  SQUID systems for ultra-low-field magnetic resonance (ULF MR) feature
superconducting pick-up coils which 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 which
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 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 dependency on the degaussing parameters. After successful degaussing,
negligible excess low frequency noise is observed.
