Nonlocal Spin Signal Research Articles

Observation and implications of magnetic domains in lateral spin valves

Co/Cu/Co lateral spin valves (LSV), with Co being the topmost layer, are in situ prepared and measured under ultrahigh vacuum conditions. The clean process yields a non-local spin signal of 0.9 mΩ. Scanning electron microscopy with polarization analysis (SEMPA) reveals domain structures in both magnetic electrodes that depend on the LSV dimensions. The spin signal correlates to SEMPA images as well as the anisotropic magnetoresistance of both Co magnets, revealing a strong impact of multi-domain states on the spin signal.

Nonlinear detection of spin currents in graphene with non-magnetic electrodes

The degree to which an electrical current is spin polarized is usually determined by how easily it travels across an interface with a magnetic contact. By using nonlinear interactions between spin and charge in graphene, the polarization of spin currents can be measured without magnetic contacts. The abilities to inject and detect spin carriers are fundamental for research on transport and manipulation of spin information1,2. Pure electronic spin currents have been recently studied in nanoscale electronic devices using a non-local lateral geometry, both in metallic systems 3 and in semiconductors4. To unlock the full potential of spintronics we must understand the interactions of spin with other degrees of freedom. Such interactions have been explored recently, for example, by using spin Hall5,6,7 or spin thermoelectric effects6,8,9. Here we present the detection of non-local spin signals using non-magnetic detectors, through an as-yet-unexplored nonlinear interaction between spin and charge. In analogy to the Seebeck effect10, where a heat current generates a charge potential, we demonstrate that a spin current in a paramagnet leads to a charge potential, if the conductivity is energy dependent. We use graphene11 as a model system to study this effect, as recently proposed12. The physical concept demonstrated here is generally valid, opening new possibilities for spintronics.

Reversal and excitations of a nanoscale magnetic domain by sustained pure spin currents

Spin-transfer effects induced by pure spin currents are explored in nonlocal spin valves by using sustained injection currents. Compared to pulsed injection currents used in previous experiments, this approach provides persistent spin-transfer torques and preserves the history of the reversal process. A nanoscale domain in a magnetic wire can be switched reversibly by the sustained pure spin currents. In addition, dips in nonlocal spin signal curves are observed at high magnetic fields for only one polarity of the injection currents. This indicates stable-state magnetization precession around the external field driven by the sustained pure spin currents.

Magnetic-field enhancement of nonlocal spin signal in Ni80Fe20/Ag lateral spin valves

We observe a magnetic-field-induced enhancement of the nonlocal spin signal in Ni{sub 80}Fe{sub 20}/Ag lateral spin valves. The enhancement depends on the bias current polarity but not on the field direction. We present a theoretical model that explains our experimental results, taking into account the electron-spin relaxation of magnetic impurities. We find that the relaxation is about an order of magnitude weaker than Elliott-Yafet relaxation.

Enhanced spin signal in nonlocal devices based on a ferromagnetic CoFeAl alloy

We systematically study the nonlocal spin signal in lateral spin valves based on CoFeAl injectors and detectors and compare the results with identically fabricated devices based on CoFe. The devices are fabricated by electron beam evaporation at room temperature. We observe a > 10-fold enhancement of the spin signal in the CoFeAl devices. We explain this increase as due to the formation of a highly spin-polarized Co2FeAl Heusler compound with large resistivity. These results suggest that Heusler compounds are promising candidates as spin polarized electrodes in lateral spin devices for future spintronic applications.

Fast electrical switching of spin injection in nonlocal spin transport devices

We present spin-injection experiments in a nonlocal spin transport device where spin is injected from a ferromagnetic FeCo electrode into a GaAs epilayer. The magnetization of the injection contact is switched by Oersted fields generated by alternating current pulses. This enables fast and offset-free measurements of nonlocal spin signals. Due to a negligible time-averaged electron spin polarization, dynamic nuclear polarization effects are small and Hanle curves measured down to T=3 K can be fit very accurately by drift-diffusion theory if a small constant Overhauser field BN=0.4 mT is accounted for.

Nonlocal spin valves with thin Co electrodes for spin-transfer

Cobalt/copper nonlocal spin valve devices with AlOx barriers are fabricated for the purpose of spin-transfer with a pure spin current. The submicron planar device consists of a Co spin injector (F1), a Co spin detector (F2), and a Cu channel. The F2 Co electrode is only 2.5 nm thick, but sizable nonlocal spin signals of 2–4 mΩ are measured at 4.2 K. The spin-transfer switching has been achieved with the assistance of a magnetic field. The magnetizations of F1 and F2 electrodes are set to an antiparallel state by sweeping the magnetic field. The magnetic field is then held at a value smaller but close to the switching field of F2. A small dc current (0.1–1 mA) is sufficient to switch F2 into being parallel with F1. Analysis has been done to rule out possible artifacts due to Oersted fields.

Manipulation of Spin Transport in Graphene by Surface Chemical Doping

The effects of surface chemical doping on spin transport in graphene are investigated by performing nonlocal measurements in ultrahigh vacuum while depositing gold adsorbates. We demonstrate manipulation of the gate-dependent nonlocal spin signal as a function of gold coverage. We discover that charged impurity scattering is not the dominant mechanism for spin relaxation in graphene, despite its importance for momentum scattering. Finally, unexpected enhancements of the spin lifetime illustrate the complex nature of spin relaxation in graphene.

Increase of nonlocal spin signal at high dc bias due to a redistribution of the injection current

The spin signals in nonlocal spin valves with 1.3 nm ${\text{AlO}}_{x}$ layers as spin-injection and spin-detection interfaces have been measured as a function of the dc bias current through the injector. Increases of the spin signals have been observed in both polarities of the injection current, with magnitudes up to 81%. This is attributed to a redistribution of the injection current due to a nonlinear resistivity at a higher bias. The baseline of the spin signal varies as a function of the bias current as well, and it is consistent with the interpretation of current redistribution. A temperature dependence of the spin signals has been conducted to demonstrate that the 81% increase of spin signal cannot be accounted for by the Joule heating of the device.

Electrical Detection of Spin Transport in Si Using High-quality Fe3Si/Si Schottky Tunnel Contacts

We investigated the epitaxial growth of high-quality Fe3Si thin films on Si(111) and demonstrated electrical injection and detection of spin-polarized electrons in Si through the Fe3Si/Si Schottky-tunnel contacts. TEM observations and analyses of electron diffraction patterns revealed that precisely controlling the Fe/Si ratio during growth is important to obtain well ordered Fe3Si structures and atomically abrupt heterointerfaces at the same time. The electrical properties of Fe3Si/Si Schottky diodes with various carrier concentrations were also examined, and highly transparent tunnel contacts were demonstrated. We were able to clearly detect nonlocal spin signals in Si at 150K using Fe3Si/Si lateral four-probe devices.

Signatures of dynamically polarized nuclear spins in all-electrical lateral spin transport devices

The effect of nuclear spins in Fe/GaAs all-electrical spin-injection devices is investigated. At temperatures below 50 K, strong modifications of the non-local spin signal are found that are characteristic for hyperfine coupling between conduction electrons and dynamically polarized nuclear spins. The perpendicular component of the nuclear Overhauser field depolarizes electron spins near zero in-plane external magnetic field, and can suppress such dephasing when antialigned with the external field, leading to satellite peaks in a Hanle measurement. The features observed agree well with a Monte Carlo simulation of the spin diffusion equation including hyperfine interaction, and are used to study the nuclear spin dynamics and relate it to the spin polarization of injected electrons.

Spin Injection and Detection in a Mesoscopic Superconductor at Low Temperatures

We theoretically study nonequilibrium spin transport in a superconducting wire connected by tunnel junctions to two ferromagnetic metal wires, each of which serves as an injector or detector of spin-polarized electron current. We present a set of Boltzmann equations to determine nonequilibrium quasiparticle distributions in this system, and obtain an analytical expression for the nonlocal spin signal in the case of small injection current. It is shown that the quasiparticle distribution in the ferromagnetic metal for detection strongly affects the magnitude of the spin signal. At low temperatures, since nonequilibrium quasiparticles created by the tunneling from the superconductor dominate thermally excited ones, the spin signal becomes independent of temperature. This explains the convergence of the spin signal with decreasing temperature observed in a recent experiment by Poli \textit{et al}.

Effect of ferromagnetic contacts on spin accumulation in an all-metallic lateral spin-valve system: Semiclassical spin drift-diffusion equations

We study the effect of the ferromagnetic (FM) contacts on the spin accumulation in the lateral spin-valve system for the collinear magnetization configurations. When an additional FM electrode is introduced in the all-metallic lateral spin-valve system, we find that the transresistance can be fractionally suppressed or very weakly influenced depending on the position of the additional FM electrode, and relative magnitudes of contact resistance and the bulk resistance defined over the spin-diffusion length. Nonlocal spin signals such as nonlocal voltage drop and leakage spin currents are independent of the magnetization orientation of the additional FM electrode. Even when the additional contact is nonmagnetic, nonlocal spin signals can be changed by the spin current leaking into the nonmagnetic electrode.

Nonlocal Spin Transport in Lateral Spin Valves with Multiple Ferromagnetic Electrodes

We study the nonlocal spin transport in a lateral spin valve with multiple ferromagnetic (FM) electrodes. When two current-injecting and two spin current-detecting electrodes are all ferromagnetic, the number of possible nonlocal spin signal states is four at maximum. In reality, this number is reduced, depending on the inter-probe distance and the relative magnitudes of the spin resistances. Our theoretical results are in agreement with recent experiments of spin injection into an Al island, a carbon nanotube, and graphene.

Spin accumulation in lateral all-ferromagnetic spin valves

Spin accumulation in magnetic tunnel junctions and lateral all-ferromagnetic spin valves is theoretically investigated by using a spin-dependent diffusive transport model. When electric current tunnels through a barrier, spin accumulation occurs in the ferromagnetic layer and can be detected with nonlocal spin valve measurements. Depending on the relative orientation of magnetization, the nonlocal spin signal has three different values and can determine the bulk spin polarization of the ferromagnetic material.

Magnetization process of a single magnetic ring detected by nonlocal spin valve measurement

We investigate the magnetization process of a 200-nm-wide Permalloy ring using a nonlocal spin-valve measurement technique in a lateral geometry. The nonlocal spin signal is found to reveal the chirality of the flux closure state in the magnetic ring. The angular dependence of the magnetization process of the Permalloy ring are also studied.