Nonlocal Spin Valve Research Articles

Oblique spin injection to graphene via geometry controlled magnetic nanowires

We exploit the geometry of magnetic nanowires, which define 1D contacts to an encapsulated graphene channel, to introduce an out-of-plane component in the polarisation of spin carriers. By design, the magnetic nanowires traverse the angled sides of the 2D material heterostructure. Consequently, the easy axis of the nanowires is inclined, and so the local magnetisation is oblique at the injection point. As a result, when performing non-local spin valve measurements we simultaneously observe both switching and spin precession phenomena, implying the spin population possesses both in-plane and out-of-plane polarisation components. By comparing the relative magnitudes of these components, we quantify the angle of the total spin polarisation vector. The extracted angle is consistent with the angle of the nanowire at the graphene interface, evidencing that the effect is a consequence of the device geometry. This simple method of spin-based vector magnetometry provides an alternative technique to define the spin polarisation in 2D spintronic devices.

Impact of material and tunnel barrier quality on spin transport in a CVD graphene non-local spin valve device array

Wafer-scale graphene films produced via chemical vapor deposition (CVD) are now commercially available, however these films inherently contain randomly distributed defects such as adlayers and grain boundaries. This report discusses the impact of these defects on the signal integrity of an array of graphene-based non-local spin valves (NLSVs). It was found that critical spin parameters fluctuate drastically between adjacent identical devices. Investigation of the channel quality indicated that adlayers do not affect spin signal significantly even when located directly in the spin transport region of the device. In contrast, grain boundary defects within the spin transport region have significant impact on spin signal. Poor tunnel barrier integrity due to residue from the fabrication process also remains a dominant factor driving device variability.

Spin injection into heavily-doped n-GaN via Schottky barrier

Spin injection and detection in bulk GaN were investigated by performing magnetotransport measurements at low temperatures. A non-local four-terminal lateral spin valve device was fabricated with Co/GaN Schottky contacts. The spin injection efficiency of 21% was achieved at 1.7 K. It was confirmed that the thin Schottky barrier formed between the heavily n-doped GaN and Co was conducive to the direct spin tunneling, by reducing the spin scattering relaxation through the interface states.

High spin polarization and spin signal enhancement in non-local spin valves with Co–Fe alloy injectors and detectors

For applications such as spin accumulation sensors for next-generation hard disk drive read heads, and for fundamental research, it is desirable to increase the spin signal in metallic non-local spin valves, which are central devices in spintronics. To this end, here, we report on the integration of high-spin-polarization Co–Fe binary alloy ferromagnetic injectors and detectors in Al-based non-local spin valves. Room-temperature deposition on amorphous substrates from an alloy target is shown to generate smooth, polycrystalline (110-textured), solid-solution body-centered-cubic Co75Fe25 films, which we characterize by energy dispersive x-ray spectroscopy, x-ray diffraction, x-ray reflectivity, atomic force microscopy, and electronic transport. Simple integration into transparent-interface Al non-local spin valves is then shown to realize up to a factor of ∼5 enhancement of the spin signal relative to Co, with full quantitative analysis yielding strikingly temperature-independent current spin polarizations exceeding 60%. We make a detailed quantitative comparison of these values with prior literature, concluding that Co–Fe alloys present a remarkably facile route to higher spin polarization and spin signals in non-local spin valves, with minimal barrier to adoption.

High-efficient and gate-tunable spin transport in GaN thin film at room temperature

The emerging semiconductor spintronics has offered a practical routine for developing high-speed and energy-efficient electronic and optoelectronic devices. GaN holds broad prospects for room-temperature spintronic applications due to its weak spin scattering and moderate spin–orbit coupling. However, the development of GaN-based spintronic devices is still hindered by the relatively low spin injection efficiency and gate controllability. In this study, gate-modulated spin transport was achieved in a highly spin-polarized GaN-based non-local spin valve. A maximum spin diffusion length of 510 nm and a high spin polarization of 14.1% was obtained with the CoFeB/MgO tunnel spin injector. By applying gate voltages from −3 to +3 V, the spin-dependent magnetoresistance can be tuned in the range of 1.6–3.9 Ω. The modulation is attributed to the controllable spin relaxation of electrons by the gate electric field. This work has demonstrated high spin polarization and exceptional electric controllability in GaN, pushing forward the research in spin field-effect transistors.

Nonlocal Spin Valves Based on Graphene/Fe3GeTe2 van der Waals Heterostructures.

With recent advances in two-dimensional (2D) ferromagnets with enhanced Curie temperatures, it is possible to develop all-2D spintronic devices with high-quality interfaces using 2D ferromagnets. In this study, we have successfully fabricated nonlocal spin valves with Fe3GeTe2 (FGT) as the spin source and detector and multilayer graphene as the spin transport channel. The nonlocal spin transport signal was found to strongly depend on temperature and disappear at a temperature below the Curie temperature of the FGT flakes, which stemmed from the temperature-dependent ferromagnetism of FGT. The spin injection efficiency was estimated to be about 1%, close to that of conventional nonlocal spin valves with transparent contacts between ferromagnetic electrodes and the graphene channel. In addition, the spin transport signal was found to depend on the direction of the magnetic field and the magnitude of the current, which was due to the strong perpendicular magnetic anisotropy of FGT and the thermal effect, respectively. Our results provide opportunities to extend the applications of van der Waals heterostructures in spintronic devices.

Magnetic control of heat generation in a magnetic tunnel contact with spin accumulation

It is described how the heat generation by an electrical current in a ferromagnetic tunnel contact to a nonmagnetic material depends on the spin accumulation in the nonmagnetic electrode for both Joule and Peltier heating. This enables the control of the heating in the tunnel contact by an external applied magnetic field that induces spin precession of the spin accumulation. Thereby, a Hanle type of spin signal is imprinted in the heating power. We derive expressions for the magnitude and sign of the magnetic-field-dependent heating power for Joule and Peltier heating, and discuss the important parameters. For contacts with small resistance-area product, the spin accumulation provides the dominant contribution to the Joule heating power, which can therefore be modulated by more than a factor of 2 with a magnetic field. The described phenomenon can also produce genuine spin signals in various devices, including nonlocal lateral spin valves.

High L21-atomic ordering and spin-polarization in Co2MnZ (Z = Ge, Sn) Heusler thin films with low-temperature annealing process

Although an enhanced magnetoresistance (MR) has been observed in many Co2-based Heusler alloys by promoting their structural ordering from B2 to L21 by post-annealing at higher temperatures (Tann > 500 °C), it is desirable to search for other Heusler alloys that crystallize in L21-order below 300 °C, as the maximum Tann is restricted for processing devices. For Co2MnZ (Z = Ge, Sn) Heusler alloys, an L21-order is expected to appear even in the as-deposited state or by a low-temperature annealing process due to their very high L21 to B2-order transition temperature (>1500 K). Here, epitaxial Co2MnZ films were grown on MgO (001) substrate at room temperature (RT) and post-annealed at Tann = 200–500 °C. Interestingly, as-sputtered films exhibit L21-ordering, which improves systematically upon increasing Tann. The spin-polarization of electric current (β) was estimated at RT using nonlocal spin-valve (NLSV) devices by measuring the spin-accumulation signal in a copper (Cu) channel. It was found that at Tann = 300 °C, the β value of Co2MnGe films is higher (∼0.65) than that of Co2FeGe0.5Ga0.5 films due to a higher degree of L21-order, which makes the Co2MnGe alloy a promising ferromagnetic electrode for spintronic device applications.

Determination of spin Hall angle in the Weyl ferromagnet Co2MnGa by taking into account the thermoelectric contributions

The spin Hall effects of various materials, including Weyl semimetals, have been studied by the spin absorption method with the nonlocal spin valve structures. Here, we study ${\mathrm{Co}}_{2}\mathrm{MnGa}$ (CMG) by using the standard spin absorption method. A considerable amount of thermoelectric signal superimposes on the inverse spin Hall signal in the measurement configuration: the applied electric current between the spin injector (permalloy) and bridge (copper) wires produces heating or cooling at the interface via the Peltier effect, which causes an out-of-plane temperature gradient in CMG. As a result, a voltage signal induced by the anomalous Nernst effect (ANE) of CMG comes into the inverse spin Hall signal. We quantitatively separate these signals by combining the experiment and a numerical simulation. About 75% of the detected signal is found attributable to the thermoelectric effects. The superposing thermoelectric contribution is a general and unavoidable problem in this method when the spin Hall materials exhibit ANE. After eliminating the thermoelectric signal, the spin Hall angle of CMG turns out to be $\ensuremath{\theta}=\ensuremath{-}0.09$ at room temperature.

Spin relaxation in copper channels with submicron cross sections

We review our recent experimental studies of the spin relaxation processes in the submicron copper channels of nonlocal spin valves (NLSV). Long spin relaxation lengths up to 3.0 μm at low temperatures are demonstrated in highly conductive copper channels with residual resistivity ratios up to 6. An unexpected anisotropy of spin relaxation is observed and can be attributed to the surface spin–orbit effects. An unusual scaling of the Kondo spin relaxation is established experimentally and can be explained by overlapping Kondo screening clouds. Our results demonstrate the technological potential of copper as a spin transport channel in spintronics and highlights unanswered fundamental questions related to the spin relaxation processes.

Tuning of spin relaxation and the Kondo effect in copper thin films by ionic gating

Spin relaxation length is a fundamental material parameter that influences all aspects of spin dependent transport. The ability to tune the spin relaxation length leads to novel spintronic phenomena and functionalities. We explore the tunability of the spin relaxation length in the mesoscopic Cu channels of nonlocal spin valves by using the ionic gating technique via a ${\mathrm{Li}}^{+}$ containing solid polymer electrolyte. At 5 K, the Cu spin relaxation length ${\ensuremath{\lambda}}_{\mathrm{Cu}}$ is tuned reversibly between 670 and 410 nm and the Cu resistivity ${\ensuremath{\rho}}_{\mathrm{Cu}}$ is tuned by 9%. The strength of the Kondo effect due to the Fe impurities in Cu is tuned by one order of magnitude. At 295 K, ${\ensuremath{\lambda}}_{\mathrm{Cu}}$ is tuned between 380 and 300 nm and ${\ensuremath{\rho}}_{\mathrm{Cu}}$ is tuned by 7%. A gradual amplification of the tuning ranges by repeated gate cycling is observed and clearly suggests an electrochemical origin. Tunable spin relaxation in simple metals enriches functionalities in metal-based spintronics and shines light on fundamental spin relaxation mechanisms.

Tuning the spintronic properties of graphene with atomically precise Au clusters

Spin relaxation is investigated in lateral nonlocal graphene spin valves with increasing densities of soft-landed Au3 and Au6 clusters. It is found that both gold clusters scatter spins via the Elliot–Yafet mechanism. The induced spin–orbit coupling strength is a few meV for both clusters, with the value for Au3 being roughly twice as large as that of Au6. A gradual increase of the deposited cluster density (up to clusters cm−2) decreases the spin and momentum lifetime of the graphene channel, with Au6 clusters affecting both spin and momentum lifetime more strongly than the Au3 clusters. Density functional theory calculations provide insights into the spin relaxation mechanism. The dependence of graphene’s electronic and spintronic properties on the density and the exact cluster size indicates the importance of the microscopic details for graphene functionalisation towards spintronic applications.

Scaling of Kondo spin relaxation: Experiments on Cu-based nonlocal spin valves with Fe impurities

The relation between the Kondo spin-relaxation rate ${{\ensuremath{\tau}}_{sK}}^{\ensuremath{-}1}$ and the Kondo momentum-relaxation rate ${{\ensuremath{\tau}}_{eK}}^{\ensuremath{-}1}$ is explored by using nonlocal spin valves with submicron copper channels that contain dilute iron impurities. A linear relation between ${{\ensuremath{\tau}}_{sK}}^{\ensuremath{-}1}$ and ${{\ensuremath{\tau}}_{eK}}^{\ensuremath{-}1}$ is established under varying temperatures for any given device. Among 20 devices, however, ${{\ensuremath{\tau}}_{sK}}^{\ensuremath{-}1}$ remains nearly constant, despite variation of ${{\ensuremath{\tau}}_{eK}}^{\ensuremath{-}1}$ by a factor of 10. This surprising relation can be understood by considering spin relaxation through overlapping Kondo screening clouds and supports the physical existence of the elusive Kondo clouds.

Origin of the magnetic field enhancement of the spin signal in metallic nonlocal spin transport devices

The nonlocal spin valve (NLSV) enables unambiguous study of spin transport, owing to its ability to isolate pure spin currents. A key principle of NLSV operation is that the ``spin signal'' is invariant under application of in-plane magnetic fields (above the ferromagnetic contact saturation field). Yet, for certain ferromagnet/normal metal pairings in NLSVs, an unexpected field enhancement of the spin signal occurs, presenting a challenge that has, thus far, been difficult to resolve with existing models. By correlating the extracted spin transport parameters with material, temperature, and field dependencies, in this work we identify field quenching of magnetic impurity scattering as the origin of this effect, confirmed by excellent agreement between our results and field-dependent Kondo theory. In addition to addressing this long-standing mystery, our findings highlight a potential systematic underestimation of spin transport parameters. By identifying signature field and temperature dependencies, we provide here a relatively simple means to isolate and quantify this additional relaxation mechanism.

Precise electrical detection of the field and current-induced switching mode of a magnetic nanodisk in a non-local spin valve

In this work, we study the manipulation and the detection of the magnetic state and switching modes of a Permalloy disk, using a non-local spin-valve with dual CoFe injectors and with the disk acting as the detector electrode. Applying simultaneously external magnetic fields and spin currents from the injectors, we demonstrate the possibility to use the spin torque created by the spin current to modulate the switching modes and switching fields of the disk, following the disentanglement of the influence of Joule heating. Moreover, by engineering the device to have a very small overlap of the spin conduit with one side of the disk, we demonstrate how, with the help of a first-order reversal curve analysis, we can extract the precise details of the switching from the monodomain to the vortex state, including the chirality of the system without the need for direct imaging.

Room-Temperature Spin Transport in Cd3As2.

As the need for ever greater transistor density increases, the commensurate decrease in device size approaches the atomic limit, leading to increased energy loss and leakage currents, reducing energy efficiencies. Alternative state variables, such as electronic spin rather than electronic charge, have the potential to enable more energy-efficient and higher performance devices. These spintronic devices require materials capable of efficiently harnessing the electron spin. Here we show robust spin transport in Cd3As2 films up to room temperature. We demonstrate a nonlocal spin valve switch from this material, as well as inverse spin Hall effect measurements yielding spin Hall angles as high as θSH = 1.5 and spin diffusion lengths of 10-40 μm. Long spin-coherence lengths with efficient charge-to-spin conversion rates and coherent spin transport up to room temperature, as we show here in Cd3As2, are enabling steps toward realizing actual spintronic devices.

Spin filtering in germanium/silicon core/shell nanowires with pseudo-helical gap

Semiconductors with strong spin–orbit interactions can exhibit a helical gap with spin-momentum locking opened by a magnetic field. Such a gap is highly spin selective as a result of a topologically protected spin-momentum locking, which can be used for spin filtering. We experimentally demonstrate such a spin filtering effect in a quasi-ballistic p-type germanium/silicon core/shell nanowire (NW), which possesses a pseudo-helical gap without the application of magnetic field. Polarized hole spin injection to the NW is achieved using cobalt ferromagnetic contacts with controlled natural surface oxide on the NW as a tunnel barrier. Local and nonlocal spin valve effects are measured as the verification of polarized spin transport in the NW outside the helical gap. We electrically tune the NW into the helical gap by scanning its chemical potential with a gate. A hysteresis loop with three resistance states is observed in the local spin valve geometry, as an evidence of spin filtering in the helical gap.

Temperature dependence of the anomalous Nernst coefficient for Ni80Fe20 determined with metallic nonlocal spin valves

The anomalous Nernst effect, which generates an out-of-plane charge voltage in response to a thermal gradient perpendicular to the magnetization of a ferromagnet, can play a significant role in many spintronic devices where large thermal gradients exist. Since they typically include features deep within the submicron regime, nonlocal spin valves can be made very sensitive to this effect by lowering the substrate thermal conductance. Here, we use nonlocal spin valves suspended on thin silicon nitride membranes to determine the temperature dependence of the anomalous Nernst coefficient of 35 nm thick permalloy (Ni80Fe20) from 78 K to 300 K. In a device with a simple ferromagnet geometry, the transverse Seebeck coefficient shows a weak temperature dependence, with values at all T near 2.5 μV/K. Assuming previously measured values of the Seebeck coefficient for permalloy, which has a near-linear dependence on T, leads to a low temperature upturn in the anomalous Nernst coefficient RN. We also show that the temperature dependence of this coefficient is different when a constricted nanowire is used as the ferromagnetic detector element.

Nonlocal Spin Dynamics in the Crossover from Diffusive to Ballistic Transport.

Improved fabrication techniques have enabled the possibility of ballistic transport and unprecedented spin manipulation in ultraclean graphene devices. Spin transport in graphene is typically probed in a nonlocal spin valve and is analyzed using spin diffusion theory, but this theory is not necessarily applicable when charge transport becomes ballistic or when the spin diffusion length is exceptionally long. Here, we study these regimes by performing quantum simulations of graphene nonlocal spin valves. We find that conventional spin diffusion theory fails to capture the crossover to the ballistic regime as well as the limit of long spin diffusion length. We show that the latter can be described by an extension of the current theoretical framework. Finally, by covering the whole range of spin dynamics, our study opens a new perspective to predict and scrutinize spin transport in graphene and other two-dimensional material-based ultraclean devices.

Modulating room temperature spin injection into GaN towards the high-efficiency spin-light emitting diodes

Spin injection performance in GaN film was systematically investigated through the three-terminal Hanle spin precession measurements with the comparison of varied tunnel barrier thickness, spin injector materials, and channel widths. Spin relaxation time and diffusion length were optimized to 37.3 ps and 139.0 nm at room temperature, respectively. With the optimal spin injector, a 12% spin polarization was obtained in a four-terminal non-local spin valve device. By applying the optimized spin injector structure to the spin-light emitting diodes, room temperature circular polarization ratios of 6.2% and 9.2% for Co and Fe spin polarizers were respectively achieved at surface-emission.