Superconductors in SQUID-Based Ultralow Field NMR—Flux-Trapping in Type-II Wires

Recent efforts in the development of ultralow field nuclear magnetic resonance (NMR) have focused on increasing prepolarizing field (B_P) strength. The strong B_P, which is up to hundreds of milliteslas, is mandatory for high-quality NMR signals. However, B_P needs to be completely removed within milliseconds so that spin relaxation signal measurement can take place before the sample magnetization wears off. In a previous study, where we compared three pick-up coils made of Nb, NbTi, and Pb, we found that only the pick-up coil made of Pb, which is a type-I superconductor, was unaffected by the strong magnetic field from the B_P coil and produced a sharp NMR signal. The other coils made of type-II superconductors all began to suffer from a degraded NMR signal with B_P above a certain threshold. Here, we show detailed measurement of magnetization loops of NbTi and Pb wires and argue that the counter pulse strategy suggested in our previous study can minimize the trapped flux and recover the spin relaxation signal. We argue that this counter pulse strategy can be applied to superconducting B_P coils, so that the counter pulses can neutralize flux trapped inside the B_P coil due to its strong internal field, thereby dramatically increasing the limit on the B_P strength.