Low-field NMR Measurement Procedure when
SQUID Detection is Used

Longqing Qiu^1,2, Yi Zhang¹, Hans-Joachim Krause¹, Alex I. Braginski¹
and Andreas Offenhäusser¹
(1) Institute of Bio- and Nanosystems, Research Center Juelich, D-52425 Juelich, Germany
(2) Pohl Institute of Solid State Physics, Tongji University, Shanghai 200092, P. R. China
e-mail: y.zhang@fz-juelich.de

Abstract - In reported low-field nuclear magnetic resonance (NMR) measurements using SQUID (Superconducting Quantum Interference Device) detection, the pre-polarizing magnetic field has been oriented orthogonal to the measuring field, B_p ⊥ B_m.  Melton et al. were first to analyze the consequences of B_p decay in time after turnoff and showed that this decay should be nonadiabatic.  We evaluated the SQUID-based measuring procedure in the light of Melton et al. analysis, found good quantitative agreement, and showed that, when the decay time constant is comparable to the precession time of the magnetic moment vector M, the optimum procedure is to orient B_p parallel to B_m and to apply a π/2 pulse to flip M, similar as in the case of conventional NMR.

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