There is a need to design and synthesise magnetic materials that can be used as models to test our understanding of fundamental magnetic phenomena. Particularly important areas of current interest include the study of systems that model aspects of the magnetic behaviour of the high-<i>T<sub>c</sub></i> superconductors (low-spin Heisenberg antiferromagnets on a square lattice) and of highly frustrated magnetic systems. A family of layered copper salts, Cu(CAP)<sub>2</sub><i>X</i><sub>4</sub>, (CAP = 5 chloro-2-aminopyridine, <i>X</i> = Cl, Br) is a promising model system for the former class of magnet. The ordered magnetic structure of this system has been studied by neutron diffraction, along with (<i>H, T</i>) phase data and appears classical in character; this picture is confirmed by MuSR data, in addition, inelastic neutron scattering gives an indication of the three dimensional nature of the exchange in this system. The jarosite family of minerals, containing kagome lattices of metal ions, commonly Fe<sup>3+</sup>, are physical realisations of highly frustrated antiferromagnets. The excitation spectra of these materials have been studied using time of flight inelastic neutron scattering. Hydronium iron jarosite appears to behave as a spin liquid to 2 K despite strong antiferromagnetic coupling between homogeneous spin centres, whereas potassium iron jarosite exhibits conventional spin waves for a kagome lattice antiferromagnet, with contributions to the Hamiltonian of either a Dzyaloshinskii-Moriya interaction or crystal field induced single ion anisotropy. SQUID magnetometry measurements made under pressure in order to perturb the system, reveal changes in ordering behaviour in potassium iron jarosite. The mineral volborthite containing a slightly distorted kagome lattice of Cu<sup>2+</sup> ions is particularly interesting as it combines both high frustration and low spin (<i>S</i> = ½). This system has been studied using various techniques including SQUID magnetometry, neutron scattering and muon spin relaxation, all of which indicate that the magnetic correlations are highly unconventional. These show that volborthite also possesses strong antiferromagnetic exchange and fluctuations remain significant down to at least 50 mK, despite the build up of short range antiferromagnetic correlations. Experiments looking at ways to minimise the hydrogen content of the various materials studied, to improve the quality of the data from neutron scattering measurements, are also reported.
| Identifer | oai:union.ndltd.org:bl.uk/oai:ethos.bl.uk:643408 |
| Date | January 2006 |
| Creators | Coomer, Fiona C. |
| Publisher | University of Edinburgh |
| Source Sets | Ethos UK |
| Detected Language | English |
| Type | Electronic Thesis or Dissertation |
| Source | http://hdl.handle.net/1842/13458 |
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