High critical current density YBCO films and fabrication of dc-SQUIDs

In order to improve the sensitivity of SQUID magnetometers made of high-T-c films, we have studied the conditions of pulsed-laser deposition of YBCO films. Among the different deposition parameters examined, extensive degassing of the vacuum chamber before and precise control of the substrate temperature during the film deposition were found effective for obtaining high critical temperature T-c and high critical current density J(c). It was also found that the residual-resistance ratio has a clear correlation with J(c), indicating that it can be a good, and easy to measure, index of the film quality. Films having T-c approximate to 89-90 K and J(c) less than or equal to 5 x 10(6) A cm(-2) at 77 K were used to fabricate SQUIDs without a pickup loop. Grain-boundary junctions formed on bicrystal substrates with a 30degrees misorientation angle exhibited IcRn values of more than 100 muV at 77 K. The well-known scaling behaviour of the relation IcRn proportional to (J(c)(GB))(1/2) was confirmed. Modulation voltages of 20-50 muV were obtained in the SQUIDs with 70 pH inductance.