The complete electronic system of a frequency-sweep 240 MHz NMR spectrometer working with a superconducting magnet is described. Full circuit and construction details are given for the rf sideband generator, pretuned coaxial cavity, af aperiodic synchronous detector, digital frequency synthesizer as well as a phase-coherent, variable-frequency, time-sharing pulse generator. This system, with multichannel irradiation possibilities uses extensively standard integrated logic circuits and can be inexpensively built by any laboratory engaged in NMR research.

The dynamics of Mn impurities in dilute CuMn (12, 20, and 43 ppm) have been studied in the spin-glass regime, down to ~T G /30. In an applied field of 275 Oe, the spins freeze gradually although their motions are highly correlated below a well-defined temperature T G , which is the same as that found in static susceptibility measurements c At T G , no anomaly is found on the impurity correlation time r e9 which is field dependent.We report on NMR studies of the longitudinal relaxation of host nuclei in dilute CuMn alloys from above to far below the onset of the spinglass regime. Measurements of T x yield information on the transverse fluctuating fields experienced by the nuclear spins at the Larmor frequency (x> n . In a dilute magnetic alloy these fields come primarily from the RKKY (Ruder man-Kittel -Kasuya-Yosida) hyperfine interactions. Their time behavior is linked to the dynamical properties of the impurities. The high-temperature regime where impurities behave as free spins has been studied by Alloul and Bernier 1 : The correlation time of paramagnetic Mn spins in CuMn is ~10-n /T sec. The work presented here has been performed in a temperature region where impurity interaction energies become comparalbe to or larger than k B T. We have reached two conclusions: (1) There exists, even in sizable applied fields, a rather well-defined temperature T Q below which all impurities interact and behave collectively. This temperature corresponds to the static susceptibility ordering temperature. Impurity clustering appears gradually as the system is cooled down to T G , becoming nearly complete at T G . (2) The impurity spin dynamics slow down considerably when the temperature is swept through T G but display no abrupt change. The observed behavior is typical of neither a well-defined cooperative transition 2 nor a glasslike freezing of thermally activated degrees of freedom. , MW-M 0 = -ik? 0 exp(-^/T 1 K ){Aexp(~^/T 1 imp ) +(1The Korringa relaxation time 7\ K (1.27/T sec) p is long in our temperature range and only yields a small correction. Parameters T^, r", «, Our host spin-lattice relaxation measurements have been performed on three dilute CuMn alloys of Mn concentration C = 12, 20, and 43 ppm. According to static susceptibility measurements, 3 these alloys order at 29, 44, and 87 mK, respectively. The temperature range of our NMR measurements extends from 2 to 300 mK on CuMn (43 ppm) at 310 kHz (# 0~2 75 Oe). Earlier measurements on all three alloys were performed from 25 mK to 2 K at 1.5 MHz (H 0 ~ 1330 Oe). 4 The shape of the copper nuclear magnetization recovery M(t) is obtained by a series of 77/2-77/2 sequences. Careful attention has been paid to heating effects at the lowest temperatures. The rotating rf field has an amplitude of 10 G; the receiver fully recovers from saturation in eight periods, There is no significant departure of the signal amplitude from Curie's law in the whole temperature range. The observed linewidth scales, as in previous work, 1 ' 4 with the static magnetization.The onset of i...