Magnetometers based on (double) relaxation oscillation SQUIDs

Overview & Synopsis

Relaxation Oscillation SQUIDs (ROS) and Double Relaxation Oscillation SQUIDS (DROS) combine the sensitivity of dc-SQUIDS with a large transfer, thus enabling flux locked loop operation with direct voltage readout by a room-temperature dc-amplifier, without flux modulation techniques. We fabricated several ROSs and DROSs with relaxation frequencies from 5 to 180 MHz. The experimental flux-to-frequency curves of the ROSs can well be fitted with theory, and the transfer (up to 170MHz/Φo) is large enough for practical use in a FLL without flux modulation. The experimental flux-to-voltage characteristics of the DROSs show very large transfer values of typically 10 to 30 mV/Φo. A DROS operated in a simple FLL with direct voltage readout showed a white noise level of about 2 µΦo/√Hz. A three channel DROS magnetometer system for biomagnetic measurements is under construction. In the DROSs that are to be used in this system, the signal flux is coupled to the SQUID by a tightly coupled 50-turn input coil.