HIGH-PERFORMANCE DC SQUID MAGNETOMETERS WITH SINGLE-LAYER YBA2CU3O7-XFLUX TRANSFORMERS

We have fabricated high-transition temperature superconducting magneto meters involving a flux transformer patterned in a single film of YBa2 Cu3O7-x, (YBCO) on a 50-mm-diam substrate. This transformer is inducti vely coupled to a loop that in tum is directly coupled to a dc superco nducting quantum interference device, patterned in a single layer of Y BCO deposited on a SrTiO3 bicrystal. At 77 K, the lowest magnetic fiel d noise achieved is 31 fT Hz-1/2 at frequencies above 5 Hz, increasing to 39 fT Hz-1/2 at 1 Hz.