Transfer function and noise properties of YBa2Cu3O7-δ direct-current superconducting-quantum-interference-device magnetometers with resistively shunted inductances

We have investigated resistively shunted direct-coupled high-T-c dc superconducting quantum interference device (SQUID) magnetometers with four different inductances (50, 100, 150, and 200 pH). The SQUIDs were based on 200 nm thick YBa2Cu3O7-delta films deposited on bicrystal substrates with a 24 degrees misorientation angle, and the shunt resistors were fabricated by masked ion damage. At T=77 K, good quantitative agreement was observed between the measured maximum voltage modulation depth Delta V and calculated values based on the theoretical predictions by Enpuku , whereas the white magnetic flux noise S-Phi at 10 kHz of all four devices was found to be a factor of 2.3 higher than predicted. The lowest white magnetic field noise of 153 fT/root Hz was obtained for the magnetometer with a SQUID inductance of 100 pH with an outer dimension of the pickup loop of just 2 mm. (C) 2000 American Institute of Physics. [S0003-6951(00)00830-5].