This thesis describes Nuclear Orientation and radiofrequency magnetic susceptibility experiments on the enhanced nuclear antiferromagnet holmium vanadate (HoVO<sub>4</sub>) from 1 K down to about 1 mK and in applied fields of up to 2.4 tesla. Adiabatic demagnetisation of single crystals of HoVO<sub>4</sub> allowed a mean nuclear spin temperature of about 1 mK to be reached. R.f. magnetic susceptibility measurements at approximately 2.4 mK and in an applied field of about 1.5 x 10<sup>-2</sup> T identified the spin 'flop' phase of the nuclear anti-ferromagnet HoVO<sub>4</sub>. The Néel temperature T<sub>N</sub> was determined to be 4.0 (2) mK which is in good agreement with the value of 4.8 mK calculated for the dipole-dipole interaction of the enhanced magnetic moments at the Ho site. Gamma-ray anisotropy measurements of <sup>166m</sup>HoVO<sub>4</sub> showed that in low applied fields (B ≮ 0.5 x 10<sup>-2</sup> T) and temperatures of about 1 mK the antiferromagnetic domains were uniformly distributed in the a-a' plane of the tetragonal zircon crystal structure. This was interpreted in terms of pinning of the nuclei due to crystalline imperfections. The spin 'flop' phase was identified in an applied field of 1.2 x 10<sup>-2</sup> T which is in good agreement with the susceptibility measurements. Demagnetisation experiments down to the relatively high residual fields of 4.2 x 10</sup>-2</sup> T and 7.4 x 10</sup>-2</sup> T and temperatures of about 2 mK showed that the effective field at the nucleus was much smaller than the applied field thus indicating that there was some antiferromagnetic ordering even at these relatively high fields. The magnetic dipole moment of <sup>166m</sup>Ho was estimated to be 3.60 (6) μ<sub>N</sub> from gamma-ray anisotropy measurements of some gamma transitions of known NO parameters. This value is in good agreement with the value of 3.69μ<sub>N</sub> calculated for the Nilsson configuration [6337./2]<sub>n</sub> [5237/2]<sub>p</sub> for the extra-core neutron and proton respectively. Mixing ratio (δ) measurements for gamma-ray transitions between the (K<sup>π</sup> = 2<sup>+</sup>) γ-vibrational and (K<sup>π</sup> = 0<sup>+</sup>) ground state rotational bands of the daughter nucleus <sup>166</sup>Er showed that Ml transition probabilities are predominantly via ΔK = 1 band mixing which is in good agreement with systematics for deformed nuclei in this mass region. Nuclear refrigeration experiments on a powdered compressed HoVO<sub>4</sub> - copper pill allowed a lattice temperature of 3.9 (3) mK to be reached. Thermal contact measurements on a similar pill showed that the effective area of contact between the salt particles and copper was only about 30 cm<sup>2</sup> which was a small fraction of the total surface area available. Preliminary work on a HoVO<sub>4</sub> - gold pill enabled a <sup>60</sup>Co ̲C̲o thermometer to be cooled to 6.0 (l) mK.
| Identifer | oai:union.ndltd.org:bl.uk/oai:ethos.bl.uk:278974 |
| Date | January 1980 |
| Creators | Nambudripad, Narayanan |
| Contributors | Allsop, A. L. |
| Publisher | University of Oxford |
| Source Sets | Ethos UK |
| Detected Language | English |
| Type | Electronic Thesis or Dissertation |
| Source | http://ora.ox.ac.uk/objects/uuid:58cb3cad-176f-46a0-ab25-5425edc17a7f |
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