Performance of high-Tc dc SQUID magnetometers with resistively shunted inductances compared to “unshunted” devices

We have investigated resistively shunted direct-coupled high-T_c dc SQUID magnetometers with different inductances. At T=77 K, good quantitative agreement was observed between the measured maximum voltage modulation depth and calculated values based on the theoretical predictions by Enpuku et al. (1995) whereas the white magnetic flux noise 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/Hz^1/2 was obtained for the magnetometer with an inductance of 100 pH and an outer dimension of the pickup loop of just 3 mm. The combined theoretical and experimental results suggest that although similar magnetic field noise values can he obtained for inductances up to 200 pH, the minimum value of the unshunted case cannot be improved upon. However, the maximum voltage modulation depth could be increased significantly at little cost to the noise compared to an unshunted device