Long-time stable HTSC DC-SQUID gradiometers with silicon dioxide passivation for measurements with superconducting flux transformers

摘要

In applications for high-Tc superconducting DC-SQUIDs such as biomagnetism,nondestructive evaluation and the relaxation of magnetic nanoparticles, it isimportant to maintain reliable sensor performance over an extended time period.We designed and produced DC-SQUID gradiometers based on YBCO thin films whichare inductively coupled to a flux transformer to achieve a higher sensitivity.The gradiometers are protected against ambient atmosphere and humidity by SiO2and amorphous YBCO layers. The noise properties of the sensor in flip-chip configuration, especially inunshielded environments, are shown. We present a comparison of TBCCO thin filmson buffered sapphire or LaAlO3 substrates for the flux transformer in shieldedand unshielded environments. We reach a low white field gradient noise of 72fT/(cm sqrt(Hz)) with the TBCCO on LaAlO3 flux transformer. The electricproperties of the gradiometers (critical current IC, normal state resistance RNand the transfer function VPhi) were measured over a period of one year and donot show significant signs of degradation.