arXiv subjects

B. J. van Wees

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

At least 109 records · Page 6Linked to original sources

arXiv2008

Spin Accumulation and Spin Relaxation in a Large Open Quantum Dot

We report electronic control and measurement of an imbalance between spin-up and spin-down electrons in micron-scale open quantum dots. Spin injection and detection was achieved with quantum point contacts tuned to have spin-selective transport, with four contacts per dot for realizing a non-local spin-valve circuit. This provides an interesting system for studies of spintronic effects since the contacts to reservoirs can be controlled and characterized with high accuracy. We show how this can be used to extract in a single measurement the relaxation time for electron spins inside the dot ~ 300 ps and the degree of spin polarization of the contacts P ~ 0.8.

cond-mat.mes-hall↗

arXiv2008

Single atom adhesion in optimized gold nanojunctions

We study the interaction between single apex atoms in a metallic contact, using the break junction geometry. By carefully 'training' our samples, we create stable junctions in which no further atomic reorganization takes place. This allows us to study the relation between the so-called jump out of contact (from contact to tunnelling regime) and jump to contact (from tunnelling to contact regime) in detail. Our data can be fully understood within a relatively simple elastic model, where the elasticity k of the electrodes is the only free parameter. We find 5 < k < 32 N/m. Furthermore, the interaction between the two apex atoms on both electrodes, observed as a change of slope in the tunnelling regime, is accounted for by the same model.

cond-mat.mes-hall↗

arXiv2008

Confinement-enhanced spin relaxation for electron ensembles in large quantum dots

We present a numerical study of spin relaxation in a semiclassical electron ensemble in a large ballistic quantum dot. The dot is defined in a GaAs/AlGaAs heterojunction system with a two-dimensional electron gas, and relaxation occurs due to Dresselhaus and Rashba spin orbit interaction. We find that confinement in a micronscale dot can result in strongly enhanced relaxation with respect to a free two-dimensional electron ensemble, contrary to the established result that strong confinement or frequent momentum scattering reduces relaxation. This effect occurs when the size of the system is on the order of the spin precession length, but smaller than the mean free path.

cond-mat.mes-hall↗

arXiv2008

Electronic spin drift in graphene field effect transistors

We studied the drift of electron spins under an applied DC electric field in single layer graphene spin valves in a field effect transport geometry at room temperature. In the metallic conduction regime ($n \simeq 3.5 \times 10^{16}$ m$^{-2}$), for DC fields of about $\pm$70 kV/m applied between the spin injector and spin detector, the spin valve signals are increased/decreased, depending on the direction of the DC field and the carrier type, by as much as $\pm$50%. Sign reversal of the drift effect is observed when switching from hole to electron conduction. In the vicinity of the Dirac neutrality point the drift effect is strongly suppressed. The experiments are in quantitative agreement with a drift-diffusion model of spin transport.

cond-mat.mes-hall↗

arXiv2008

Anisotropic spin relaxation in graphene

Spin relaxation in graphene is investigated in electrical graphene spin valve devices in the non-local geometry. Ferromagnetic electrodes with in-plane magnetizations inject spins parallel to the graphene layer. They are subject to Hanle spin precession under a magnetic field $B$ applied perpendicular to the graphene layer. Fields above 1.5 T force the magnetization direction of the ferromagnetic contacts to align to the field, allowing injection of spins perpendicular to the graphene plane. A comparison of the spin signals at B = 0 and B = 2 T shows a 20 % decrease in spin relaxation time for spins perpendicular to the graphene layer compared to spins parallel to the layer. We analyze the results in terms of the different strengths of the spin orbit effective fields in the in-plane and out-of-plane directions.

cond-mat.mes-hall↗

arXiv2008

Non-local detection of resistance fluctuations of an open quantum dot

We investigate quantum fluctuations in the non-local resistance of an open quantum dot which is connected to four reservoirs via quantum point contacts. In this four-terminal quantum dot the voltage path can be separated from the current path. We measured non-local resistance fluctuations of several hundreds of Ohms, which have been characterized as a function of bias voltage, gate voltage and perpendicular magnetic field. The amplitude of the resistance fluctuations is strongly reduced when the coupling between the voltage probes and the dot is enhanced. Along with experimental results, we present a theoretical analysis based on the Landauer-Büttiker formalism. While the theory predicts non-local resistance fluctuations of considerably larger amplitude than what has been observed, agreement with theory is very good if an additional dephasing mechanism is assumed.

cond-mat.mes-hall↗

arXiv2007

The influence of device geometry on many-body effects in quantum point contacts: Signatures of the 0.7 anomaly, exchange and Kondo

The conductance of a quantum point contact (QPC) shows several features that result from many-body electron interactions. The spin degeneracy in zero magnetic field appears to be spontaneously lifted due to the so-called 0.7 anomaly. Further, the g-factor for electrons in the QPC is enhanced, and a zero-bias peak in the conductance points to similarities with transport through a Kondo impurity. We report here how these many-body effects depend on QPC geometry. We find a clear relation between the enhanced g-factor and the subband spacing in our QPCs, and can relate this to the device geometry with electrostatic modeling of the QPC potential. We also measured the zero-field energy splitting related to the 0.7 anomaly, and studied how it evolves into a splitting that is the sum of the Zeeman effect and a field-independent exchange contribution when applying a magnetic field. While this exchange contribution shows sample-to-sample fluctuations and no clear dependence on QPC geometry, it is for all QPCs correlated with the zero-field splitting of the 0.7 anomaly. This provides evidence that the splitting of the 0.7 anomaly is dominated by this field-independent exchange splitting. Signatures of the Kondo effect also show no regular dependence on QPC geometry, but are possibly correlated with splitting of the 0.7 anomaly.

cond-mat.mes-hall↗

arXiv2007

Persistence of the 0.7 anomaly of quantum point contacts in high magnetic fields

The spin degeneracy of the lowest subband that carries one-dimensional electron transport in quantum point contacts appears to be spontaneously lifted in zero magnetic field due to a phenomenon that is known as the 0.7 anomaly. We measured this energy splitting, and studied how it evolves into a splitting that is the sum of the Zeeman effect and a field-independent exchange contribution when applying a magnetic field. While this exchange contribution shows sample-to-sample fluctuations, it is for all QPCs correlated with the zero-field splitting of the 0.7 anomaly. This provides evidence that the splitting of the 0.7 anomaly is dominated by this field-independent exchange splitting.

cond-mat.mes-hall↗

arXiv2006

Large cone angle magnetization precession of an individual nanomagnet with dc electrical detection

We demonstrate on-chip resonant driving of large cone-angle magnetization precession of an individual nanoscale permalloy element. Strong driving is realized by locating the element in close proximity to the shorted end of a coplanar strip waveguide, which generates a microwave magnetic field. We used a microwave frequency modulation method to accurately measure resonant changes of the dc anisotropic magnetoresistance. Precession cone angles up to $9^{0}$ are determined with better than one degree of resolution. The resonance peak shape is well-described by the Landau-Lifshitz-Gilbert equation.

cond-mat.mes-hall↗

arXiv2006

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 d.c. 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 d.c. voltage.

cond-mat.mes-hall↗

arXiv2006

An electron spin injection driven, paramagnetic solid-state MASER device

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 spin injection-driven paramagnetic MASER device.

cond-mat.mtrl-sci↗

arXiv2006

The role of Joule heating in the formation of nanogaps by electromigration

We investigate the formation of nanogaps in gold wires due to electromigration. We show that the breaking process will not start until a local temperature of typically 400 K is reached by Joule heating. This value is rather independent of the temperature of the sample environment (4.2-295 K). Furthermore, we demonstrate that the breaking dynamics can be controlled by minimizing the total series resistance of the system. In this way, the local temperature rise just before break down is limited and melting effects are prevented. Hence, electrodes with gaps < 2 nm are easily made, without the need of active feedback. For optimized samples, we observe quantized conductance steps prior the gap formation.

cond-mat.mtrl-sci↗

arXiv2006

Microwave spectroscopy on magnetization reversal dynamics of nanomagnets with electronic detection

We demonstrate a detection method for microwave spectroscopy on magnetization reversal dynamics of nanomagnets. Measurement of the nanomagnet anisotropic magnetoresistance was used for probing how magnetization reversal is resonantly enhanced by microwave magnetic fields. We used Co strips of 2 um x 130 nm x 40 nm, and microwave fields were applied via an on-chip coplanar wave guide. The method was applied for demonstrating single domain-wall resonance, and studying the role of resonant domain-wall dynamics in magnetization reversal.

cond-mat.mtrl-sci↗

arXiv2006

On-chip detection of ferromagnetic resonance of a single submicron permalloy strip

We measured ferromagnetic resonance of a single submicron ferromagnetic strip, embedded in an on-chip microwave transmission line device. The method used is based on detection of the oscillating magnetic flux due to the magnetization dynamics, with an inductive pick-up loop. The dependence of the resonance frequency on applied static magnetic field agrees very well with the Kittel formula, demonstrating that the uniform magnetization precession mode is being driven.

cond-mat.mes-hall↗

arXiv2006

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 $λ_\mathrm{sf} = 400 \pm 50$~nm and an interface spin current polarization $P = (10 \pm 1)%$ at low temperature and $λ_\mathrm{sf} = 350 \pm 50$~nm, $P = (8 \pm 1)%$ at room temperature.

cond-mat.mes-hall↗

arXiv2006

The Magneto-coulomb effect in spin valve devices

We discuss the influence of the magneto-coulomb effect (MCE) on the magnetoconductance of spin valve devices. We show that MCE can induce magnetoconductances of several per cents or more, dependent on the strength of the coulomb blockade. Furthermore, the MCE-induced magnetoconductance changes sign as a function of gate voltage. We emphasize the importance of separating conductance changes induced by MCE from those due to spin accumulation in spin valve devices.

cond-mat.mes-hall↗

arXiv2005

Unified description of bulk and interface-enhanced spin pumping

The dynamics of non-equilibrium spin accumulation generated in metals or semiconductors by rf magnetic field pumping is treated within a diffusive picture. The dc spin accumulation produced in a uniform system by a rotating applied magnetic field or by a precessing magnetization of a weak ferromagnet is in general given by a (small) fraction of hbar 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, the spin accumulation is dramatically enhanced at the interface, saturating at the universal value hbar omega in the limit of long spin relaxation time. This effect can be maximized when the system dimensions are of the order of sqrt(2pi D omega), where D is the diffusion constant. We compare our results to the interface spin pumping theory of A. Brataas et al. [Phys. Rev. B 66, 060404(R) (2002)].

cond-mat.mes-hall↗

arXiv2005

Separating spin and charge transport in single wall carbon nanotubes

We demonstrate spin injection and detection in single wall carbon nanotubes using a 4-terminal, non-local geometry. This measurement geometry completely separates the charge and spin circuits. Hence all spurious magnetoresistance effects are eliminated and the measured signal is due to spin accumulation only. Combining our results with a theoretical model, we deduce a spin polarization at the contacts of approximately 25 %. We show that the magnetoresistance changes measured in the conventional two-terminal geometry are dominated by effects not related to spin accumulation.

cond-mat.mes-hall↗