Multilevel YBaCuO flux transformers with high T_c SQUIDs: A prototype high T_c SQUID magnetometer working at 77 K

We have constructed optimized high Tc YBa2Cu 3O7 (YBCO) multiturn flux transformers using laser-deposited films patterned with ion milling and only photolithographic masking. A five-turn flux transformer with a 0.5×0.5 mm2 pickup loop was coupled to a YBCO superconducting quantum interference device (SQUID) made on a separate substrate to demonstrate a prototype high Tc SQUID magnetometer with all components working at 77 K. There was no additional noise associated with the input coil and transformer. The magnetic field sensitivity of the 0.5×0.5 mm2 magnetometer was 3.8 pT/√Hz at 77 K and 1 kHz.