Highly sensitive YBa₂Cu₃O₇ dc SQUID magnetometer with thin‐film flux transformer Available to Purchase

We have designed and fabricated a thin‐film flux transformer by a YBa₂Cu₃O₇/SrTiO₃ (YBCO/STO) multilayer process. The flux transformer consists of a 20‐turn input coil of 10 μm linewidth and a single‐turn pickup loop surrounding an area of 7.5 by 7.5 mm². All device levels are patterned by standard photolithography and Ar‐ion‐beam etching. The flux transformer has been combined in flip‐chip configuration with the square washer of a low‐noise YBCO dc SQUID. The dc SQUID magnetometer exhibits a magnetic field sensitivity of 1.4 nT/Φ₀. The intrinsic white flux noise level of 70μΦ₀/√Hz at 77 K is mainly dominated by the SQUID corresponding to a magnetic field resolution of the magnetometer of 100 fT/√Hz for frequencies above 40 Hz. At 1 Hz we measured 200 fT/√Hz. Biomagnetic measurements were performed in a magnetically shielded chamber.