We have fabricated several low-noise direct-current superconducting quantum interference device (SQUID) magnetometers from single layers of YBa2Cu3O7−δ on 10 mm×10 mm bicrystal substrates. The magnetometer design consists of a single-turn pickup loop that is directly coupled to the SQUID inductance. At 77 K, these magnetometers exhibit large voltage modulation with applied flux of over 40 μV. The minimum flux noise, measured at 77 K using conventional flux-locked loop electronics with bias current reversal, is 3.5×10−6 Φ0/√Hz above 10 kHz and 6.5×10−6 Φ0/√Hz at 1 Hz. The field-to-flux conversion efficiency is measured to be 10 nT/Φ0, resulting in a white magnetic field noise of 35 fT/√Hz above 10 kHz, increasing to 65 fT/√Hz at 1 Hz.

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.