The pure nuclear quadrupole spin—lattice relaxation times for 127I in SnI4 have been measured over the temperature range from 4.2° to 300°K. Measurements were performed on both the 208- and 416-Mc/sec transitions. From the solution of the rate equations the transition probabilities, W1 and W2, corresponding to Δm=±1 and Δm=±2 transitions, are evaluated using the experimentally determined relaxation times. The ratio of the two required relaxation times is found to have a rather insensitive dependence upon W2/W1 and could be determined only at temperatures below 77°K. For temperatures below about 15°K a third relaxation time was required to describe the observed relaxation curves. Since cross-relaxation or spin—diffusion effects were not observed, the determination of the spin—lattice relaxation times from the stimulated echo was found to give far more accurate results than the conventional two-pulse technique.
The theory of quadrupolar relaxation developed by Bayer and the modification given by Woessner and Gutowsky have been reformulated for nuclei with spin 53. The third time constant required to fit the low-temperature data is predicted by either theory even though the details of the theory do not agree well with experiment results. The ratio W2/W1 is found from the experimental observations to increase by a factor of about 2 as the temperature decreases from 77° to about 7°K. The results obtained from Bayer and from Woessner and Gutowsky indicate an opposite effect. However, by assuming a temperature dependence for the molecular vibrational frequencies, better agreement can be obtained between the expressions for W2/W1 and the experimentally determined values. The temperature dependence of the lifetime of the molecular torsional oscillations has not been determined in detail and remains as a second parameter of poor definition. Because of the limited information describing the molecular torsional oscillations, relatively poor agreement is obtained between the theoretical and experimental determinations of the individual W1 and W2 transitions.
This paper reports the results of the measurement of nuclear spin-lattice relaxation rates in KMnF 3 , which indicate that there are at least two distinct relaxation processes in the ordered state. The high-temperature rate has a power-law temperature dependence, while the low-temperature rate has an approximate exp(~a/T) dependence, indicating the effect of a magnon energy gap.The first measurement 1 of relaxation times in CuCl 2 • 2H 2 0 led to the theoretical work of Moriya, 2 Van Kranendonk and Bloom, 3 and Mitchell. 4 These papers treated a magnon-Raman relaxation process and yielded the result (neglecting spin-wave interactions, using the longwavelength limit, and assuming quantization of the electronic and nuclear spins along different directions in the crystal) that the nuclear relaxation rates should be proportional to T 3 for temperature above T^g, and proportional to T 2 exp(-T^Lg/T) for temperatures below ^AE> wnere kl AE * S * ne width of the magnon energy gap. Most experimental data have been in poor agreement with these predictions.Pincus and Winter 5 have described a mechanism by which thermal phonons can participate directly in the nuclear relaxation process. This mechanism, applicable only when T «T^E, yields a linear temperature dependence for the direct process and a T 7 dependence for the Raman process. In CuCl 2 • 2H 2 0, where Phys. Rev. J.08, 475 (1957). 12 B. S. Charidrasekhar and J. A. Rayne, Phys. Rev.
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