Low-field NMR measurement procedure when SQUID detection is used.

In reported low-field nuclear magnetic resonance (NMR) measurements using Superconducting Quantum Interference Device (SQUID) detection, the pre-polarizing magnetic field has been usually oriented orthogonal to the measuring field, B(p) perpendicular B(m). Melton et al. systematically analyzed the consequences of B(p) decay in time after turnoff and showed that this decay should be nonadiabatic. We evaluated our measuring procedure in the light of that analysis, and found good quantitative agreement. It was showed that, when the decay time constant is comparable to the precession period of the magnetization of the sample, M, the optimum procedure is to orient B(p) parallel to B(m) and to apply a pi/2 pulse to flip M, similar as in the case of conventional NMR.