SQUID DETECTED NMR AND NQR

The dc Superconducting QUantum Interference Device (SQUID) is a sensit ive detector of magnetic flux, with a typical flux noise of the order 1 mu Phi(0) Hz(-1/2) at liquid helium temperatures. Here Phi(0) = (h) over bar/2e is the flux quantum. In our NMR or NQR spectrometer, a nio bium wire coil wrapped around the sample is coupled to a thin film sup erconducting coil deposited on the SQUID to form a flux transformer. W ith this untuned input circuit the SQUID measures the flux, rather tha n the rate of change of flux, and thus retains its high sensitivity do wn to arbitrarily low frequencies. This feature is exploited in a cw s pectrometer that monitors the change in the static magnetization of a sample induced by radio frequency irradiation. Examples of this techni que are the detection of NQR in Al-27 in sapphire and B-11 in boron ni tride, and a level crossing technique to enhance the signal of N-14 in peptides. Research is now focused on a SQUID-based spectrometer for p ulsed 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 N-14 in NH4ClO4, 63 +/- 6 ms and 22 +/ - 2 ms, respectively. With the aid of two-frequency pulses to excite t he 359 kHz and 714 kHz resonances in ruby simultaneously, it is possib le to obtain a two-dimensional NQR spectrum. As a third example, the p ulsed spectrometer is used to study NMR spectrum of Xe-129 after polar iza-tion with optically pumped Rb. The NMR line can be detected at fre quencies 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 mu m particle of Pt-195 at 6 mT and 4.2 K. (C) 1998 Elsevier Sci ence B.V.