Modification of a FIT rock magnetometer using non-cryogenic magnetic field sensors and their potential for high-sensitivity measurements

We present options for modifying a F.I.T. Messtechnik GmbH SQUID rotation magnetometer (FIT) for measurement of rock samples, using modern non-cryogenic magnetic field sensors. These include three types of fluxgate magnetometers by Stefan Mayer Instruments, and a novel optically pumped magnetometer from QuSpin Inc. All sensors may be interchanged easily with the original SQUID sensor and between them. We installed these sensors and determined the resulting sensitivity and measurement precision when used in the FIT magnetometer system, without any other modifications. The sensors tested have a wide range of cost, between about $90 and $10,000, and system noise levels vary accordingly from about 10 –3 Am −1 for the most economic down to 2•10 –7 Am −1 (0.7•10 –12 Am 2 ) for the most expensive. These noise levels depend on the operation parameters such as the integration time of the lock-in amplifier, the distance of the sensor from the rotating sample, or gain settings. All fluxgate sensors cover large intensity ranges up to > 10 3 Am −1 , but the atomic pumping magnetometer QZFM Gen.2 can only be used at low intensities, 6 to 75•10 –3 Am −1 depending on the gain setting. Moving the sensor 8 cm away from the sample extends the range of the magnetization intensity that could be measured up to 3.6 Am −1 ; this is easy to do and only requires a recalibration of the system under such a changed condition. Measuring in the close position five times at 5 s integration time remanent magnetization of a sample of 9.4•10 –6 Am −1 intensity provided a mean direction with a confidence angle α 95 = 4.1°, demonstrating that a very high sensitivity of the FIT may be achieved installing the QZFM sensor, e.g., suitable for measuring low intensity rocks like stalagmites. Another good choice is the Stefan Mayer Instruments FL1-100 sensor resulting in a system noise level of > 5 •10 –6 Am −1 , which allows measuring many sediments and igneous rocks up to magnetizations of 1000 Am −1 . Even the most inexpensive fluxgate sensor would be well suitable for measuring lava flow samples, as it provides a noise level of 0.4–1 •10 –3 Am −1 .