A Cryotiger® gas-mixture cooler was applied for cooling of three high-Tc SQUID magnetometers. These SQUID magnetometers were mounted on an alumina holder in an axial gradiometer configuration. From 20 Hz upward, the system noise was about 0.1 pT/√Hz. Below this frequency, the noise gradually increased to a level of 10 pT/√Hz at 1 Hz. This low-frequency excess noise appeared to be due to remnant magnetization of the Cryotiger cold head. Movement of magnetic cold-head parts with respect to the SQUIDs are induced by pressure fluctuations in the heat exchanger lines. By using one SQUID as a reference for the cooler noise, a first-order gradiometer can be formed in which the cooler noise is eliminated. To establish a proper second-order gradiometer either a fourth SQUID has to be added, or the spatial separation between cold head and SQUIDs has to be increased significantly.
Rijpma, A. P., ter Brake, H. J. M., de Vries, E., Holland, H. J., & Rogalla, H. (2005). Low-frequency magnetic interference in a SQUID gradiometer caused by a Cryotiger® gas-mixture cooler. Cryogenics, 45(4), 317-322. https://doi.org/10.1016/j.cryogenics.2004.11.009