Low-noise, single-layer YBa/sub 2/Cu/sub 3/O/sub 7-x/ DC SQUID magnetometers at 77 K

We describe a low-noise dc Superconducting QUantum Interference Device (SQUID) magnetometer that is fabricated from a single layer of YBa/sub 2/Cu/sub 3/O/sub 7-x/ on a 2 cm/spl times/2 cm bicrystal substrate. The magnetometer design consists of a single-turn pickup loop that is directly coupled to a low inductance SQUID. Using conventional flux-locked loop electronics with bias current reversal, the white flux noise of several of these magnetometers operated at 77 K is observed to be as low as 2.2 /spl mu//spl Phi//sub 0///spl radic/Hz above 10 kHz, increasing to about 5.7 /spl mu//spl Phi//sub 0///spl radic/Hz at 1 Hz. The field-to-flux conversion efficiency is measured to be 4.6 nT//spl Phi//sub 0/, resulting in a white magnetic field noise of to fT//spl radic/Hz above 10 kHz, increasing to 26 fT//spl radic/Hz at 1 Hz.>