DIRECT-CURRENT SUPERCONDUCTING QUANTUM INTERFERENCE DEVICE SPECTROMETER FOR PULSED NUCLEAR-MAGNETIC-RESONANCE AND NUCLEAR-QUADRUPOLE RESONANCE AT FREQUENCIES UP TO 5 MHZ

A spectrometer based on a de superconducting quantum interference devi ce (SQUID) has been developed for the direct detection of nuclear magn etic resonance (NMR) or nuclear quadrupole resonance (NQR) at frequenc ies up to 5 MHz. The sample is coupled to the input coil of the niobiu m-based SQUID via a nonresonant superconducting circuit. The flux lock ed loop involves the direct offset integration technique with addition al positive feedback in which the output of the SQUID is coupled direc tly to a low-noise preamplifier. Precession of the nuclear quadrupole spins is induced by a magnetic field pulse with the feedback circuit d isabled; subsequently, flux locked operation is restored and the SQUID amplifies the signal produced by the nuclear free induction signal. T he spectrometer has been used to detect Al-27 NQR signals in ruby (Al2 O3[Cr3+]) at 359 and 714 kHz. (C) 1996 American Institute of Physics.