The dc Superconducting QUantum Interference Device SQUID is a sensitive detector of magnetic flux, with a typical flux noise of the order 1 mF Hz y1r2 at liquid helium temperatures. Here F s "r2 e is the flux quantum. In our NMR or 0 0 NQR spectrometer, a niobium wire coil wrapped around the sample is coupled to a thin film superconducting coil deposited on the SQUID to form a flux transformer. With this untuned input circuit the SQUID measures the flux, rather than the rate of change of flux, and thus retains its high sensitivity down to arbitrarily low frequencies. This feature is exploited in a cw spectrometer that monitors the change in the static magnetization of a sample induced by radio frequency irradiation.
Examples of this technique are the detection of NQR in 27 Al in sapphire and 11 B in boron nitride, and a level crossing technique to enhance the signal of 14 N in peptides. Research is now focused on a SQUID-based spectrometer for pulsed NQR and NMR, which has a bandwidth of 0-5 MHz. This spectrometer is used with spin-echo techniques to measure the NQR longitudinal and transverse relaxation times of 14 N in NH ClO , 63" 6 ms and 22 " 2 ms, respectively. With the aid 4 4
of two-frequency pulses to excite the 359 kHz and 714 kHz resonances in ruby simultaneously, it is possible to obtain a two-dimensional NQR spectrum. As a third example, the pulsed spectrometer is used to study NMR spectrum of 129 Xe after polariza-tion with optically pumped Rb. The NMR line can be detected at frequencies as low as 200 Hz. At fields below about 2 mT the longitudinal relaxation time saturates at about 2000 s. Two recent experiments in other laboratories have extended these pulsed NMR techniques to higher temperatures and smaller samples. In the first, images were obtained of mineral oil floating on water at room temperature. In the second, a SQUID configured as a thin film gradiometer was used to detect NMR in a 50 mm particle of 195 Pt at 6 mT and 4.2 K. q 1998 Elsevier Science B.V.