Effect of HTS Superconductors on Homogeneity of Measurement Field in Low Field Nuclear Magnetic Resonance Detection

In recent years, superconducting quantum interference devices (SQUIDs) have been demonstrated to be useful in the low field nuclear magnetic resonance (NMR) measurements. The high temperature superconducting (HTS) SQUID used in our experiments has a frequency-independent sensitivity of 40-50 fT/Hz^1/2. When a liquid nitrogen cooled LC circuit is employed to form a tuned circuit with the SQUID, the sensitivity of the system can be further enhanced. The LC circuit consists of a capacitor and a coil made of copper wire or HTS tape, which is inductively coupled to the SQUID. However, the homogeneity of the measurement field deteriorates because of the HTS tape coil in the proximity of the sample. In contrast, the thin film SQUID with a washer area of 1 cm² has no effect on the NMR signal. Therefore, the impairment of the measurement field homogeneity in the case of different superconducting elements nearby is discussed by examining the free induction decay signals at 9 kHz. It is found that a square superconducting film with an area of 1 cm² may compensate for the inhomogeneity of the measurement field after the adjustment of its position.