Spin injection and spin accumulation in permalloy-copper mesoscopic spin valves

We study the electrical injection and detection of spin currents in a lateral spin valve device, using permalloy (Py) as ferromagnetic injecting and detecting electrodes and copper (Cu) as non-magnetic metal. Our multi-terminal geometry allows us to experimentally distinguish different magneto resistance signals, being 1) the spin valve effect, 2) the anomalous magneto resistance (AMR) effect and 3) Hall effects. We find that the AMR contribution of the Py contacts can be much bigger than the amplitude of the spin valve effect, making it impossible to observe the spin valve effect in a 'conventional' measurement geometry. However, these 'contact' magneto resistance signals can be used to monitor the magnetization reversal process, making it possible to determine the magnetic switching fields of the Py contacts of the spin valve device. In a 'non local' spin valve measurement we are able to completely isolate the spin valve signal and observe clear spin accumulation signals at T=4.2 K as well as at room temperature. We obtain spin diffusion lengths in copper of 1 micrometer and 350 nm at T=4.2 K and room temperature respectively.Comment: 8 pages (incl. figures), 7 figures, RevTex, conferenc

Spin resolved Andreev reflection in ferromagnet-superconductor junctions with Zeeman splitting

Andreev reflection in ferromagnet-superconductor junctions is derived in a regime in which Zeeman splitting dominates the response of the superconductor to an applied magnetic field. Spin-up and spin-down Andreev reflections are shown to be resolved as voltage is increased. In the metallic limit, the transition from Andreev to tunnel conductivity in the spin-up channels has a non trivial behavior when spin polarization is increased. The conductance is asymmetric in a voltage reversal.Comment: RevTex. 13 pages. 3 figures include

Spin accumulation probed in multiterminal lateral all-metallic devices

We study spin accumulation in an aluminium island, in which the injection of a spin current and the detection of the spin accumulation are done by means of four cobalt electrodes that connect to the island through transparent tunnel barriers. Although the four electrodes are designed as two electrode pairs of the same shape, they nonetheless all exhibit distinct switching fields. As a result the device can have several different magnetic configurations. From the measurements of the amplitude of the spin accumulation, we can identify these configurations, and using the diffusion equation for the spin imbalance, we extract the spin relaxation length $\lambda_\mathrm{sf} = 400 \pm 50$~nm and an interface spin current polarization $P = (10 \pm 1)$ at low temperature and $\lambda_\mathrm{sf} = 350 \pm 50$~nm, $P = (8 \pm 1)$ at room temperature

Direct demonstration of decoupling of spin and charge currents in nanostructures

The notion of decoupling of spin and charge currents is one of the basic principles underlying the rapidly expanding feld of Spintronics. However, no direct demonstration of the phenomenon exists. We report a novel measurement, in which a non-equilibrium spin population is created by a point-like injection of current from a ferromagnet across a tunnel barrier into a one dimensional spin channel, and detected differentially by a pair of ferromagnetic electrodes placed symmetrically about the injection point. We demonstrate that the spin current is strictly isotropic about the injection point and, therefore, completely decoupled from the uni-directional charge current.Comment: 13 pages, 3 figures; accepted for publication in Nano Letter

Current distribution inside Py/Cu lateral spin-valve device

We have investigated experimentally the non-local voltage signal (NLVS) in the lateral permalloy (Py)/Cu/Py spin valve devices with different width of Cu stripes. We found that NLVS strongly depends on the distribution of the spin-polarized current inside Cu strip in the vicinity of the Py-detector. To explain these data we have developed a diffusion model describing spatial (3D) distribution of the spin-polarized current in the device. The results of our calculations show that NLVS is decreased by factor of 10 due to spin flip-scattering occurring at Py/Cu interface. The interface resistivity on Py/Cu interface is also present, but its contribution to reduction of NLVS is minor. We also found that most of the spin-polarized current is injected within the region 30 nm from Py-injector/Cu interface. In the area at Py-detector/Cu interface, the spin-polarized current is found to flow mainly close on the injector side, with 1/e exponential decay in the magnitude within the distance 80 nm.Comment: 10 pages, 14 figure

Spin relaxation in mesoscopic superconducting Al wires

We studied the diffusion and the relaxation of the polarized quasiparticle spins in superconductors. To that end, quasiparticles of polarized spins were injected through an interface of a mesoscopic superconducting Al wire in proximity contact with an overlaid ferromagnetic Co wire in the single-domain state. The superconductivity was observed to be suppressed near the spin-injecting interface, as evidenced by the occurrence of a finite voltage for a bias current below the onset of the superconducting transition. The spin diffusion length, estimated from finite voltages over a certain length of Al wire near the interface, was almost temperature independent in the temperature range sufficiently below the superconducting transition but grew as the transition temperature was approached. This temperature dependence suggests that the relaxation of the spin polarization in the superconducting state is governed by the condensation of quasiparticles to the paired state. The spin relaxation in the superconducting state turned out to be more effective than in the normal state.Comment: 9 pages, 8 figure

Spin Injection and Detection in Magnetic Nanostructures

We study theoretically the spin transport in a nonmagnetic metal connected to ferromagnetic injector and detector electrodes. We derive a general expression for the spin accumulation signal which covers from the metallic to the tunneling regime. This enables us to discuss recent controversy on spin injection and detection experiments. Extending the result to a superconducting device, we find that the spin accumulation signal is strongly enhanced by opening of the superconducting gap since a gapped superconductor is a low carrier system for spin transport but not for charge. The enhancement is also expected in semiconductor devices.Comment: 4 pages, 3 figure

First- and Second-Order Phase Transitions, Fulde-Ferrel Inhomogeneous State and Quantum Criticality in Ferromagnet/Superconductor Double Tunnel Junctions

First- and second-order phase transitions, Fulde-Ferrel (FF) inhomogeneous superconducting (SC) state and quantum criticality in ferromagnet/superconductor/ferromagnet double tunnel junctions are investigated. For the antiparallel alignment of magnetizations, it is shown that a first-order phase transition from the homogeneous BCS state to the inhomogeneous FF state occurs at a certain bias voltage $V^{\ast}$; while the transitions from the BCS state and the FF state to the normal state at $V_{c}$ are of the second-order. A phase diagram for the central superconductor is presented. In addition, a quantum critical point (QCP), $$, is identified. It is uncovered that near the QCP, the SC gap, the chemical potential shift induced by the spin accumulation, and the difference of free energies between the SC and normal states vanish as $$ with the quantum critical exponents $z\nu =1/2$, 1 and 2, respectively. The tunnel conductance and magnetoresistance are also discussed.Comment: 5 pages, 4 figures, Phys. Rev. B 71, 144514 (2005

Sign of the crossed conductances at a FSF double interface

Crossed conductance in hybrid Ferromagnet / Superconductor / Ferromagnet (FSF) structures results from the competition between normal transmission and Andreev reflection channels. Crossed Andreev reflection (CAR) and elastic cotunneling (EC) between the ferromagnets are dressed by local Andreev reflections, which play an important role for transparent enough interfaces and intermediate spin polarizations. This modifies the simple result previously obtained at lowest order, and can explain the sign of the crossed resistances in a recent experiment [D. Beckmann {\sl et al.}, cond-mat/0404360]. This holds both in the multiterminal hybrid structure model (where phase averaging over the Fermi oscillations is introduced ``by hand'' within the approximation of a single non local process) and for infinite planar interfaces (where phase averaging naturally results in the microscopic solution with multiple non local processes).Comment: 9 pages, 7 figure

Spin injection and spin accumulation in all-metal mesoscopic spin valves

We study the electrical injection and detection of spin accumulation in lateral ferromagnetic metal-nonmagnetic metal-ferromagnetic metal (F/N/F) spin valve devices with transparent interfaces. Different ferromagnetic metals, permalloy (Py), cobalt (Co) and nickel (Ni), are used as electrical spin injectors and detectors. For the nonmagnetic metal both aluminium (Al) and copper (Cu) are used. Our multi-terminal geometry allows us to experimentally separate the spin valve effect from other magneto resistance signals such as the anomalous magneto resistance (AMR) and Hall effects. We find that the AMR contribution of the ferromagnetic contacts can dominate the amplitude of the spin valve effect, making it impossible to observe the spin valve effect in a 'conventional' measurement geometry. In a 'non local' spin valve measurement we are able to completely isolate the spin valve signal and observe clear spin accumulation signals at T=4.2 K as well as at room temperature (RT). For aluminum we obtain spin relaxation lengths (lambda_{sf}) of 1.2 mu m and 600 nm at T=4.2 K and RT respectively, whereas for copper we obtain 1.0 mu m and 350 nm. The spin relaxation times tau_{sf} in Al and Cu are compared with theory and results obtained from giant magneto resistance (GMR), conduction electron spin resonance (CESR), anti-weak localization and superconducting tunneling experiments. The spin valve signals generated by the Py electrodes (alpha_F lambda_F=0.5 [1.2] nm at RT [T=4.2 K]) are larger than the Co electrodes (alpha_F lambda_F=0.3 [0.7] nm at RT [T=4.2 K]), whereas for Ni (alpha_F lambda_F<0.3 nm at RT and T=4.2 K) no spin signal is observed. These values are compared to the results obtained from GMR experiments.Comment: 16 pages, 12 figures, submitted to PR