Radiative Detection of Pulsed Nuclear Magnetic Resonance.

NASA Astrophysics Data System (ADS)

The problem investigated is nuclear magnetic resonance (NMR) as used in conjunction with nuclear orientation (NO) studies of nuclei at low temperatures. Both theoretical and practical aspects are covered. Chapter 1 consists of a historical review, and a survey of concepts in NMR, NO and the combined subject NMRNO. This includes NMR in ferromagnets and ferromagnetic polarization, as a preamble to the experimental Chapter 5, where an experiment is described using ('60)Co nuclei in the ferromagnetic host nickel. Sections 1.4.3. and 1.4.4. are used to obtain the anisotropy as a function of inverse temperature, as shown in figure 5.1.. Single passage and pulsed NMRNO is reviewed in section 1.5., and pulsed NMRNO is carried out on ('60)Co in polycrystalline nickel, a host which had not previously been used with success by Barclay (1969). From a theoretical point of view, section 1.3.6(B) shows that when the quadrupole interactions of the nuclei are ignored, the quantum mechanical solution for the time dependence of the spin magnetization is identical with that from a classical treatment. Consequently, Chapter 2 has a brief review of three dimensional rotation matrices, used in sections 2.4. to 2.8. inclusive, and from which the analytic expressions are obtained for general pulse sequences, using the Fourier transform discussed in section 2.3.3.. These expressions assume that the linewidth of the distribution of nuclei is negligible compared with the enhanced RF field which the nuclei experience. Although, after correcting for experimental resolution, the exact lineshape function obtained in an experiment for a particular sample should be used, possibly including satellite lines, this is a system-dependent calculation, and is not amenable to analytic solution. For a more general calculation, we have used a Gaussian linefunction, as described in section 2.3., and we have furthermore used the dimensionless variables of section 2.3.2.. An interesting adjunct to two pulse NMRNO (section 2.5.) is the Shirley two pulse equivalent of section 2.5.3., which shows explicitly how the multipole effects may be observed. Since the Jaynes and Bloom papers are landmarks in the development of signal calculations in pulsed NMR, Chapter 3 describes their method in detail, and shows that the results obtained are consistent for one to five pulses, with those obtained in Chapter 2. In Chapter 4, a computer program which had been developed to calculate broad line pulsed NMRNO signals, and checked against the analytic expressions, yields and signals S(,k) for k = 1 to 4. Other more general features, such as mixed multipole signals, asymptotic signals for multiple pulse, and the general detection angle are also discussed. In Chapter 5, as previously mentioned, experiments on the ('60)Co in Ni systems are described, and also, we estimate the thermal link time for the salt-coldfinger -sample, both theoretically and experimentally. General results obtained experimentally for ('60)Co Fe were a thermal link time of order 9 sec. at 68.6 MHz, which is the resonant frequency for the ('60)Co Ni system, and a nominal 10 mK. The ratio, R, for the effective RF field B(,1) to the linewidth obtained was 0.14, significantly lower than the value, R = 0.45, obtained at 165 MHz in similar studies on ('60)Co Fe (Foster, 1979). Consequently, a 90(DEGREES) pulse corresponding to 6 (mu)sec for ('60)Co Ni was much longer than the 0.8 (mu)sec for ('60)Co Fe. From the theoretical point of view, the appearance of subechoes, and their subsequent experimental observation for ('60)Co Fe, together with their time shifts, made the calculations well worth while. Chapter 6 concludes the thesis, and presents some possibilities for further work, some of which has already been initiated.