A multichannel low transition temperature SQUID magnetometer system with integrated superconducting magnetic shielding is described, which is intended for recording the magnetocardiogram (MCG) of mice in a biolaboratory environment. The Dewar has a horizontal warm bore surrounded by a superconducting niobium shield. The mouse is situated in the center of the warm bore close to the SQUID magnetometers while its MCG is measured. We have characterized the system by determining the shielding behavior of the magnetic shield and by measuring the magnetic noise. We determined the attenuation of an external magnetic dipole field to be better than 10 -6 in the warm bore and better than 10 -7 at the magnetometer positions. The magnetic flux density noise of the system above 10 Hz is 2.7 fT/ radicHz, thus about twice as high as the intrinsic noise of the magnetometers. We determined the system performance by detecting the MCG of mice. Important physiological features like P-wave, QRS complex and T-wave could be resolved.
NanoSQUID sensors (Superconducting Quantum Interference Devices with nanoscale dimensions) are regarded as a useful tool for sensitive magnetic measurements and have achieved a low noise level. The nanoSQUIDs comprising nanobridge junctions are fabricated by photolithography followed by focused ion beam patterning. While previously such self-shunted nanobridge junctions have shown hysteretic characteristics at 4.2 K, we propose an approach to increase the operational temperature in order to reduce the critical current of the device. We report the design of a readout system, comprising all necessary components to achieve a low noise readout, which is limited by the nanoSQUID noise. It consists of a variable temperature (5–10 K) liquid helium probe stick, a coil system and a SQUID preamplifier. The orthogonal coil system enables particle magnetization up to 100 mT in parallel to and a flux bias of up to 28 mT perpendicular to the nanoSQUID, to set the working point. The nanoSQUID achieves a low white noise level of 0.23 $\mu\Phi_{0}/\sqrt{\rm Hz}$ including a readout contribution of 0.10 $\mu\Phi_{0}/\sqrt{\rm Hz}$ .
Das Photoscanning-Verfahren dient der zerstörungsfreien Detektion von Defekten in Solarzellen. Die Daten ermöglichen Rückschlüsse auf grundlegende physikalische Parameter der Zelle, wie zum Beispiel den Wirkungsgrad, bzw. mechanische Störungen. Es erlaubt eine umfassende Charakterisierung einer Vielzahl von unterschiedlichen Solarzellentypen. Innerhalb des Projektes werden diese Einsatzmöglichkeiten des Photoscanning-Verfahren verifiziert. Seit 2011 wurde die Weiterentwicklung des Verfahrens innerhalb einer Kooperation mit der Firma SENTECH als Projektträger, der Physikalisch Technischen Bundesanstalt und der TH Wildau als Projektbeteiligte fortgesetzt.
We report the use of an ultralow noise nano-superconducting quantum interference device (nanoSQUID) to measure the hysteretic magnetization behavior of a single FePt nanobead at a temperature of around 7 K in a magnetic field of only ∼10 mT. We also show that the nanobead can be accurately positioned with respect to the SQUID loop and then removed without affecting SQUID performance. This system is capable of further development with wide applications in nanomagnetism.
Wir berichten uber die Eigenschaften von rauscharmen, an der PTB entwickelten SQUID-Stromsensoren sowie deren Anwendung fur die Auslesung von nanostrukturierten Hot-Spot-Detektoren. Die hierfur verwendeten SQUID-Arrays zeichnen sich durch Robustheit und einfache Handhabung bei gleichzeitig sehr gunstigen Rauscheigenschaften aus. Die SQUID-Arrays, die aus 16 bzw. 25 einzelnen, in Reihe geschalteten SQUIDs bestehen, haben niedrige Eingangsinduktivitaten von etwa 3 nH und besitzen typische Stromrauschwerte von 8 pA Hz -1/2 bei 4,2 K sowie 5 pA Hz -1/2 bei 100 mK. Sie konnen im Erdmagnetfeld ohne Abschirmung abgekuhlt und betrieben werden: Eine Degradierung der V(_)-Kennlinie durch Dephasierung der Einzel-SQUID-Beitrage tritt nicht auf. Das kompakte gradiometrische Design erlaubt daruber hinaus die Montage der Sensor-Chips direkt auf dem Kupfertrager in unmittelbarer Nahe zum Detektor, ohne dass das Rauschen zunimmt.
Erste Messungen am Detektor mit SQUID-Auslesung wurden bei 5 K unter der Verwendung der kommerziell erhaltlichen DC-SQUID-Elektronik Magnicon XXF-1 durchgefuhrt. Einzelne vom Detektor absorbierte Photonen im sichtbaren bzw. nahen infraroten Spektralbereich erzeugen am Ausgang der SQUID-Elektronik Spannungspulse von einigen 10 mV. Deren minimale Dauer wird dabei durch die Bandbreite der SQUID-Elektronik begrenzt. In geschlossener Regelschleife (FFL-Mode) haben die Spannungspulse eine mittlere Dauer (FWHM) von 35 ns bei einer Anstiegszeit von 16 ns. Im Kleinsignalbetrieb (Amp-Mode) sinkt die Anstiegszeit auf etwa 8 ns. Die Amplitude der Spannungspulse hangt von der Energie der Photonen ab, sodass der Detektor auch eine spektrale Auflosung besitzt. Die Messung mit SQUID-Vorverstarker bei 5 K lieferte einen Wert von ungefahr 1.
We report the design and performance of thin-film microsusceptometers intended for magnetic measurements on samples at variable temperature down to the low mK range and excitation frequencies of up to about 1 MHz. The devices are realized as first-order gradiometers with two circular loops of 60 μm or 30 μm average diameter resulting in a total inductance of 360 pH or 250 pH, respectively. An integrated excitation coil generates a magnetic field with a sensitivity of 0.1 T/A at the sample position, whereas the Josephson junctions are located in a field-reduced area. The susceptometers are fabricated by a conventional Nb/AlOx/Nb trilayer process. In order to enhance the sensitivity to the level required for the measurement of sub-μm samples, an extra detection loop of about 450 nm inner diameter was integrated into one of the pickup loops by using a focused ion beam (FIB). We show that this device is able of detecting signals from very small permalloy samples. An improved susceptometer design for equipment with integrated nanoloops is also presented, for which a total inductance of 50 pH is predicted.
We have developed a family of HfTi nanoSQUID gradiometers for different applications. These Nb-based nanoSQUIDs contain overdamped superconductor–normal conductor–superconductor (SNS) Josephson junctions with HfTi as a normal conducting barrier. The lateral dimensions of the junctions are about 200 nm × 200 nm, and the barrier thickness is nominally 30 nm. In order to enhance their practical use, the nanoSQUIDs are implemented with gradiometric SQUID and feedback loops, gradiometric transformers, and rf filters. The devices can be operated in an excitation field of up to a few mT with very low levels of nonlinearity. Due to the small loop size and the resulting low loop inductance, a white noise level down to 110 nΦ0/√Hz was achieved. The 1/f noise with a typical corner frequency below 1 kHz is dominated by critical current fluctuations. It can be reduced by applying bias reversal. A noise level of 600 nΦ0/√Hz was achieved at 1 Hz in a two-stage flux locked loop with bias reversal.
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