High performance dc SQUID magnetometers with single layer YBa2Cu3O7−x flux transformers

We have fabricated high-transition temperature superconducting magnetometers involving a flux transformer patterned in a single film of YBa2Cu3O7−x (YBCO) on a 50-mm-diam substrate. This transformer is inductively coupled to a loop that in turn is directly coupled to a dc superconducting quantum interference device, patterned in a single layer of YBCO deposited on a SrTiO3 bicrystal. At 77 K, the lowest magnetic field noise achieved is 31 fT Hz−1/2 at frequencies above 5 Hz, increasing to 39 fT Hz−1/2 at 1 Hz.