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, SmBa 2 Cu 3 O y (SmBCO) and La 0.1 -Er 0.95 Ba 1.95 Cu 3 O 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 muPhi 0 /Hz 1/2 at 1 kHz. The magnetometer with an integrated 20-turn flux transformer and the effective area of about 1.8 mm 2 exhibited a field noise of 25 fT/Hz 1/2 or less at 1 kHz.