D. Grundler et al., HIGHLY SENSITIVE YBA2CU3O7 DC SQUID MAGNETOMETER WITH THIN-FILM FLUX TRANSFORMER, Applied physics letters, 63(19), 1993, pp. 2700-2702
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.