Key elements for a sensitive 77 K direct current superconducting quantum interference device magnetometer

Abstract

We have made sensitive dc superconducting quantum interference device (SQUID) magnetometers that operate at 77 K have been made. By using bicrystal grain boundary Josephson junctions and a single layer of YBa2Cu3O7 thin film with optimized processing and an optimized directly coupled design, we have produced magnetometers with very low noise and a large transfer function. At 77 K, the transfer function is 100 μV/Φ0; the white field noise of the magnetometer is less than 10 fT/√Hz above 10 kHz, rising to 26 fT/√Hz at 1 Hz. The corresponding white noise energy has the low value of 2×10−31 J s; a value very similar to that of commercial Nb SQUIDS. Key elements required to achieve this performance are discussed.