LOW-NOISE YBA2CU3O7-PRBA2CU3O7 MULTITURN FLUX TRANSFORMERS

Epitaxial thin-film YBa2Cu3O7 flux transformers with multiturn input c oils have been constructed on 1 cm 2MgO substrates using PrBa2Cu3O7 as an insulator. By coupling these to high-temperature superconducting q uantum interference devices (SQUIDs) using a flip-chip technology, sev eral low-noise magnetometers and a gradiometer have been constructed. The yield of fully working flux transformers on 1 cm 2 substrates exce eds 80%, and field sensitivities at 77 K of (520 +/- 30)fT/square-root Hz at 1 Hz and (165 +/- 8)fT/square-root Hz above 10 Hz have been ach ieved. No evidence for excess low-frequency noise above that of the un coupled SQUID has been found in the flip-chip magnetometers.