High-Tc rf SQUIDs for operation in magnetic fields. Effect of thermal fluctuations

Abstract

Optimization of bulk high-Tc rf SQUID characteristics is carried out in the limit of strong thermal fluctuations. It is shown that such fluctuations affect the impedance of interferometers so that they operate only in quasi-nonhysteretic mode. Textured ceramics can be used to produce SQUIDs capable of stable operation in unshielded environment and/or in strong magnetic fields (H=150–200 A/m) with a sensitivity about 2×10−13 T/Hz1/2. These SQUIDs form the basis of a portable susceptometer designed and tested in constant (up to 150 A/m) and varying (up to 4 A/m) magnetic fields with a magnetic moment sensitivity not lower than 1.6×10−10 A⋅m2.