Non-local Spin Valve Structures Research Articles

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.

Homoepitaxial graphene tunnel barriers for spin transport

Tunnel barriers are key elements for both charge-and spin-based electronics, offering devices with reduced power consumption and new paradigms for information processing. Such devices require mating dissimilar materials, raising issues of heteroepitaxy, interface stability, and electronic states that severely complicate fabrication and compromise performance. Graphene is the perfect tunnel barrier. It is an insulator out-of-plane, possesses a defect-free, linear habit, and is impervious to interdiffusion. Nonetheless, true tunneling between two stacked graphene layers is not possible in environmental conditions usable for electronics applications. However, two stacked graphene layers can be decoupled using chemical functionalization. Here, we demonstrate that hydrogenation or fluorination of graphene can be used to create a tunnel barrier. We demonstrate successful tunneling by measuring non-linear IV curves and a weakly temperature dependent zero-bias resistance. We demonstrate lateral transport of spin currents in non-local spin-valve structures, and determine spin lifetimes with the non-local Hanle effect. We compare the results for hydrogenated and fluorinated tunnel and we discuss the possibility that ferromagnetic moments in the hydrogenated graphene tunnel barrier affect the spin transport of our devices.

Spin relaxation characteristics in Ag nanowire covered with various oxides

We have studied spin relaxation characteristics in a Ag nanowire covered with various oxide layers of Bi2O3, Al2O3, HfO2, MgO, or AgOx by using non-local spin valve structures. The spin-flip probability, a ratio of momentum relaxation time to spin relaxation time at 10 K, exhibits a gradual increase with an atomic number of the oxide constituent elements, Mg, Al, Ag, and Hf. Surprisingly, the Bi2O3 capping was found to increase the probability by an order of magnitude compared with other oxide layers. This finding suggests the presence of an additional spin relaxation mechanism such as Rashba effect at the Ag/Bi2O3 interface, which cannot be explained by the simple Elliott-Yafet mechanism via phonon, impurity, and surface scatterings. The Ag/Bi2O3 interface may provide functionality as a spin to charge interconversion layer.

Contact-induced charge contributions to non-local spin transport measurements in Co/MgO/graphene devices

Recently, it has been shown that oxide barriers in graphene-based non-local spin-valve structures can be the bottleneck for spin transport. The barriers may cause spin dephasing during or right after electrical spin injection which limits spin transport parameters such as the spin lifetime of the whole device. An important task is to characterize the quality of the oxide barriers of both spin injection and detection contacts in a fabricated device. To address this issue, we discuss the influence of spatially inhomogeneous oxide barriers and especially conducting pinholes within the barriers on the background signal in non-local measurements of graphene/MgO/Co spin-valve devices. By both simulations and reference measurements on devices with non-ferromagnetic electrodes, we demonstrate that the background signal can be caused by an inhomogeneous current flow through the oxide barriers. As a main result, we demonstrate the existence of charge accumulation besides the actual spin accumulation signal in non-local voltage measurements, which can be explained by a redistribution of charge carriers by a perpendicular magnetic field similar to the classical Hall effect. Furthermore, we present systematic studies of the phase of the low frequency non-local ac voltage signal which is measured in non-local spin measurements when applying ac lock-in techniques. This phase has so far widely been neglected in the analysis of non-local spin transport. We demonstrate that this phase is another hallmark of the homogeneity of the MgO spin injection and detection barriers. We link backgate dependent changes of the phase to the interplay between the capacitance of the oxide barrier and the quantum capacitance of graphene.

Homoepitaxial tunnel barriers with functionalized graphene-on-graphene for charge and spin transport

The coupled imperatives for reduced heat dissipation and power consumption in high-density electronics have rekindled interest in devices based on tunnelling. Such devices require mating dissimilar materials, raising issues of heteroepitaxy, layer uniformity, interface stability and electronic states that severely complicate fabrication and compromise performance. Two-dimensional materials such as graphene obviate these issues and offer a new paradigm for tunnel barriers. Here we demonstrate a homoepitaxial tunnel barrier structure in which graphene serves as both the tunnel barrier and the high-mobility transport channel. We fluorinate the top layer of a graphene bilayer to decouple it from the bottom layer, so that it serves as a single-monolayer tunnel barrier for both charge and spin injection into the lower graphene channel. We demonstrate high spin injection efficiency with a tunnelling spin polarization >60%, lateral transport of spin currents in non-local spin-valve structures and determine spin lifetimes with the Hanle effect.

Calculation of Nonlocal Spin Transfer Torque

We microscopically derive the spin-transfer torque due to diffusive spin current arising from nonequilibrium spin accumulation as realized in systems with nonlocal spin valve structure. Spin relaxation effects in spin transport is taken into account by introducing magnetic impurities.

Nonvolatile multiple-valued memory device using lateral spin valve

The authors propose a nonvolatile multiple-valued memory based on a nonlocal spin valve structure. Multibit informations are formed by changing the magnetization configuration in a nonlocal voltage probe consisting of a magnetic multilayer. A simple calculation method for the spin-accumulation voltage induced in lateral ferromagnetic/nonmagnetic multilayered hybrid structures is also developed on the basis of the spin resistance model. The developed model enables us to find the thickness of each ferromagnetic layer for the optimized operation of the multiple-valued memory.

Domain-Wall Depinning Assisted by Pure Spin Currents

We study the depinning of domain walls by pure diffusive spin currents in a nonlocal spin valve structure based on two ferromagnetic Permalloy elements with copper as the nonmagnetic spin conduit. The injected spin current is absorbed by the second Permalloy structure with a domain wall, and from the dependence of the wall depinning field on the spin current density we find an efficiency of 6×10{-14} T/(A/m{2}), which is more than an order of magnitude larger than for conventional current induced domain-wall motion. Theoretically we find that this high efficiency arises from the surface torques exerted by the absorbed spin current that lead to efficient depinning.

Suppression of spin accumulation in nonmagnet due to ferromagnetic ohmic contact

We experimentally demonstrate that the ohmic contact of a Ni–Fe wire additionally connected to a Cu strip between an injector and detector in a nonlocal spin-valve structure signicantly suppresses the spin polarization induced in the Cu strip. This behavior is attributable to spin current absorption into the connected additional Ni–Fe wire.