Verlag des Forschungszentrums Jülich
JUEL-3826
Selders, Peter
Keramische Supraleiter-Quanteninterferometer im Magnetfeld: Rauschreduktion durch gezielte Flußssfadenverankerung mittels Antidots
IV, 131 S., 2000
Topic of this work is the analysis and improvement of the sensitivity of high-Tc superconducting rf-SQUIDs for operation in unshielded environment. The increase of the low-frequency noise due vortex motion in an applied magnetic field leads to a drastic reduction of sensibility of the sensors, which affects possible application of SQUIDs. Aim of this work was the reduction of the vortex motion by simple measures, i. e. specially positioned micrometer holes (antidots), in order to achieve a field independent noise performance of the SQUIDs. This work can be divided into three major parts:
1. The penetration and motion of vortices in rf-Washer SQUIDs with applied magnetic field has been examined and compared with existing models of the surface barrier and the thermal activated vortex motion.
2. Concepts for the reduction of the low frequency noise due to vortex motion have been developed. These are based on the pinning of vortices by artificial defects especially micrometer holes, so called antidots.
3. The noise reduction due to vortex pinning by antidots in rf-Washer SQUIDs in an applied magnetic field has been experimentally demonstrated in field-cooled and zero-field cooled measurements.
In the frame of this work the connections between the penetration and motion of vortices into and in the washer of the SQUID on one hand and the low frequency noise on the other hand are proved and illuminated by different measurements. The penetration and motion of single vortices into and in the washer of the SQUIDs has been detected. The reduction of the low frequency noise in applied magnetic fields to the zero field level is realized by special arrangements of antidots. A shift of the onset of the increase of the low frequency noise from 8µT for the SQUIDs without antidots to 40µT in field-cooled measurements and to 25µT in zero-field cooled measurements with SQUIDs with antidots is achieved. The developed measurement techniques are well suited for the further examination of the properties of vortices in rf-Washer SQUIDs and the vortex antidot interaction.
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