide range of magnetic fields. Signal analysis reveals that for the interpretation of the two-terminal magnetoresistance and the nonlocal spin signals, one needs to consider various superimposed contributions, namely, (i) spin signals arising from spin transport of mobile carriers through the Si channel from one ferromagnetic contact to the other, thus depending on the relative magnetization of the two contacts, (ii) spin signals also arising from the spin accumulation of mobile carriers in the Si channel but generated at each of the ferromagnetic contacts separately, and (iii) spin signals originating from spin accumulation of carriers that are confined at or near the Si/MgO interface of the magnetic tunnel contacts, with rather different spin precession characteristics. Perhaps surprisingly, in the nonlocal spin signal a clear broad Hanle signal from confined electrons is also observed, and argued to be mediated by heat flow from the injector to the nonlocal detector.

Effect of Fe atomic layers at the ferromagnet–semiconductor interface on temperature-dependent spin transport in semiconductors

Using artificially controlled ferromagnet (FM)–semiconductor (SC) interfaces, we study the decay of the nonlocal spin signals with increasing temperature in SC-based lateral spin-valve devices. When more than five atomic layers of Fe are inserted at the FM/SC interfaces, the temperature-dependent spin injection/detection efficiency (Pinj/det) can be interpreted in terms of the T32 law, meaning a model of the thermally excited spin waves in the FM electrodes. For the FM/SC interfaces with the insufficient insertion of Fe atomic layers, on the other hand, the decay of Pinj/det is more rapid than the T32 curve. Using magneto-optical Kerr effect measurements, we find that more than five atomic layers of Fe inserted between FM and SC enable us to enhance the ferromagnetic nature of the FM/SC heterointerfaces. Thus, the ferromagnetism in the ultra-thin FM layer just on top of SC is strongly related to the temperature-dependent nonlocal spin transport in SC-based lateral spin-valve devices. We propose that the sufficient ferromagnetism near the FM/SC interface is essential for high-performance FM–SC hybrid devices above room temperature.

Thermal Stability at the Interface between Ferromagnetic Alloys and Germanium for Semiconductor Spintronics Devices

Spin MOSFET has drawn much attention as an attractive candidate for future semiconductor devices because of its nonvolatility and reconfigurability [1]. For realization of the spin MOSFET, highly efficient spin injection from ferromagnets to semiconductors with low parasitic resistance is one of the most important key technologies. Recently, we demonstrated room-temperature spin transport in germanium (Ge) based lateral spin valve devices with Schottky-tunnel contact, where the interface resistance at the ferromagnet/Ge is much smaller than that with conventional insulating tunnel barriers [2, 3]. However, when the thermal annealing was conducted to fabricate gate-stack structures for the spin MOSFET, the spin injection/detection efficiency was degraded due to the formation of reaction layers at the ferromagnetic alloys/Ge interface [4]. In this study, we explore the enhancement in the thermal stability of the ferromagnet/Ge interface and demonstrate room-temperature spin signals even for the lateral spin device annealed at 250 oC by an insertion of five Fe atomic layers between the ferromagnetic alloys and Ge.We fabricated lateral spin valve devices with Co2FeAl0.5Si0.5(CFAS)/Fe (5 atomic-layers)/Ge Schottky tunnel junction [Fig. 1(a)]. A 70-nm-thick phosphorus (P) doped Ge layer was grown on Ge/Si(111) virtual substrate at 350 oC as a spin transport layer by molecular beam epitaxy (MBE). On the P doped Ge layer, five Fe atomic layers and an epitaxial CFAS layer were formed using low temperature MBE. Beneath the ferromagnetic contacts, the P δ-doping with a thin Si layer was inserted to promote the tunnel conduction. The CFAS/Fe and δ-doped Ge layers were patterned into the contacts with 0.4 × 5.0 μm2 and 1.0 × 5.0 μm2, where the edge-to-edge distances between the CFAS/n+-Ge contacts are designed to be 0.45 μm. As a reference sample, a device without the Fe atomic layers was also fabricated. The devices with and without the Fe atomic layers were annealed at 250 oC in N2 atmosphere.Figures 1(b) and (c) show the nonlocal spin signals at 77 K for devices annealed at 250 oC without and with an Fe insertion, respectively, together with schematics of the CFAS/Ge and CFAS/Fe/Ge interfaces after the annealing. For the device without an Fe insertion [Fig. 1(b)], the spin signal is significantly degraded (~97 %) compared with the as prepared device, where the Ge out-diffusion causes the degradation of the spin injection/detection efficiency [4]. On the other hand, even after the annealing, clear hysteretic curves related to the magnetization states of the ferromagnetic injector and detector contacts can be observed for the device with an insertion of Fe atomic layers [Fig. 1(c)]. Although there is ~50 % decrease of the spin signal by the annealing, the magnitude of nonlocal spin signals is ~ 140 mΩ due to the significant improvement of the spin injection efficiency [5] and of the thermal stability of the CFAS/Fe/Ge interface. As shown in schematics of Figs 1(b) and 1(c), the inserted Fe atomic layers can suppress the Ge out-diffusion into CFAS and maintain the highly efficient spin injection. For the device with an Fe atomic insertion annealed at 250 oC, a spin signal of ~7 mΩ can be obtained even at room temperature.This work was partly supported by Grants-in-Aid for Scientific Research (A) (No. 16H02333) and (S) (No. 17H06120 and 19H05616) from the Japan Society for the Promotion of Science (JSPS). M.Y. acknowledges JSPS Research Fellowships for Young Scientists (No.18J00502).[1] S. Sugahara and M. Tanaka, Appl. Phys. Lett. 84, 2307 (2004).[2] M. Yamada et al., Appl. Phys. Express 10, 093001 (2017).[3] M. Tsukahara et al., Appl. Phys. Express 12, 033002 (2019).[4] B. Kuerbanjiang et al., Phys. Rev. B 98, 115304 (2018).[5] M. Yamada et al., (submitted). Figure 1

Suppression of Donor-Driven Spin Relaxation in Strained Si0.1Ge0.9

We experimentally study the strain effect on electron spin relaxation in a semiconductor using nonlocal spin-transport measurements in lateral spin-valve devices. Application of in-plane and biaxial tensile strain to a (111)-oriented and heavily doped $n\text{\ensuremath{-}}\mathrm{type}\phantom{\rule{0.2em}{0ex}}{\mathrm{Si}}_{0.1}{\mathrm{Ge}}_{0.9}$ layer leads to lifting of the valley degeneracy in the conduction band and to reduction of the electron's effective mass, resulting in increased electron mobility. Nonlocal four-terminal spin signals in the strained-${\mathrm{Si}}_{0.1}{\mathrm{Ge}}_{0.9}$ lateral spin-valve devices are markedly enhanced and the estimated spin lifetime becomes 3 times longer than that in strain-free devices at low temperatures. On the basis of a comparison of the experimental data and recent theories, we propose that only the donor-driven intervalley spin-flip scattering of electrons at low temperatures is partly suppressed for the strained and heavily doped ${\mathrm{Si}}_{0.1}{\mathrm{Ge}}_{0.9}$.

Effect of the low-resistance tunnel barriers induced inhomogeneous spin current distribution in graphene crossed configuration lateral spin valve

The nonlocal spin valve configuration consists of two ferromagnetic and nonmagnetic channels, which is an effective configuration for determining spin injection and accumulation. Here, we report that a reversed nonlocal spin signal was detected by changing the voltage probe configurations in graphene (Py/MgO/graphene/MgO/Py) lateral spin valves. The abnormal reversed spin-dependent nonlocal voltage is attributed to the nonuniform pinhole at the interface of the low-resistance tunnel barrier, which makes the charge current flow through the detection electrode and return to the graphene channel. We demonstrate that the channel-width induced spin-polarized current inhomogeneity significantly contributes to nonlocal resistance. A detailed description and simulated results of the tunnel junctions provide evidence for the reversal of the nonlocal voltage sign induced by the low-resistance tunnel barriers. Our work sheds light on the understanding of the spatial distribution of the spin current and the effect of the tunnel barrier, which are essential for the development of spintronic devices.

Two-Dimensional Flexible High Diffusive Spin Circuits.

Owing to their unprecedented electronic properties, graphene and two-dimensional (2D) crystals have brought fresh opportunities for advances in planar spintronic devices. Graphene is an ideal medium for spin transport while being an exceptionally resilient material for flexible nanoelectronics. However, these extraordinary traits have never been combined to create flexible graphene spin circuits. Realizing such circuits could lead to bendable strain-spin sensors, as well as a unique platform to explore pure spin current based operations and low-power 2D flexible nanoelectronics. Here, we demonstrate graphene spin circuits on flexible substrates for the first time. Despite the rough topography of the flexible substrates, these circuits prepared with chemical vapor deposited monolayer graphene reveal an efficient room temperature spin transport with distinctively large spin diffusion coefficients ∼0.2 m2 s-1. Compared to earlier graphene devices on Si/SiO2 substrates, such values are up to 20 times larger, leading to one order higher spin signals and an enhanced spin diffusion length ∼10 μm in graphene-based nonlocal spin valves fabricated using industry standard systems. This high performance arising out of a characteristic substrate terrain shows promise of a scalable and flexible platform towards flexible 2D spintronics. Our innovation is a key step for the exploration of strain-dependent 2D spin phenomena and paves the way for flexible graphene spin memory-logic units and planar spin sensors.

Crystal orientation effect on spin injection/detection efficiency in Si lateral spin-valve devices

We show evident crystal orientation effect on pure spin current transport in Si-based lateral spin-valve (LSV) devices with epitaxially grown CoFe/MgO tunnel contacts. When we compare nonlocal spin signals between LSV devices along 1 0 0 (Si1 0 0) and 1 1 0 (Si1 1 0), we find that the magnitude of the spin signals for Si1 0 0 LSV devices is always larger than that for Si1 1 0 LSV devices. The analyses based on the one-dimensional spin diffusion model reveal that the spin diffusion length and spin lifetime between Si1 0 0 and Si1 1 0 LSV devices are comparable, while the spin injection/detection efficiency in Si1 0 0 LSV devices is evidently larger than that in Si1 1 0 ones. Possible origins of the difference in the spin injection/detection efficiency between Si1 0 0 and Si1 1 0 LSV devices are discussed.

Bias-dependent spin injection into graphene on YIG through bilayer hBN tunnel barriers

We study the spin injection efficiency into single and bilayer graphene on the ferrimagnetic insulator Yttrium-Iron-Garnet (YIG) through an exfoliated tunnel barrier of bilayer hexagonal boron nitride (hBN). The contacts of two samples yield a resistance-area product between 5 and 30 k$\Omega\mu$m$^2$. Depending on an applied DC bias current, the magnitude of the non-local spin signal can be increased or suppressed below the noise level. The spin injection efficiency reaches values from -60% to +25%. The results are confirmed with both spin valve and spin precession measurements. The proximity induced exchange field is found in sample A to be (85 $\pm$ 30) mT and in sample B close to the detection limit. Our results show that the exceptional spin injection properties of bilayer hBN tunnel barriers reported by Gurram et al. are not limited to fully encapsulated graphene systems but are also valid in graphene/YIG devices. This further emphasizes the versatility of bilayer hBN as an efficient and reliable tunnel barrier for graphene spintronics.

Efficient Injection and Detection of Out-of-Plane Spins via the Anomalous Spin Hall Effect in Permalloy Nanowires.

We report a novel mechanism for the electrical injection and detection of out-of-plane spin accumulation via the anomalous spin Hall effect (ASHE), where the direction of the spin accumulation can be controlled by manipulating the magnetization of the ferromagnet. This mechanism is distinct from the spin Hall effect (SHE), where the spin accumulation is created along a fixed direction parallel to an interface. We demonstrate this unique property of the ASHE in nanowires made of permalloy (Py) to inject and detect out-of-plane spin accumulation in a magnetic insulator, yttrium iron garnet (YIG). We show that the efficiency for the injection/detection of out-of-plane spins can be up to 50% of that of in-plane spins. We further report the possibility to detect spin currents parallel to the Py/YIG interface for spins fully oriented in the out-of-plane direction, resulting in a sign reversal of the nonlocal magnon spin signal. The new mechanisms that we have demonstrated are highly relevant for spin torque devices and applications.

Probing tunneling spin injection into graphene via bias dependence

The bias dependence of spin injection in graphene lateral spin valves is systematically studied to determine the factors affecting the tunneling spin injection efficiency. Three types of junctions are investigated, including MgO and hexagonal boron nitride (hBN) tunnel barriers and direct contacts. A DC bias current applied to the injector electrode induces a strong nonlinear bias dependence of the nonlocal spin signal for both MgO and hBN tunnel barriers. Furthermore, this signal reverses its sign at a negative DC bias for both kinds of tunnel barriers. The analysis of the bias dependence for injector electrodes with a wide range of contact resistances suggests that the sign reversal correlates with bias voltage rather than current. We consider different mechanisms for nonlinear bias dependence and conclude that the energy-dependent spin-polarized electronic structure of the ferromagnetic electrodes, rather than the electrical field-induced spin drift effect or spin filtering effect of the tunnel barrier, is the most likely explanation of the experimental observations.

Spin Absorption Effect at Ferromagnet/Ge Schottky-Tunnel Contacts

We study the influence of the junction size in ferromagnet (FM)/semiconductor (SC) contacts on four-terminal nonlocal spin signals in SC-based lateral spin-valve (LSV) structures. When we use FM/Ge Schottky-tunnel junctions with relatively low resistance-area products, the magnitude of the nonlocal spin signal depends clearly on the junction size, indicating the presence of the spin absorption effect at the spin-injector contact. The temperature-dependent spin signal can also be affected by the spin absorption effect. For SC spintronic applications with a low parasitic resistance, we should consider the influence of the spin absorption on the spin-transport signals in SC-based device structures.

Interfacial Crystal Structures and Non-Local Spin Signals of Co2FeAl0.5Si0.5/n-GaAs Junctions

We have investigated interfacial crystal structures and non-local spin signals $\Delta R$ of Co2FeAl0.5Si0.5 (CFAS)/n-GaAs junctions. Cross-sectional transmission electron microscopy observations indicated that with the exception of Ga diffusion into CFAS of the sample deposited at 400 °C, the interfacial structure of the junctions and defect density at the interface were not very different for different CFAS fabrication temperatures of the substrate ( $T_{\mathrm {CFAS}})$ . The obtained reflection high-energy electron diffraction patterns showed that all samples fabricated at $T_{\mathrm {CFAS}}$ varying from room temperature to 400 °C exhibited the L21 ordered structure in the vicinity of CFAS/n-GaAs junctions. It is found that the junctions with larger rectifying characteristic as indicated by the conduction ratio $G(0.5~ \text {V})/G(-0.5~ \text {V})$ show larger spin signal $\Delta R$ . This may strongly affect the spin injection/detection efficiency.

Effects of interface geometry on spin injection

Spin-injection efficiency may be affected by interface geometry and it was investigated theoretically by considering a nonmagnetic (NM) sphere embedded in a ferromagnetic (FM) host. When spin-polarized current is injected into the NM sphere from the FM host, it is found that spinsplitting of the electrochemical potential is enhanced at the interface while the spin polarization of the electrical current is reduced compared with the flat interface. In the Co/Cu and the Py/Cu systems, the values are different from those of the flat interface even when the sphere radius is 3 μm. Attachment of another NM electrode to the NM sphere changes the spin-splitting of the electrochemical potential, which may be critical for the nonlocal spin signal in the lateral spin valve.

Noise-based approximation to thermal spin-injection in Fe/GaAs

We analyze the prospects for thermal spin-injection from iron into gallium arsenide via the application of electrical noise. By estimating the applied effective temperature-equivalent gradients, we characterize the magnitude of any electrical part of the thermal spin-injection efficiency or the spin-dependent Seebeck effect. The magnitude of the non-local spin signal associated with this effect suggests that temperature differences on the order of ∼100 K would be needed for true thermal spin-injection experiments. The large size of the effective temperature gradients induced by the noise-based method means that even very small thermo-electric effects can be quantified.

Strong Modulation of Spin Currents in Bilayer Graphene by Static and Fluctuating Proximity Exchange Fields.

Two-dimensional materials provide a unique platform to explore the full potential of magnetic proximity-driven phenomena, which can be further used for applications in next-generation spintronic devices. Of particular interest is to understand and control spin currents in graphene by the magnetic exchange field of a nearby ferromagnetic material in graphene-ferromagnetic-insulator (FMI) heterostructures. Here, we present the experimental study showing the strong modulation of spin currents in graphene layers by controlling the direction of the exchange field due to FMI magnetization. Owing to clean interfaces, a strong magnetic exchange coupling leads to the experimental observation of complete spin modulation at low externally applied magnetic fields in short graphene channels. Additionally, we discover that the graphene spin current can be fully dephased by randomly fluctuating exchange fields. This is manifested as an unusually strong temperature dependence of the nonlocal spin signals in graphene, which is due to spin relaxation by thermally induced transverse fluctuations of the FMI magnetization.

Large impact of impurity concentration on spin transport in degenerate n -Ge

We experimentally show that the spin relaxation in degenerate $n$-type germanium (${n}^{+}$-Ge) depends strongly on the concentration of the donor impurity (${N}_{\mathrm{d}}$) at low temperatures. From measuring nonlocal spin signals for various lateral spin-valve devices at 8 and 77 K, the spin diffusion length (${\ensuremath{\lambda}}_{\mathrm{Ge}}$) of ${n}^{+}$-Ge can be estimated as a function of carrier concentration ($n$), i.e., ${N}_{\mathrm{d}}\ensuremath{\approx}n$ ($\ensuremath{\sim}{10}^{18}$ ${\mathrm{cm}}^{\ensuremath{-}3}$). We clearly find a large change in ${\ensuremath{\lambda}}_{\mathrm{Ge}}$ from 1.43 to 0.56 $\ensuremath{\mu}\mathrm{m}$ within a relatively narrow range of $n$ at 8 K. The experimental findings are interpreted quantitatively in terms of a recent theory based on donor-driven spin scattering in multivalley conduction bands in Ge.

Nonlocal magnon spin transport in NiFe2O4 thin films

We report magnon spin transport in nickel ferrite platinum (Pt)/(NiFe2O4, NFO) bilayer systems at room temperature. A nonlocal geometry is employed, where the magnons are excited by the spin Hall effect or by the Joule heating induced spin Seebeck effect at the Pt injector and detected at a certain distance away by the inverse spin Hall effect at the Pt detector. The dependence of the nonlocal magnon spin signals as a function of the magnetic field is closely related to the NFO magnetization behavior. In contrast, we observe that the magnetoresistance measured locally at the Pt injector does not show a clear relationship with the average NFO magnetization. We obtain a magnon spin relaxation length of 3.1 ± 0.2 μm in the investigated NFO samples.

Robust spin-current injection in lateral spin valves with two-terminal Co2FeSi spin injectors

We demonstrate generation and detection of pure spin currents by combining a two-terminal spin-injection technique and Co2FeSi (CFS) spin injectors in lateral spin valves (LSVs). We find that the two-terminal spin injection with CFS has the robust dependence of the nonlocal spin signals on the applied bias currents, markedly superior to the four-terminal spin injection with permalloy reported previously. In our LSVs, since the spin transfer torque from one CFS injector to another CFS one is large, the nonlocal magnetoresistance with respect to applied magnetic fields shows large asymmetry in high bias-current conditions. For utilizing multi-terminal spin injection with CFS as a method for magnetization reversals, the terminal arrangement of CFS spin injectors should be taken into account.