Contactless measurement of electric current using magnetic sensors

We review recent advances in magnetic sensors for DC/AC current transducers, especially novel AMR sensors and integrated fluxgates, and we make critical comparison of their properties. Most contactless electric current transducers use magnetic cores to concentrate the flux generated by the measured current and to shield the sensor against external magnetic fields. In order to achieve this, the magnetic core should be massive. We present coreless current transducers which are lightweight, linear and free of hysteresis and remanence. We also show how to suppress their weak point: crosstalk from external currents and magnetic fields

Giant Magnetoresistive Biosensors for Time-Domain Magnetorelaxometry: A Theoretical Investigation and Progress Toward an Immunoassay.

Magnetorelaxometry (MRX) is a promising new biosensing technique for point-of-care diagnostics. Historically, magnetic sensors have been primarily used to monitor the stray field of magnetic nanoparticles bound to analytes of interest for immunoassays and flow cytometers. In MRX, the magnetic nanoparticles (MNPs) are first magnetized and then the temporal response is monitored after removing the magnetic field. This new sensing modality is insensitive to the magnetic field homogeneity making it more amenable to low-power portable applications. In this work, we systematically investigated time-domain MRX by measuring the signal dependence on the applied field, magnetization time, and magnetic core size. The extracted characteristic times varied for different magnetic MNPs, exhibiting unique magnetic signatures. We also measured the signal contribution based on the MNP location and correlated the coverage with measured signal amplitude. Lastly, we demonstrated, for the first time, a GMR-based time-domain MRX bioassay. This approach validates the feasibility of immunoassays using GMR-based MRX and provides an alternative platform for point-of-care diagnostics

Quantum properties of atomic-sized conductors

Using remarkably simple experimental techniques it is possible to gently break a metallic contact and thus form conducting nanowires. During the last stages of the pulling a neck-shaped wire connects the two electrodes, the diameter of which is reduced to single atom upon further stretching. For some metals it is even possible to form a chain of individual atoms in this fashion. Although the atomic structure of contacts can be quite complicated, as soon as the weakest point is reduced to just a single atom the complexity is removed. The properties of the contact are then dominantly determined by the nature of this atom. This has allowed for quantitative comparison of theory and experiment for many properties, and atomic contacts have proven to form a rich test-bed for concepts from mesoscopic physics. Properties investigated include multiple Andreev reflection, shot noise, conductance quantization, conductance fluctuations, and dynamical Coulomb blockade. In addition, pronounced quantum effects show up in the mechanical properties of the contacts, as seen in the force and cohesion energy of the nanowires. We review this reseach, which has been performed mainly during the past decade, and we discuss the results in the context of related developments.Comment: Review, 120 pages, 98 figures. In view of the file size figures have been compressed. A higher-resolution version can be found at: http://lions1.leidenuniv.nl/wwwhome/ruitenbe/review/QPASC-hr-ps-v2.zip (5.6MB zip PostScript

Spin-Mediated Transport in Superconducting and Spin-Polarized Systems

The effects of spin-imbalance on the electronic transport properties of spin-polarized and superconducting systems have been studied in detail. The transport properties of the quaternary Heusler alloys Co2MnSi1-xAlx (0≤x≤1), which have been theoretically predicted to develop a half-metallic band structure as x→0, were investigated. Resistivity versus temperature measurements as a function of Al concentration (x) revealed a systematic reduction in the residual resistivity ratio as well as a transition from weakly-localized to half-metallic conduction as x→0. From measurements of the ordinary and anomalous Hall effects, the charge carrier concentration was found to increase, while the anomalous Hall coefficient decreased by nearly an order of magnitude with each sample as x→0 (∆x=0.25). Scaling of the anomalous Hall effect with longitudinal resistivity reveals that both the skew-scattering and intrinsic contributions grow quickly as x→1, indicating that disorder and band-structure effects cause the large anomalous Hall effect magnitudes observed for Co2MnAl. The non-equilibrium behavior of disordered superconducting Al films in high Zeeman fields has also been investigated. The tunneling density-of-states of the films were measured through the first-order Zeeman critical field transition. It is found that films with sheet resistances of a few hundred ohms exhibit large avalanche-like collapses of the condensate on the super-heating branch of the critical field hysteresis loop. In contrast, the transition back into the superconducting phase (i.e., along the super-cooling branch) is always continuous. These avalanches are suppressed by tilting the field as little as 1.5° and disappear above T = 300 mK, although the transition remains hysteretic. The fact that the condensate follows an unstable trajectory to the normal state suggests that the order-parameter in the hysteretic regime is not homogeneous. It is argued that this unusual behavior is a manifestation of the disordered Larkin-Ovchinnikov phase, which is a disordered remnant of the elusive, spin-imbalanced superconducting state known as the Fulde-Ferrell-Larkin-Ovchinnikov phase

Entwicklung eines magnetoresistiven Biosensors zur Detektion von Biomolekülen

Schotter J. Development of a magnetoresistive biosensor for the detection of biomolecules. Bielefeld (Germany): Bielefeld University; 2004.Diese Arbeit präsentiert eine neue Nachweismöglichkeit für Biomoleküle, in deren Rahmen Sub-Mikrometer große magnetische Marker and magnetoresistive Sensoren in einen magnetischen Biochip integriert werden. Die interessierenden Moleküle werden an Oberflächen-immobilisierte Proben hybridisiert und spezifisch mit magnetischen Partikeln markiert. Im Folgenden werden die Streufelder der magnetischen Marker als Widerstandsänderung in einem eingebetteten magnetoresistiven Sensor nachgewiesen. Jedes einzelne Sensorelement deckt die Fläche eines typischen Proben-DNA Spots ab, und über 200 Sensorelemente sind in einen magnetischen Sensor-Prototypen integriert, wodurch er kompatibel zu DNA-Microarray Applikationen ist. Die Eigenschaften verschiedener kommerziell erhältlicher magnetischer Partikel werden verglichen und hinsichtlich ihrer Eignung als Marker für magnetische Biosensoren untersucht. Sensoren, welche entweder auf dem Riesen-Magnetowiderstand oder dem Tunnel-Magnetowiderstand basieren, werden präsentiert, und ihre Reaktion auf lokale Streufelder, welche von magnetischen Markern auf ihrer Oberfläche induziert werden, wird untersucht. DNA-Hybridisierungsexperimente werden präsentiert, die zeigen, dass unser Prototyp eines magnetischen Biosensors komplexe DNA-Sequenzen mit einer Länge von tausend Basen bis herab zu einer Konzentration von etwa 20 pM nachweisen kann. Ein direkter Vergleich unserer magnetoresistiven und einer Fluoreszenz-basierten Detektionsmethode zeigt, dass unser magnetischer Biosensor bei dem Nachweis geringer DNA-Konzentrationen überlegen ist. Außerdem weist der magnetische Biosensor eine kompakte Größe auf und übersetzt die vorhandene Menge einer bestimmten Sorte Biomoleküle direkt in ein elektronisches Signal, wodurch dies eine sehr vielversprechende Wahl für die Detektionseinheit eines zukünftigen Lab-on-a-Chip Gerätes darstellt.In this thesis, a new sensing scheme for biomolecules is presented that combines sub-micron sized magnetic markers and magnetoresistive sensors into a magnetic biochip. The molecules of interest are hybridized to surface-immobilized probes and get specifically labeled by magnetic markers. Afterwards, the stray fields of the magnetic markers are detected as a resistance change by an embedded magnetoresistive sensor. Each sensor element covers the area of a typical probe DNA spot, and over 200 sensor elements are integrated into a magnetic biosensor prototype, thus making it compatible to standard DNA microarray applications. The properties of different commercially available magnetic particles are investigated and compared with respect to their suitability for magnetic biosensor applications. Sensors based both on giant and tunneling magnetoresistance are presented, and their response to local stray fields induced by magnetic markers on their surface is studied. DNA hybridization experiments are presented that prove that our prototype magnetic biosensor can detect complex DNA with a length of one thousand bases down to a concentration of about 20 pM. A direct comparison of the magnetoresistive and a fluorescent detection methods shows that our magnetic biosensor is superior to standard fluorescent detection at low DNA concentrations. Furthermore, the magnetic biosensor has compact size and directly translates the abundance of desired biomolecules into an electronic signal, thus making it a very promising choice for the detection unit of future lab-on-a-chip devices

The effects of a magnetic barrier and a nonmagnetic spacer in tunnel structures

The spin-polarized transport is investigated in a new type of magnetic tunnel junction which consists of two ferromagnetic electrodes separated by a magnetic barrier and a nonmagnetic metallic spacer. Based on the transfer matrix method and the nearly-free-electron-approximation the dependence of the tunnel magnetoresistance (TMR) and electron-spin polarization on the nonmagnetic layer thickness and the applied bias voltage are studied theoretically. The TMR and spin polarization show an oscillatory behavior as a function of the spacer thickness and the bias voltage. The oscillations originate from the quantum well states in the spacer, while the existence of the magnetic barrier gives rise to a strong spin polarization and high values of the TMR. Our results may be useful for the development of spin electronic devices based on coherent transport.Comment: 15 pages, 5 figure

Vertical polymer tunneling sensor platform by hot embossing technique

Recent development in microfabrication technology has brought much attention to the development of miniaturized, inexpensive and high-accuracy MEMS devices and microsystems. The ultimate goal of our project is to develop a versatile, three-dimensional, high precision sensor platform, which can be used for displacement, velocity or acceleration measurement. The first step, on which this dissertation is based, is to fabricate a one-dimensional (parallel with the Z axis) tunneling sensor, which in turn can be developed into two- and three-dimensional sensor platforms through structural and functional integration. Since the invention of mini-structured high-sensitivity silicon-based tunneling sensor in 1993, the synthesis and fabrication of PMMA-based tunneling sensors still remains an over-looked area. Compared with traditional silicon-based tunneling sensors, PMMA is less expensive, has little stiffness, and is easier to work with micro-machining process. Moreover, this all-PMMA-based tunneling sensor is one of the first generations of functional micro-sensors/devices for organic compatible applications. The hot embossing technique, one of the most widely used micromachining approaches in “soft-lithography”, was chosen for its fast turnaround, fewer processing parameters, and simplicity. Because the mold can be used repeatedly, the potential of mass-production is further highlighted in this dissertation. All-PMMA-based tunneling vertical sensors have been successfully fabricated. The overall size of the packaged sensor is 8 mm x 8 mm x 1 mm, with the measurement circuits bounded together. The natural frequency of the sensor structure is 133 Hz. The bandwidth of the feedback system is 6.3 kHz with voltage over acceleration sensitivity of 20.6 V/g. The resolution at 192 Hz is 0.2485 μg/[special characters omitted]. Compared with the silicon-based tunneling sensor, the PMMA sensor\u27s apparent advantages are: low cost, less processing time, less processing instruments, high yields, wider bandwidth, and theoretically lower noise level. Given all our research results, we can expect that the PMMA-based tunneling sensor platform to become the base for the next generation of highly sensitive micro-sensors in many important areas, notably in chemical, magnetic, infrared, and organic applications