HIGHLY SENSITIVE YBA2CU3O7 DC SQUID MAGNETOMETER WITH THIN-FILM FLUX TRANSFORMER

We have designed and fabricated a thin-film flux transformer by YBa2Cu 3O7/SrTiO3 (YBCO/STO) multilayer process. The flux transformer consist s of a 20-turn input coil of 10 mum linewidth and a single-turn pickup loop surrounding an area of 7.5 by 7.5 mm2. All device levels are pat terned by standard photolithography and Ar-ion-beam etching. The flux transformer has been combined in flip-chip configuration with the squa re washer of a low-noise YBCO dc SQUID. The dc SQUID magnetometer exhi bits a magnetic field sensitivity of 1.4 nT/PHI0. The intrinsic white flux noise level of 70muPHI0/ square-root Hz at 77 K is mainly dominat ed by the SQUID corresponding to a magnetic field resolution of the ma gnetometer of 100 fT/ square-root Hz for frequencies above 40 Hz. At 1 Hz we measured 200 fT/ square-root Hz. Biomagnetic measurements were performed in a magnetically shielded chamber.