Ultra-sensitive SQUID Systems for Pulsed Fields—Degaussing Superconducting Pick-Up Coils

SQUID systems for ultra-low-field magnetic resonance (ULF MR) feature superconducting pick-up coils that must tolerate exposure to pulsed fields of up to 100 mT. Using type II superconductor niobium (Nb) field distortions due to trapped vortices in the wire result. In addition, their rearrangement after quick removal of the pulsed field leads to excess low-frequency noise that limits the signal-to-noise ratio. In contrast, type I superconductors, such as lead (Pb), do not exhibit vortices but form an intermediate state with the coexistence of normal and superconducting domains. We measured the magnetization loops of superconducting wire samples of Nb and Pb together with their noise behavior after pulsed fields. Pb also exhibits significant excess low-frequency noise once the wire has been driven into the intermediate state. To avoid this problem, we removed the field not abruptly but in a linearly decaying sinusoidal manner, thereby degaussing the wire. After the application of 57 mT, we found that Nb can be degaussed within at least 50 ms, the shortest time used in this study. Pb can also be degaussed, albeit within 100 ms and a more complex dependence on the degaussing parameters. After successful degaussing, negligible excess low-frequency noise is observed.