Various types of thin-film dc gradiometer SQUIDs with integrated pickup loops were investigated. The SQUIDs were prepared on a 5 mm × 10 mm chip in the well developed Nb-NbO x -Pb/Au/In technology. It was shown that the SQUIDs work in an unshielded environment. Our best SQUIDs have a field gradient sensitivity better then 30 fT cm −1 Hz −1/2 . All gradiometers have some response to a homogeneous field, i.e., a parasitic area. The effective pickup area, the field gradient sensitivity, and the components of the parasitic area for the field perpendicular and parallel to the gradiometer plane were measured for all types of gradiometer. The calculations of flux coupling efficiency are compared with experimental results. The origins of parasitic area are discussed. A further reduction of external disturbances was achieved by using an electronic second-order gradiometer. Examples of magnetocardiographic measurement in a physical laboratory without any shielding with first and second-order gradiometers are shown.
Absfract-A SQUIDmeasurement system for magnetorelaxometry 0 in disturbed environment is presented. It was developed for the determination of the distribution of magnetic nanoparticles in large objects like animals. The system is based on a thin film LTS SQUID gradiometer with a baseline of 5 mm and a field gradient sensitivity of about 30 f'l'/cmWn. The entire system i s PC-controlled and includes a nonmagnetic x-y stage. A Helmholtz coil of 80 cm diameter allows the magnetization of the samples with fields up to 7 mT. First measurements in a disturbed environment are presented.
A system for spatially resolved magnetorelaxometry in spatially expanded objects (e.g. for in vivo investigations) in a disturbed environment is presented. The system is based on an LTS SQUID gradiometer with a field gradient sensitivity of 30 fT cm-1 Hz-1/2 and an intrinsic balance of some 10-5. A Helmholtz coil of 80 cm diameter allows the magnetization of the samples with fields up to 7 mT. A nonmagnetic x-y stage (driven by step motors) covers a scanning area of 40 cm × 25 cm. We present spatially resolved measurements of the Néel relaxation of samples (dried ferrofluid) and compare them with model calculations. The data acquisition and monitoring as well as the filtering and fitting procedure are discussed.
An approximate analytical calculation of the current-voltage characteristics of SQUIDs with parasitic capacitances and inductances is presented. The analytical treatment of the I -V curves is based on the approximation of harmonic signals. For bias currents I B > 1.5 I 0 (critical current of the SQUID) the approximated and the numerically calculated I -V curves are in very good agreement. From the analytical treatment of the characteristics we have derived analytical formulae for three voltages, where there is no flux modulation at the I -V characteristic. The comparison of numerically calculated and experimental characteristics shows that the equation of the upper point without flux modulation of the I -V curves is suitable for calculating parameters such as inductances and capacitances of SQUIDs with parasitic elements.
The influence on thel-Vcurves and on the noise properties of parasitic capacitances caused by a covered slit of a Ketchen-type SOUID has been studied. The experimental results are compared with numerical simulations based on a lumpedcircuit-element model. In the C V characteristics we found two operating points, with minimum white noise, that were independent of the parasitic capacitance.
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