YBa₂Cu₃O_7-δ thin-film gradiometers: Fabrication and performance

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.