Thin-film flux transformers of YBa2Cu3O7-x

A three-layer in situ laser deposition process was used to construct superconducting thin-film flux transformers of YBa₂Cu₃O_7-x (YBCO). The transformers are designed for efficient coupling to a planar thin-film DC SQUID, have 0.7-cm² magnetic field pickup areas, and have ten-turn input coils. When coupled to low transition temperature (T_c) SQUID, the resulting hybrid magnetometer exhibits excess low-frequency flux noise which arises in the transformer. This noise depends on the geometry of the flux transformer, and the observed behavior agrees with model calculations of the expected contribution from flux motion in the YBCO. The hybrid magnetometer attains a magnetic field sensitivity of about 0.9 pTHz^-1/2 at 1 Hz with the transformer at 60 K; the RMS noise decreases as 1/f ^1/2 at 1 Hz up to a frequency f≈1 kHz. It is believed that the sensitivity is high enough for use, for example, in magnetocardiography