77 K RF SQUID with 1/f noise only below 5 Hz

The authors report the results of developing RF SQUIDs, operating at liquid nitrogen temperatures, with 1/f noise only below about 5 Hz. To their knowledge, this is the best result among all published data for high-T_c RF SQUIDs. These particular devices were made from bars of Y₁Ba₂Cu₃O_{7- delta} . The ceramic was prepared by the conventional ceramic fabrication method. The only difference was that the samples were pressed at higher pressure than usual-140 approximately 300 N/cm². As a result, a high-density ceramic was obtained, which is very stable in water and other corrosive substances. This kind of ceramic possesses a small critical current density which is very important for producing a good SQUID. The SQUIDs are of the Zimmerman two-hole type. They use bridges with typical dimensions of about 100 mu m, fabricated by mechanical erosion with a diamond-coated wire. Because the critical current of such ´microbridges´ was too high for proper SQUID operation, the bridges were additionally weakened by thermal treatment.