Noise and dc characteristics of thin-film Bi-Sr-Ca-Cu-oxide dc SQUIDs

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Abstract
Highly oriented Bi‐Sr‐Ca‐Cu‐oxide thin films with Tc≥100 K and large (10–20 μm) grains were used to produce thin‐film dc superconducting quantum interference devices (SQUIDs). The best SQUID, which operated up to 75 K, had an inside loop diameter of ∼18 μm and an estimated loop inductance of 30 pH. The device was patterned by standard photolithography and had weak links that were likely due to existing grain boundaries around the loop. The effective loop area, calculated from the period of the SQUID modulation, is roughly consistent with the geometrical area of the SQUID loop. Significant hysteresis was observed in the flux‐voltage characteristic which depended on the amplitude of the magnetic field sweep. The 1/f noise levels measured at 100 Hz in this device gave a flux noise Sφ of 1.0×10−8 Φ20/Hz at 4.8 K and 2.3×10−7 Φ20/Hz at 60 K. The detailed noise measurements which we report for this SQUID are the first for any Bi‐Sr‐Ca‐Cu‐oxide based SQUID.