Long-time stable HTSC DC-SQUID gradiometers with sil- icon dioxide passivation for measurements with supercon- ducting flux transformers

In applications for high-Tc superconducting DC-SQUIDs such as biomagnetism, nondestructive evaluation and the relaxation of magnetic nanoparticles, it is important to maintain reliable sensor performance over an extended time period. We designed and produced DC-SQUID gradiometers based on YBa2Cu3O7−x (YBCO) thin films which are inductively coupled to a flux transformer to achieve a higher sensitivity. The gradiometers are protected against ambient atmosphere and humidity by SiO2 and amorphous YBCO layers. The noise properties of the sensor in flip-chip configuration, especially in unshielded environments, are shown. We present a comparison of Tl2Ba2CaCu2O8+x (TBCCO) thin films on buffered sapphire or LaAlO3 substrates for the flux transformer in shielded and unshielded environments. We reach a low white field gradient noise of 72 fT/(cm · Hz) with the TBCCO on LaAlO3 flux transformer. The electric properties of the gradiometers (critical current IC , normal state resistance RN and the transfer function VΦ) were measured over a period of one year and do not show significant signs of degradation.