YBa2Cu3O7−δ thin-film gradiometers: Fabrication and performance

Abstract
We have fabricated and tested superconducting first‐order gradiometers made from YBa2Cu3O7−δ (YBCO) thin‐film flux transformers coupled to YBCO dc SQUIDs. The flux transformers consisted of 10‐turn input coils with 5‐μm linewidth and two pick‐up loops with 600‐μm linewidth, each of which surrounded an area of 2.1 by 2.8 mm. All levels were patterned by wet etching. A magnetic field gradient of 1 nT/m resulted in a flux of 5.8×10−4 Φ in the SQUID which corresponds to a responsivity of 1.21×10−9 m3. The 1/f noise level at 77 K and 10 Hz was 0.6 nT/m√Hz and was dominated by SQUID noise.