Using SQUIDs for AC measurements

Experiments in which a general-purpose SQUID (superconducting quantum interference device) magnetometer has been equipped with an AC drive coil show that the system can operate from 0.001 Hz to 1.0 kHz with a sensitivity roughly independent of frequency above 0.1 Hz. This is consistent with the noise characteristics of the SQUID detection system. As in conventional AC measurement systems, determination of the relative phase between the applied AC field and the sample response is critical in determining the out-of-phase component of the signal. Initial experiments show that the system can provide significant improvement over conventional AC detection systems at low frequencies and small signal amplitudes. Extensions of this work may allow SQUIDs to function as high-sensitivity detectors at frequencies up to several megahertz for such applications as low-field nuclear magnetic resonance and nuclear quadrupole resonance