Lateral Spin-valve Devices Research Articles

Room-temperature local magnetoresistance effect in n-Ge devices with low-resistive Schottky-tunnel contacts

Two-terminal local magnetoresistance (MR) effect in n-type germanium (Ge) based lateral spin-valve (LSV) devices can be observed at room temperature. By using phosphorus δ-doped Heusler-alloy/Ge Schottky-tunnel contacts, the resistance-area product of the contacts is able to be less than 0.20 kΩ μm2, which is the lowest value in semiconductor based LSV devices. From the one-dimensional spin drift-diffusion model, the interface spin polarization of the Heusler-alloy/Ge contacts in the present LSV devices can be estimated to be ∼0.018 at room temperature. We experimentally propose that it is important for enhancing the local MR ratio in n-Ge based LSV devices to improve the interface spin polarization of the Heusler-alloy/Ge contacts.

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.

Observation of local magnetoresistance signals in a SiGe-based lateral spin-valve device

Utilizing a Si0.1Ge0.9 layer grown on Ge/Si(111) as a spin-transport channel and a Co2FeAl0.5Si0.5 ferromagnetic epilayer as a spin injector and detector, we demonstrate two-terminal local magnetoresistance signals at low temperatures in SiGe based lateral spin-valve devices. The magnitude of the local magnetoresistance signals is twice as large as that of nonlocal signals below 50 K. The local magnetoresistance signal can be observed up to 225 K, at which the nonlocal magnetoresistance signals already disappear. To observe the local magnetoresistance signal at higher temperatures, we should consider the spin detection sensitivity of the spin detector contact in the used lateral spin-valve devices.

Correlation between spin transport signal and Heusler/semiconductor interface quality in lateral spin-valve devices

We show direct evidence for the impact of Heusler/semiconductor interfaces atomic structure on the spin transport signals in semiconductor-based lateral spin-valve (LSV) devices. Based on atomic scale Z-contrast scanning transmission electron microscopy and energy dispersive x-ray spectroscopy we show that atomic order/disorder of ${\mathrm{Co}}_{2}{\mathrm{FeAl}}_{0.5}{\mathrm{Si}}_{0.5}$ (CFAS)/$n$-Ge LSV devices is critical for the spin injection in Ge. By conducting a postannealing of the LSV devices, we find 90% decrease in the spin signal while there is no difference in the electrical properties of the $\mathrm{CFAS}\text{/}n$-Ge contacts and in the spin diffusion length of the $n$-Ge layer. We show that the reduction in the spin signals after annealing is attributed to the presence of intermixing phases at the Heusler/semiconductor interface. First-principles calculations show how that intermixed interface region has drastically reduced spin polarization at the Fermi level, which is the main cause for the significant decrease of the spin signal in the annealed devices above $300{\phantom{\rule{0.16em}{0ex}}}^{\ensuremath{\circ}}\mathrm{C}$.

Unveiling the mechanisms of the spin Hall effect in Ta

Spin-to-charge current interconversions are widely exploited for the generation and detection of pure spin currents and are key ingredients for future spintronic devices including spin-orbit torques and spin-orbit logic circuits. In the case of the spin Hall effect, different mechanisms contribute to the phenomenon and determining the leading contribution is peremptory for achieving the largest conversion efficiencies. Here, we experimentally demonstrate the dominance of the intrinsic mechanism of the spin Hall effect in highly resistive Ta. We obtain an intrinsic spin Hall conductivity for \ensuremath{\beta}-Ta of $\ensuremath{-}820\ifmmode\pm\else\textpm\fi{}120(\ensuremath{\hbar}/e)\phantom{\rule{0.16em}{0ex}}{\mathrm{\ensuremath{\Omega}}}^{\ensuremath{-}1}\phantom{\rule{0.16em}{0ex}}\mathrm{c}{\mathrm{m}}^{\ensuremath{-}1}$ from spin absorption experiments in a large set of lateral spin valve devices. The predominance of the intrinsic mechanism in Ta allows us to linearly enhance the spin Hall angle by tuning the resistivity of Ta, reaching up to \ensuremath{-}35 \ifmmode\pm\else\textpm\fi{} 3%, the largest reported value for a pure metal.

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.

Magnetic modulation of inverse spin Hall effect in lateral spin-valves

We analytically investigated the spin-dependent transport properties in a lateral spin-valve device comprising pinned ferromagnetic electrodes allowing the injection of a spin current in a spin conducting channel where spin orbit scattering takes place. This produces an inverse spin Hall (ISHE) voltage across the thickness of the spin conducting channel. It is shown that by adding an extra soft ferromagnetic electrode with rotatable magnetization along the spin conducting channel, the ISHE generated voltage can be magnetically modulated by changing the magnetization orientation of this additional electrode. The dependence of the ISHE voltage on the direction of magnetization of the ferromagnetic electrode with rotatable magnetization was calculated in various configurations. Our results suggest that such structures could be considered as magnetic field sensors in situations where the total thickness of the sensor is constrained such as in hard disk drive readers.

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.

Large spin-valve effect in a lateral spin-valve device based on ferromagnetic semiconductor GaMnAs

We investigate the spin-dependent transport properties of a lateral spin-valve device based on the ferromagnetic semiconductor GaMnAs. This device is composed of a GaMnAs channel layer grown on GaAs with a narrow trench across the channel. Its current–voltage characteristics show tunneling behavior. Large magnetoresistance (MR) ratios of more than ∼10% are obtained. These values are much larger than those (∼0.1%) reported for lateral-type spin metal–oxide–semiconductor field-effect transistors. The magnetic field direction dependence of the MR curve differs from that of the anisotropic magnetoresistance of GaMnAs, which confirms that the MR signal originates from the spin-valve effect between the GaMnAs electrodes.

Read sensor technology for ultrahigh density magnetic recording

Abstract

Inverse spin-valve effect in nanoscale Si-based spin-valve devices

We investigated the spin-valve effect in nano-scale silicon (Si)-based spin-valve devices using a Fe/MgO/Ge spin injector/detector deposited on Si by molecular beam epitaxy. For a device with a 20 nm Si channel, we observed clear magnetoresistance up to 3% at low temperature when a magnetic field was applied in the film plane along the Si channel transport direction. A large spin-dependent output voltage of 20 mV was observed at a bias voltage of 0.9 V at 15 K, which is among the highest values in lateral spin-valve devices reported so far. Furthermore, we observed that the sign of the spin-valve effect is reversed at low temperatures, suggesting the possibility of a spin-blockade effect of defect states in the MgO/Ge tunneling barrier.

Ballistic transport in spin-valve Si channel demonstrates feasibility of nanoscale spin-MOSFETs

Ballistic electron motion in nanoscale Si channels of a lateral spin-valve device, in addition to improved conductivity matching, advances spintronics technology towards spin-MOSFETS.

Observation of Magnetoresistance Effect in $n$ -Type Non-Degenerate Germanium With Co2Fe0.4Mn0.6Si Heusler Alloy Electrodes

The local 3-terminal magnetoresistance properties of n-Ge/MgO/Co2Fe0.4Mn0.6Si lateral spin-valve devices were systematically investigated. In the spin extraction condition, clear steep voltage changes were successfully observed. We measured the bias voltage and temperature dependences of the spin resistance-area product. At a high bias voltage, the spin signal increased with increasing voltage, reaching a maximum value of $7.0~\Omega \mu \text{m}^{{{2}}}$ at $V_{{\rm {bias}}} = 663$ mV. The signal decreased with increasing temperature but was still observed up to 160 K.

Room-temperature spin transport in n-Ge probed by four-terminal nonlocal measurements

We demonstrate electrical spin injection and detection in n-type Ge (n-Ge) at room temperature using four-terminal nonlocal spin-valve and Hanle-effect measurements in lateral spin-valve (LSV) devices with Heusler-alloy Schottky tunnel contacts. The spin diffusion length (λGe) of the Ge layer used (n ∼ 1 × 1019 cm−3) at 296 K is estimated to be ∼0.44 ± 0.02 µm. Room-temperature spin signals can be observed reproducibly in the low bias voltage range (≤0.7 V) for LSVs with relatively low resistance–area product (RA) values (≤1 kΩ µm2). This means that the Schottky tunnel contacts used here are more suitable than ferromagnet/MgO tunnel contacts (RA ≥ 100 kΩ µm2) for developing Ge spintronic applications.

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.

In-plane isotropic magnetic and electrical properties of MnAs/InAs/GaAs[formula omitted]B hybrid structure

We characterized in-plane magnetic and electrical properties of MnAs/InAs/GaAs(111)B hybrid structure grown by molecular beam epitaxy (MBE). We observed isotropic easy magnetization in two crystallographic in-plane directions, [2̅110] and [01̅10] of hexagonal MnAs i.e. [1̅10] and [112̅] of cubic InAs. We also fabricated transmission line model (TLM) devices, and observed almost isotropic electrical properties in two crystallographic in-plane directions, [1̅10] and [112̅] of cubic InAs. Also we tried to fabricate and characterize lateral spin-valve (LSV) devices from the hybrid structure. We could roughly estimate the spin injection efficiency and the spin diffusion length at room temperature in [112̅] direction. We believe that the hybrid structures are helpful to design spintronic device with good flexibility in-plane.

Tuning the spin Hall effect of Pt from the moderately dirty to the superclean regime

We systematically measure and analyze the spin diffusion length and the spin Hall effect in Pt with a wide range of conductivities using the spin absorption method in lateral spin valve devices. We observe a linear relation between the spin diffusion length and the conductivity, evidencing that the spin relaxation in Pt is governed by the Elliott-Yafet mechanism. We find a single intrinsic spin Hall conductivity ($\sigma_{SH}^{int}=1600\pm150\: \Omega^{-1}cm^{-1}$) for Pt in the full range studied which is in good agreement with theory. For the first time we have obtained the crossover between the moderately dirty and the superclean scaling regimes of the spin Hall effect by tuning the conductivity. This is equivalent to that obtained for the anomalous Hall effect. Our results explain the spread of the spin Hall angle values in the literature and find a route to maximize this important parameter.

Non-Local Lateral Spin-Valve Devices Fabricated With a Versatile Top-Down Fabrication Process

We report a method that uses top-down techniques for fabrication of non-local lateral spin-valve devices. Using this process, we demonstrate the fabrication of non-local lateral spin valves with Cu channels and Co nanopillar structures down to 75 nm × 100 nm. This will provide an appealing, cost-effective approach for industrial applications and allow for high control over material interface properties and device design. The nanopillar structures are essential to the scalability of devices, which is required for magnetic read head and logic applications to be competitive with current technologies.

Knight shift and nuclear spin relaxation inFe/n-GaAs heterostructures

We investigate the dynamically polarized nuclear-spin system in Fe/\emph{n}-GaAs heterostructures using the response of the electron-spin system to nuclear magnetic resonance (NMR) in lateral spin-valve devices. The hyperfine interaction is known to act more strongly on donor-bound electron states than on those in the conduction band. We provide a quantitative model of the temperature dependence of the occupation of donor sites. With this model we calculate the ratios of the hyperfine and quadrupolar nuclear relaxation rates of each isotope. For all temperatures measured, quadrupolar relaxation limits the spatial extent of nuclear spin-polarization to within a Bohr radius of the donor sites and is directly responsible for the isotope dependence of the measured NMR signal amplitude. The hyperfine interaction is also responsible for the $2\text{ kHz}$ Knight shift of the nuclear resonance frequency that is measured as a function of the electron spin accumulation. The Knight shift is shown to provide a measurement of the electron spin-polarization that agrees qualitatively with standard spin transport measurements.

Spin-related thermoelectric conversion in lateral spin-valve devices with single-crystalline Co2FeSi electrodes

We demonstrate the conversion between a heat current and a spin current in Cu-based lateral spin valves (LSVs) with single-crystalline Co2FeSi (CFS) electrodes. We can observe the thermally induced spin injection from CFS into Cu resulting from the spin-dependent Seebeck effect, and the heat current generated by the spin-dependent Peltier effect can be detected even in the LSV structures. This study is an important step toward understanding heat-spin conversion in single-crystalline materials with various electronic band structures.