arXiv subjects

B. J. van Wees

Publications and source records attributed to B. J. van Wees.

125 records · Page 7Linked to original sources

arXiv2004

Spin injection, accumulation and spin precession in a mesoscopic non-magn etic metal island

We experimentally study spin accumulation in an aluminium island with all dimensions smaller than the spin relaxation length, so that the spin imbalance throughout the island is uniform. Electrical injection and detection of the spin accumulation are carried out in a four terminal geometry by means of four cobalt electrodes connected to the island through tunnel barriers. We model the system theoretically and we investigate the role of the ferromagnetic electrodes on the spin accumulation at the limit at which the electron diffusion time can be neglected. We present measurements of spin accumulation at room temperature and at 4.2 K: in both cases the spin accumulation signal is larger than the Ohmic resistance of the aluminium island. From magnetisation precession measurements at room temperature, we extract a spin relaxation time of 60 ps and a polarisation P = 8% for tunnel barriers with resistances as low as 20 Ωμm^2.

cond-mat.mes-hall↗

arXiv2003

Zero-dimensional spin accumulation and spin dynamics in a mesoscopic metal island

We have measured electron spin accumulation at 4.2 K and at room temperature in an aluminium island with all dimensions (400nm x 400nm x 30nm) smaller than the spin relaxation length. For the first time, we obtain uniform spin accumulation in a four-terminal lateral device with a magnitude exceeding the ohmic resistance in the island. By controlling the magnetisation directions of the four magnetic electrodes that contact the island, we have performed a detailed study of the spin accumulation. Spin precession measurements confirm the uniformity of our system and provide an accurate method to extract the spin relaxation time.

cond-mat.mes-hall↗

arXiv2002

Electrical detection of spin accumulation and spin precession at room temperature in metallic spin valves

We have fabricated a multi terminal lateral mesoscopic metallic spin valve demonstrating spin precession at room temperature, using tunnel barriers in combination with metallic ferromagnetic electrodes as spin injector and detector. The observed modulation of the output signal due to the spin precession is discussed and explained in terms of a time of flight experiment of electrons in a diffusive conductor. The obtained spin relaxation length lambda_{sf}=500 nm in an Al strip will make possible detailed studies of spin dependent transport phenomena and allow to explore the possibilities of the electron spin for new electronic applications at RT.

cond-mat.mes-hall↗

arXiv2002

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.

cond-mat.mes-hall↗

arXiv2002

Direct demonstration of circulating currents in a controllable $π$-SQUID generated by a 0 to $π$ transition of the weak links

A controllable $π$-SQUID is a DC SQUID with two controllable $π$-junctions as weak links. A controllable $π$-junction consists of a superconducting - normal metal - superconducting Josephson junction with two additional contacts to the normal region of the junction. By applying a voltage $V_c$ over these contacts it is possible to control the sate of the junction, i.e. a conventional (0) state or a $π$-state, depending on the magnitude of $V_c$. We demonstrate experimentally that, by putting one junction into a $π$-state, a screening current is generated around the SQUID loop at integer external flux. To be able to do this, we have fabricated controllable $π$-junctions, based on Cu-Nb or Ag-Nb, in a new geometry. We show that at 1.4 K only the Nb-Ag device shows the transition to a $π$-state as a function of $V_c$ consistent with theoretical predictions. In a controllable $π$ SQUID based on Nb-Ag we observe, a part from a screening current at integer external flux, a phase shift of $π$ of the $V_{SQUID}-B$ oscillations under suitable current bias, depending on the magnitude of $V_c$.

cond-mat.supr-con↗

arXiv2001

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.

cond-mat.mes-hall↗

arXiv2001

The controllable pi - SQUID

We have fabricated and studied a new kind of DC SQUID in which the magnitude and sign of the critical current of the individual Josephson junctions can be controlled by additional voltage probes connected to the junctions. We show that the amplitude of the voltage oscillations of the SQUID as a function of the applied magnetic field can be tuned and that the phase of the oscillations can be switched between 0 and $π$ in the temperature range of 0.1 - 4.2 K using a suitable control voltage. This is equivalent to the external application of (n+1/2) flux quantum.

cond-mat.mes-hall↗

arXiv2000

Basic obstacle for electrical spin-injection from a ferromagnetic metal into a diffusive semiconductor

We have calculated the spin-polarization effects of a current in a two dimensional electron gas which is contacted by two ferromagnetic metals. In the purely diffusive regime, the current may indeed be spin-polarized. However, for a typical device geometry the degree of spin-polarization of the current is limited to less than 0.1%, only. The change in device resistance for parallel and antiparallel magnetization of the contacts is up to quadratically smaller, and will thus be difficult to detect.

cond-mat.mes-hall↗

arXiv2000

Electrical spin injection and detection in a semiconductor. Is it feasible?

The electrical injection of spin polarized electrons in a semiconductor can be achieved in principle by driving a current from a ferromagnetic metal, where current is known to be significantly spin polarized, into the semiconductor via ohmic conduction. For detection a second ferromagnet can be used as drain. We studied submicron lateral spin valve junctions, based on high mobility InAs/AlSb two-dimensional electron gas (2DEG), with Ni, Co and Permalloy as ferromagnetic electrodes. In the standard geometry it is very difficult to separate true spin injection from other effects, including local Hall effect, anomalous magnetoresistance (AMR) contribution from the ferromagnetic electrodes and weak localization/anti-localization corrections, which can closely mimic the signal expected from spin valve effect. The reduction in size, and the use of a multiterminal non-local geometry allowed us to reduce the unwanted effects to a minimum. Despite all our efforts, we have not been able to observe spin injection. However, we find that this 'negative' result in these systems is actually consistent with theoretical predictions for spin transport in diffusive systems.

cond-mat.mes-hall↗

arXiv2000

Comment on "Observation of Spin Injection at a Ferromagnet-Semiconductor Interface, by P.R. Hammar et al

In a recent Letter Hammar et al. claim the observation of injection of a spin-polarized current in a two-dimensional electron gas (2DEG). This is an important observation, since, despite considerable effort of several groups, all attempts to realize spin-injection into a 2DEG using purely electrical measurements have failed sofar. However, in my opinion the claim made is not correct, and the observed behaviour can be explained by a combination of a magneto resistance (Hall) effect (e.g. generated by the fringe magnetic fields present at the edges of the ferromagnetic electrode), with a {\it spin-independent} rectification effect due to the presence of a metal- semiconductor junction.

cond-mat↗

arXiv1999

Probing the electron-electron interaction in a diffusive gold wire using a controllable Josephson junction

We have studied the critical current of a diffusive superconductor - normal metal - superconductor (SNS) Josephson junction as a function of the electron energy distribution in the normal region. This was realized in a 4 terminal device, in which a mesoscopic gold wire between two electron reservoirs is coupled in its center to two superconducting electrodes. By varying the length of the wire and applying a voltage over it we are able to control the electron distribution function in the center of the wire, which forms the normal region of the SNS junction. The observed voltage and temperature dependence are in good agreement with the existing theory on diffusive SNS junctions, except for low energies. However, an electron-electron interaction time $τ_0$ =10 ps was found, which is three orders of magnitude faster than expected from theory.

cond-mat.mes-hall↗

arXiv1999

Spin accumulation induced resistance in mesoscopic ferromagnet/ superconductor junctions

We present a description of spin-polarized transport in mesoscopic ferromagnet-superconductor (F/S) systems, where the transport is diffusive, and the interfaces are transparent. It is shown that the spin reversal associated with Andreev reflection generates an excess spin density close to the F/S interface, which leads to a spin contact resistance. Expressions for the contact resistance are given for two terminal and four terminal geometries. In the latter the sign depends on the relative magnetization of the ferromagnetic electrodes.

cond-mat.mes-hall↗

arXiv1997

Suppression of the Coulomb interaction contribution to the conductance by a parallel magnetic field

The Coulomb interaction contribution to the conductance is investigated in a phase-coherent disordered 2-dimensional electron gas, which resistance can be varied by an overall gate electrode. Its magnitude of dGeei=-0.3 e^2/h is obtained by applying a bias voltage to suppress the Coulomb anomaly. In contrast to theoretical predictions, dGeei is suppressed by a parallel magnetic field. The zero-bias magnetoresistance exhibits reproducible fluctuations in perpendicular magnetic fields on a field scale much larger than that expected for universal conductance fluctuations, which might be attributed to fluctuations in the Coulomb interaction contribution.

cond-mat.mes-hall↗

arXiv1997

Reentrant behavior in the superconducting phase-dependent resistance of a disordered 2-dimensional electron gas

We have investigated the bias-voltage dependence of the phase-dependent differential resistance of a disordered T-shaped 2-dimensional electron gas coupled to two superconducting terminals. The resistance oscillations first increase upon lowering the energy. For bias voltages below the Thouless energy, the resistance oscillations are suppressed and disappear almost completely at zero bias voltage. We find a qualitative agreement with the calculated reentrant behavior of the resistance and discuss quantitative deviations.

cond-mat.mes-hall↗

arXiv1997

Comment on "Conductance fluctuations in mesoscopic normal-metal/superconductor samples"

Recently, Hecker et al. [Phys. Rev. Lett. 79, 1547 (1997)] experimentally studied magnetoconductance fluctuations in a mesoscopic Au wire connected to a superconducting Nb contact. They claimed to have observed an enhancement of the rms magnitude of these conductance fluctuations in the superconducting state (rms(Gns)) relative to that in the normal state (rms(Gn)) by a factor of 2.8. In this comment, we argue that the measured rms(Gns) is NOT significantly enhanced compared to rms(Gn) when we correct for the presence of an incoherent series resistance from the contacts, which is different when Nb is in the superconducting or normal state.

cond-mat.mes-hall↗

arXiv1995

Experimental determination of the quasi-particle decay length $ξ_{\text{Sm}}$ in a superconducting quantum well

We have investigated experimentally the electronic transport properties of a two-dimensional electron gas (2DEG) present in an AlSb/InAs/AlSb quantum well, where part of the toplayer has been replaced by a superconducting Nb strip, with an energy gap $Δ_0$. By measuring the lateral electronic transport underneath the superconductor, and comparing the experimental results with a model based on the Bogoliubov-de Gennes equation and the Landauer-Büttiker formalism, we obtain a decay length $ξ_{\text{Sm}} \approx 100~\text{nm}$ for electrons. This decay length corresponds to an interface transparency $T_{\text{SIN}}=0.7$ between the Nb and InAs. Using this value, we infer an energy gap in the excitation spectrum of the SQW of $Δ_{\text{eff}} = 0.97 Δ_0 = 0.83~\text{meV}$.

cond-mat↗

arXiv1994

Proximity and Josephson effects in superconductor - two dimensional electron gas planar junctions

The DC Josephson effect is theoretically studied in a planar junction in which a two dimensional electron gas (2DEG) infinite in lateral directions is in contact with two superconducting electrodes placed on top of the 2DEG. An energy gap in the excitation spectrum is created in the 2DEG due to the proximity effect. It is shown that under certain conditions, the region of the 2DEG underneath the superconductors is analogous to a superconducting region with an order parameter $\varepsilon_g\exp(iϕ)$\/, where $\varepsilon_g~(\varepsilon_g<Δ)$\/ depends on the interface transmittance and the Fermi velocity mismatch between the superconductors and the 2DEG.

cond-mat↗