Nonlocal Spin Valve Signal Research Articles

Spin injection and detection in MnAs/GaAs/InAs hybrid system on GaAs(111)B through lateral non-local spin valve measurement at varied temperature

We successfully confirmed spin injection and detection in the MnAs/GaAs/InAs hybrid system on GaAs(111)B through lateral nonlocal spin valve (NLSV) measurement at 1.5 K and 77 K. Surprisingly, we found larger NLSV signals at 77 K than that at 1.5 K. This seems to be interesting behavior compared to typical temperature dependent spin injection and detection study. We studied in depth the NLSV signals to extract spin parameters such as spin diffusion length and spin injection efficiency in the hybrid system. We found spin diffusion length to be ∼10 µm and ∼7 µm at 1.5 K and 77 K, respectively, and also found spin injection efficiency to be ∼1.6% and ∼2.5% at 1.5 K and 77 K, respectively. The reason behind higher injection efficiency at 77 K comes from better impedance matching between MnAs and InAs at 77 K than that at 1.5 K due to large temperature variation of MnAs resistivity.

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.

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.

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.

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.

Control of spin current by a magnetic YIG substrate in NiFe/Al nonlocal spin valves

We study the effect of a magnetic insulator [yttrium iron garnet (YIG)] substrate on the spin-transport properties of ${\mathrm{Ni}}_{80}{\mathrm{Fe}}_{20}/\mathrm{Al}$ nonlocal spin valve (NLSV) devices. The NLSV signal on the YIG substrate is about two to three times lower than that on a nonmagnetic ${\mathrm{SiO}}_{2}$ substrate, indicating that a significant fraction of the spin current is absorbed at the Al/YIG interface. By measuring the NLSV signal for varying injector-to-detector distances and using a three-dimensional spin-transport model that takes spin-current absorption at the Al/YIG interface into account, we obtain an effective spin-mixing conductance ${G}_{\ensuremath{\uparrow}\ensuremath{\downarrow}}\ensuremath{\simeq}5--8\ifmmode\times\else\texttimes\fi{}{10}^{13}\phantom{\rule{0.28em}{0ex}}{\ensuremath{\Omega}}^{\ensuremath{-}1}\phantom{\rule{0.16em}{0ex}}{\mathrm{m}}^{\ensuremath{-}2}$. We also observe a small, but clear, modulation of the NLSV signal when rotating the YIG magnetization direction with respect to the fixed spin polarization of the spin accumulation in the Al. Spin relaxation due to thermal magnons or roughness of the YIG surface may be responsible for the observed small modulation of the NLSV signal.

Electrical spin injection into high mobility 2D systems.

We report on spin injection into a high mobility 2D electron system confined at an (Al,Ga)As/GaAs interface, using (Ga,Mn)As Esaki diode contacts as spin aligners. We measured a clear nonlocal spin valve signal, which varies nonmonotonically with the applied bias voltage. The magnitude of the signal cannot be described by the standard spin drift-diffusion model, because at maximum this would require the spin polarization of the injected current to be much larger than 100%, which is unphysical. A strong correlation of the spin signal with contact width and electron mean free path suggests that ballistic transport in the 2D region below ferromagnetic contacts should be taken into account to fully describe the results.

Significant modulation of electrical spin accumulation by efficient thermal spin injection

We have investigated the dc bias current dependence of the nonlocal spin valve signal in CoFeAl/Cu lateral spin valves. The spin signal is found to increase monotonically with the bias current. Surprisingly, the modulation amplitude from $\ensuremath{-}1$ mA to 1 mA exceeds 30 percent of the spin signal at low bias current. From the analysis based on the one-dimensional spin diffusion model and considering the bias-current heating effect, we find that the contribution of the thermal spin injection is much larger than the influence of the reduction of the spin diffusion length due to the Joule heating. We also show that the second harmonic lock-in signal precisely extracts the contribution of the thermal spin injection from the mixed spin signal.

Spin injection and detection in In0.53Ga0.47As nanomembrane channels transferred onto Si substrates

We investigate the electrical spin injection and detection in In0.53Ga0.47As nanomembranes, which are originally grown on InP substrates and subsequently heterogeneously integrated on SiO2/Si substrates via a transfer printing technique. Through local and nonlocal spin valve measurements employing the In0.53Ga0.47As nanomembrane channels on SiO2/Si substrates, we successfully observe the electrical detection of spin injection from Ni81Fe19 ferromagnetic metal electrodes into the channels. Furthermore, nonlocal spin valve signals are detected up to T = 300 K without mixing with anisotropic magnetoresistance, which is evidently verified by observing a memory effect.

Electrical spin injection from ferromagnet into an InAs quantum well through a MgO tunnel barrier

The electrical spin injection from Ni81Fe19 (NiFe) into an InAs quantum well through a MgO tunneling barrier has been investigated for potential application to an InAs-based spin field-effect transistor. The insertion of a 2-nm-thick MgO tunnel barrier between NiFe and InAs increased the junction resistance by two orders of magnitude compared with that without a MgO barrier. The sample with a MgO barrier showed a clear nonlocal spin-valve signal at 1.4 K, possibly due to the alleviation of the impedance mismatching problem. The estimated spin polarization was 8.1%, which is higher than any reported in the literature for a NiFe/InGaAs Schottky junction.

Effect of CoFe insertion in Co2MnSi/CoFe/n-GaAs junctions on spin injection properties

The CoFe thickness (tCoFe) dependence of spin injection efficiency was investigated for Co2MnSi/CoFe/n-GaAs junctions. The ΔVNL/I value, which is a measure of spin injection efficiency, strongly depended on tCoFe, where ΔVNL is the amplitude of a nonlocal spin-valve signal, and I is an injection current. Importantly, the maximum value of ΔVNL/I for a Co2MnSi/CoFe/n-GaAs junction was one order of magnitude higher than that for a CoFe/n-GaAs junction, indicating that a Co2MnSi electrode works as a highly polarized spin source. No clear spin signal, on the other hand, was observed for a Co2MnSi/n-GaAs junction due to diffusion of Mn atoms into the GaAs channel. Secondary ion mass spectrometry analysis indicated that the CoFe insertion effectively suppressed the diffusion of Mn into GaAs, resulting in improved spin injection properties compared with those for a Co2MnSi/n-GaAs junction.

Qualitative study of temperature-dependent spin signals in n-Ge-based lateral devices with Fe3Si/n+-Ge Schottky-tunnel contacts

We study electrical spin injection and detection in n-Ge across Fe3Si/n+-Ge Schottky tunnel barriers. Spin-accumulation signals detected electrically by the three-terminal Hanle-effect measurements have large temperature dependence, and the spin signals disappear at around 200 K. We find that the temperature variation in the spin signals is strongly related to that in the interface resistance of the Fe3Si/n+-Ge contacts. We also observe marked reduction in nonlocal spin-valve signals with increasing temperature in the four-terminal device with almost the same Fe3Si/n+-Ge/n-Ge heterostructure. Note that the nonlocal spin signals depend on not only the measurement temperature but also on the electrical characteristics of the spin injector. Considering the results of three-terminal and four-terminal measurements, we conclude that temperature dependence of the spin signals in our Fe3Si/n+-Ge/n-Ge devices is governed by the electrical characteristics of the spin injector. This study means that it is necessary to consider the effective Schottky-tunnel contacts with the stability on temperature variations for next-generation Ge-based spintronic applications.

Magnetic field dependence of non-local lateral spin-valve signals beyond the Hanle effect

We present a theoretical model of spin transport in metallic lateral valves that takes into account spin scattering on magnetic impurities. We show that the model agrees with recent experimental findings of increasing non-local spin signals by in-plane magnetic field, which is parallel to the injected spins. The increase arises due to reduction of conduction electron spin flips on magnetic impurities present at the metal–ferromagnet interfaces as they freeze out under application of the magnetic field.

Electrical Spin Injection and Detection in Silicon Nanowires through Oxide Tunnel Barriers

We demonstrate all-electrical spin injection, transport, and detection in heavily n-type-doped Si nanowires using ferromagnetic Co/Al(2)O(3) tunnel barrier contacts. Analysis of both local and nonlocal spin valve signals at 4 K on the same nanowire device using a standard spin-transport model suggests that high spin injection efficiency (up to ~30%) and long spin diffusion lengths (up to ~6 μm) are achieved. These values exceed those reported for spin transport devices based on comparably doped bulk Si. The spin valve signals are found to be strongly bias and temperature dependent and can invert sign with changes in the dc bias current. The influence of the nanowire morphology on field-dependent switching of the contacts is also discussed. Owing to their nanoscale geometry, ~5 orders of magnitude less current is required to achieve nonlocal spin valve voltages comparable to those attained in planar microscale spin transport devices, suggesting lower power consumption and the potential for applications of Si nanowires in nanospintronics.

Spin injection properties in trilayer graphene lateral spin valves

We report on the electrical injection and detection of spin accumulation in trilayer-graphene/MgO/Permalloy lateral spin-valve (LSV) structure. Non-local spin valve signal is clearly observed in the LSV, indicating that spin coherence extends underneath all ferromagnetic contacts. We also show that low-resistivity graphene/MgO/Py junctions enable efficient spin injection and detection in LSV with high applied current density, which leads to large spin accumulation of 120 μV at room temperature. A spin diffusion length of 1.5 μm was obtained for the injector-detector separation dependence of spin valve signal measurements carried out at room temperature, while at T = 10 K, the diffusion length increases to 2.3 μm.

Estimation of the spin polarization for Heusler-compound thin films by means of nonlocal spin-valve measurements: Comparison of Co2FeSi and Fe3Si

We study room-temperature generation and detection of pure spin currents using lateral spin-valve devices with electrodes formed from the Heusler compounds Co${}_{2}$FeSi (CFS) or Fe${}_{3}$Si (FS). The magnitude of the nonlocal spin-valve signals is strongly affected by resistivity variations observed particularly in low-temperature-grown Heusler compounds containing ordered structures. From an analysis based on a one-dimensional spin diffusion model, we find that the spin polarization monotonically increases with decreasing resistivity, which depends on the structural ordering, for both the CFS and FS electrodes, and show that CFS has a larger spin polarization than FS.

Chirality control of magnetic vortex in a square Py dot using current-induced Oersted field

We have proposed a method for controlling the vortex chirality in a squared permalloy dot by using the circular Oersted field locally induced by flowing a DC current across a small Py/Cu junctions. The reliability of the chirality control has been evaluated by measuring the nonlocal spin valve signal. The desired vortex chirality has been obtained when the injecting DC current has a moderate magnitude. However, the large DC current is found to reduce the control reliability. Another possibility for controlling the vortex structure using the large DC current injection was also discussed.

Enhanced spin signals due to native oxide formation in Ni80Fe20/Ag lateral spin valves

Large nonlocal spin valve signals are reported in mesoscopic Ni80Fe20/Ag lateral spin valves upon exposing them to air. Magnetotransport measurements combined with transmission electron microscopy show that the formation of a native oxide layer at the Ni80Fe20/Ag interface is responsible for the large signals. The results indicate that lateral spin valves with superior performance to those based on high-resistance tunnel barriers can be achieved via controllable growth of native permalloy oxides.