Elimination of 1/f Noise in Gradiometers for SQUID-based Ultra-Low Field Nuclear Magnetic Resonance

Superconducting wire may trap magnetic flux after being exposed to strong magnetic fields, for example when used as gradiometer material in a SQUID-based ultra-low field nuclear magnetic resonance system. In this work we investigated noise caused by the dynamics of trapped flux in superconducting gradiometers made from different materials. Niobium (Nb), niobium-titanium (NbTi), and tantalum (Ta) wires were tested. Small wire-wound gradiometers were connected to a SQUID sensor. Nb and NbTi wires showed increasing 1/f noise after exposure to magnetic fields of 5 mT and above. A larger second-order gradiometer made of Ta wire was tested inside a magnetically shielded room. A heating wire was placed on the outside of the Ta wire. Thermo-cycling of the Ta gradiometer significantly decreased the 1/f noise caused by its exposure to a magnetic field.