Two-Stage Directly-Coupled Superconducting Quantum Interference Device Array Magnetometer

The characteristics of the voltage-biased superconducting quantum interference device (SQUID) were investigated. Under the proper bias conditions, the flux-to-current transfer coefficient in a voltage-bias mode is the same as that in a current bias mode. To enhance the flux-to-current transfer coefficient, we proposed the parallel SQUID array connected by wire bonding. The enhancement in current modulation amplitude with the parallel array is verified experimentally. The probabilities of finding successful SQUIDs to form the array are also calculated. According to the theoretical analysis, the field noise of the two-stage directly-coupled SQUID magnetometer is less than that of the conventional directly-coupled SQUID magnetometer. The proposed design can be implemented by using the high-T_c junction technologies.