Noise performance of SQUID magnetometers as a function of critical current value: A statistical-like approach

The critical current value is a key parameter of a Superconducting Quantum Interference Device. In this paper we investigate the effects of different critical current values on the noise performance of SQUID magnetometers. By using a statistical-like approach, we measure the spectral density of magnetic field noise of 200 magnetometers relating the values to the critical current change. The magnetometer consists of a dc-SQUID in a washer shape magnetically coupled to a square superconducting pickup coil. The fully integrated design includes a feedback coil to work in Flux Locked Loop mode and an Additional Positive Feedback circuit to increase its voltage responsivity. The main application field of these sensors is the magnetoencephalography that exploits multichannel systems based on SQUID magnetometers, in which the uniformity of noise performance of the sensors is needed. We found that a best value of critical current is 20 μα and also that a value spread ranging from 8 to 40 μm, does not affect in an evident way the sensitivity of such magnetometers. The corresponding noise spectral density remains within the range 2 - 6 fT/Hz.