Sensitive YBa sub 2 Cu sub 3 O sub 7 minus x thin-film magnetometer

Our YBa{sub 2}Cu{sub 3}O{sub 7{minus}{ital x}} thin-film magnetometer consists of a dc superconducting quantum interference device with bi-epitaxial Josephson junctions, fabricated on one chip, and a flux transformer with a multiturn input coil, fabricated on a second chip. Photolithographic processing is used to pattern all layers. The magnetometer operates in a flux-locked loop at temperatures up to 81 K, with the flux transformer improving the magnetic field sensitivity by a factor of 83{plus minus}3. The low-frequency rms magnetic field noise scales approximately as 1/{ital f}{sup 1/2}, where {ital f} is the frequency, with a magnitude of 0.6 pT Hz{sup {minus}1/2} at 10 Hz and 0.09 pT Hz{sup {minus}1/2} at 1 kHz with the magnetometer immersed in liquid N{sub 2}.