Magnetometers using RF‐driven squids and their applications in rock magnetism and paleomagnetism

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Abstract
A variety of magnetometer systems have been constructed utilizing superconducting quantum interference device (Squid) sensors. The magnetometers have high sensitivity and fast response times and eliminate the need for spinning or vibrating the sample to make measurements. The magnetometers have superconducting sense coils in which a dc persistent current is induced by the insertion of the sample. The current gives, via a flux transformer, a field at the Squid sensor, which is detected as the measure of the magnetization of the sample. The sense coils, the transformer, and the detector are housed within a superconducting shield. It was the advent of the Josephson junction devices which permitted the use of critical currents so small that flux‐switching sensors such as those used in the magnetometers could be developed. The particular Squid used in these magnetometers is an RF‐driven weak link sensor. The magnetometers have been used to observe magnetic transitions to identify magnetic phases. A high‐field susceptometer with a superconducting magnet permits determination of saturation magnetization and magnetic characteristics. Standard paleomagnetic samples can be measured at a magnetic moment sensitivity of 10−8 G cm³. Small samples have been measured with a 3‐mm access system whose noise level is 2 × 10−10 G cm³. Long cores can be measured by using a horizontal open access system. Continuous thermal demagnetization and intensity determinations are being developed.

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