Development of Integrated HTS SQUIDs With a Multilayer Structure and Ramp-Edge Josephson Junctions

We have fabricated high-temperature superconductor (HTS) dc superconducting quantum interference devices (SQUIDs) with a multilayer structure and ramp-edge Josephson junctions. For stable operation at 77 K, SmBa₂Cu₃O_y (SmBCO) and La_0.1-Er_0.95Ba_1.95Cu₃O_y (La-ErBCO) films with a T _c above 90 K were employed as materials for the base-electrode and counter-electrode layers, respectively. Junctions were fabricated by employing a Cu-poor La-ErBCO layer as a precursor of the barrier. The fabricated parallel gradiometers with a baseline of 1-3 mm exhibited the field-induced modulation voltage of 20-50 muV at 77 K, and a clear modulation up to 86.8 K. Noise measurements at 77 K revealed a white flux noise of 4.5-10 muPhi₀/Hz^1/2 at 1 kHz. The magnetometer with an integrated 20-turn flux transformer and the effective area of about 1.8 mm² exhibited a field noise of 25 fT/Hz^1/2 or less at 1 kHz.