Biomedical subjects

B J van Wees

Publications and source records attributed to B J van Wees.

8 recordsLinked to original sources

PubMed2006

Electrical detection of spin pumping due to the precessing magnetization of a single ferromagnet.

We report direct electrical detection of spin pumping, using a lateral normal-metal/ferromagnet/normal-metal device, where a single ferromagnet in ferromagnetic resonance pumps spin-polarized electrons into the normal metal, resulting in spin accumulation. The resulting backflow of spin current into the ferromagnet generates a dc voltage due to the spin-dependent conductivities of the ferromagnet. By comparing different contact materials (Al and/or Pt), we find, in agreement with theory, that the spin-related properties of the normal metal dictate the magnitude of the dc voltage.

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PubMed2006

A solid state paramagnetic maser device driven by electron spin injection.

In response to an external, microwave-frequency magnetic field, a paramagnetic medium will absorb energy from the field that drives the magnetization dynamics. Here we describe a new process by which an external spin-injection source, when combined with the microwave field spin pumping, can drive the paramagnetic medium from one that absorbs microwave energy to one that emits microwave energy. We derive a simple condition for the crossover from absorptive to emissive behavior. Based on this process, we propose a solid-state, paramagnetic device in which microwave amplification by stimulated emission of radiation is driven by spin injection.

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PubMed2006

Unified description of bulk and interface-enhanced spin pumping.

We describe a mechanism for generating nonequilibrium electron-spin accumulation in semiconductors or metals by rf magnetic field pumping. With a semiclassical model we show that a rotating applied magnetic field (or the processing magnetization inside a weak ferromagnet) generates a dc spin accumulation. For bulk systems this spin accumulation is in general given by a small fraction of h omega, where omega is the rotation or precession frequency. With the addition of a neighboring, field-free region, and allowing for the diffusion of spins across the interface, the spin accumulation is dramatically enhanced towards h omega near the interface. The interface-enhanced spin accumulation obtained within our bulk-oriented model is surprisingly similar to predictions based on interface-scattering theory [A. Brataas, Phys. Rev. B 66, 060404(R) (2002)].

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PubMed2003

One-way optoelectronic switching of photochromic molecules on gold.

We investigate photochromic molecular switches that are self-assembled on gold. We use two experimental techniques; namely, the mechanically controllable break-junction technique to measure electronic transport, and UV/Vis spectroscopy to measure absorption. We observe switching of the molecules from the conducting to the insulating state when illuminated with visible light (lambda=546 nm), in spite of the gold surface plasmon absorption present around this wavelength. However, we fail to observe the reverse process which should occur upon illumination with UV light (lambda=313 nm). We attribute this to quenching of the excited state of the molecule in the open form by the presence of gold.

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PubMed2003

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 aluminum island with all dimensions (400 nm x 400 nm x 30 nm) 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 magnetization 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.

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PubMed2002

Direct observation of the transition from the conventional superconducting state to the pi state in a controllable Josephson junction.

We measure the full supercurrent-phase relation of a controllable pi junction around the transition from the conventional 0 state to the pi state. We show that around the transition the Josephson supercurrent-phase relation changes from I(sc) approximately I(c)sin((phi) to I(sc) approximately I(c)sin((2phi). This implies that, around the transition, two minima in the junction free energy exist, one at phi=0 and one at phi=pi, whereas only one minimum exists at the 0 state (at phi=0) and at the pi state (at phi=pi). Theoretical calculations based on the quasiclassical theory are in good agreement with the observed behavior.

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PubMed2002

Electrical detection of spin precession in a metallic mesoscopic spin valve.

To study and control the behaviour of the spins of electrons that are moving through a metal or semiconductor is an outstanding challenge in the field of 'spintronics', where possibilities for new electronic applications based on the spin degree of freedom are currently being explored. Recently, electrical control of spin coherence and coherent spin precession during transport was studied by optical techniques in semiconductors. Here we report controlled spin precession of electrically injected and detected electrons in a diffusive metallic conductor, using tunnel barriers in combination with metallic ferromagnetic electrodes as spin injector and detector. The output voltage of our device is sensitive to the spin degree of freedom only, and its sign can be switched from positive to negative, depending on the relative magnetization of the ferromagnetic electrodes. We show that the spin direction can be controlled by inducing a coherent spin precession caused by an applied perpendicular magnetic field. By inducing an average precession angle of 180 degrees, we are able to reverse the sign of the output voltage.

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PubMed2001

Electrical spin injection and accumulation at room temperature in an all-metal mesoscopic spin valve.

Finding a means to generate, control and use spin-polarized currents represents an important challenge for spin-based electronics, or 'spintronics'. Spin currents and the associated phenomenon of spin accumulation can be realized by driving a current from a ferromagnetic electrode into a non-magnetic metal or semiconductor. This was first demonstrated over 15 years ago in a spin injection experiment on a single crystal aluminium bar at temperatures below 77 K. Recent experiments have demonstrated successful optical detection of spin injection in semiconductors, using either optical injection by circularly polarized light or electrical injection from a magnetic semiconductor. However, it has not been possible to achieve fully electrical spin injection and detection at room temperature. Here we report room-temperature electrical injection and detection of spin currents and observe spin accumulation in an all-metal lateral mesoscopic spin valve, where ferromagnetic electrodes are used to drive a spin-polarized current into crossed copper strips. We anticipate that larger signals should be obtainable by optimizing the choice of materials and device geometry.

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