Performance of Pickup Coil Made of Litz Wire and Coupled to HTS SQUID

Performance of a copper pickup coil coupled to HTS SQUID was studied. The coil was made of Litz wire to prevent the eddy current loss at high frequencies, and was cooled at T = 77 K to reduce the thermal noise. The coil with average diameter D = 50 mm and number of turns N = 150 was made with the Litz wire, and was resonantly coupled to HTS SQUID made of ramp-edge junction. The 59-turn input coil was made of superconducting thin film and coupled to the SQUID with flip chip coupling scheme, which gave the mutual inductance of M=1700 pH. The field noise of the magnetometer was measured to be about 1 fT/Hz1/2 for f > 20 kHz. The experimental results agreed reasonably well with the designed values.