Spin ordering at low temperatures

This thesis investigates some aspects of the ordering of nuclear spin systems at low temperatures. The thesis is divided into two parts. In part one some nuclear orientation experiments of 74As in Fe and 206Bi and Ni and Fe are described. These nuclei are orientated by the large hyperfine field inside Ni and Fe. The orientation is detected by the anisotropy of the emitted gamma-rays and in the case of 74As by the destruction of this anisotropy by NMR. Chapter one reviews the theory of the nuclear orientation method. To aid in the interpretation of the 206Bi results, some channelling experiments were done. A review of the method of atom location by channelling is given in chapter two. Chapter three describes the apparatus and technique used. The results of the 74AsFe experiments are given in chapter four. In these experiments the magnetic moment of the ground state of 74As was measured to be μ = 1.597(3), the sign of the hyperfine field was found to be positive, and the spin-lattice relaxation time was found to be 120(30) s. The results of the 206BiNi experiments are given in chapter five. The hyperfine was found to be 390(15) kOe for both diffused polycrystalline and implanted single crystal samples. The multipolarity mixing ratios of many of the gamma,-rays were found. Reorientation of the metastable state of 206Pb was observed, which had not been expected. On the basis of the channelling experiments, this reorientation has been explained by the theory that the Ri nuclei experience a large electric field gradient because they have been displaced from substitutional sites by dislocation loops. It was found also that 23(3)% of the implanted EL ions were on the surface of the Ni, where they had not been expected. This has been accounted for by the sputtering of the Ni surface. It was also found that the Ni migrates out of the Ni single crystals when heated to 600 C. This was contrary to expectations since Bi is reported as being soluble in Ni. The results of the 206BiFe experiments are reported in chapter six. In samples slowly cooled from melts, none of the Bi activity remained. In samples rapidly cooled from melts, 77(3)% of the Bi were in sites with a hyperfine field of 900(140) kOe. The rest were in low field sites. In the implanted samples, 91(8)% of the Bi were in 1400(30) kOe sites. The amount of reorientation was different in the two cases. In the former, the reorientation could be accounted for if the crystal structure near the Bi was similar to that in Ni. In the latter case, the channelling experiments showed that some of the Bi were in sites near single vacancies, which would account for the electric field gradient. In part two, the development of an apparatus for nuclear cooling experiments is described. Chapter one reviews the theory of nuclear cooling and chapter two reviews the principles of NMR thermometry. The apparatus was designed to achieve a spin temperature -6K in Cu nuclei to observe spin ordering effects caused by the internal interaction. Because of numerous difficulties the experiment has not been done. In chapters three and four, the apparatus is described, along with an account of the many difficulties encountered in building it. A description of the methods of overcoming these difficulties is also given.