This thesis presents the results of muon-spin relaxation (µ<sup>+</sup><abbr>SR</abbr>) studies into magnetic materials, and demonstrates how these results can be exploited to quantify the materials’ low moments and reduced dimensionality. Dipole-field simulations, traditionally used to estimate likely muon sites within a crystal structure, are described. A novel Bayesian approach is introduced which allows bounds to be extracted on magnetic moment sizes and magnetic structures—previously very difficult using µ<sup>+</sup><abbr>SR</abbr>—based on reasonable assumptions about positions in which muons are likely to stop. The simulations are introduced along with relevant theory, and MµCalc, a platform-independent program which I have developed for performing the calculations is described. The magnetic ground states of the isostructural double perovskites Ba<sub>2</sub>NaOsO<sub>6</sub> and Ba<sub>2</sub>LiOsO<sub>6</sub> are investigated with µ<sup>+</sup><abbr>SR</abbr>. In Ba<sub>2</sub>NaOsO<sub>6</sub> long-range magnetic order is detected via the onset of a spontaneous muon-spin precession signal below <var>T</var><sub>c</sub> = 7.2(2) K, while in Ba<sub>2</sub>LiOsO<sub>6</sub> a static but spatially-disordered internal field is found below 8 K. Bayesian analysis is used to show that the magnetic ground state in Ba<sub>2</sub>NaOsO<sub>6</sub> is most likely to be low-moment (˜ 0.2<var>µ</var><sub>B</sub>) ferromagnetism and not canted antiferromagnetism. Ba<sub>2</sub>LiOsO<sub>6</sub> is antiferromagnetic and a spin-flop transition is found at 5.5 T. A reduced osmium moment is common to both compounds, probably arising from a combination of spin–orbit coupling and frustration. Results are also presented from µ<sup>+</sup><abbr>SR</abbr> investigations concerning magnetic ordering in several families of layered, quasi–two-dimensional molecular antiferromagnets based on transition metal ions such as <var>S</var> = ½ Cu<sup>2+</sup> bridged with organic ligands such as pyrazine. µ<sup>+</sup><abbr>SR</abbr> allows us to identify ordering temperatures and study the critical behaviour close to <var>T</var><sub>N</sub> , which is difficult using conventional probes. Combining this with measurements of in-plane magnetic exchange <var>J</var> and predictions from quantum Monte Carlo simulations allows assessment of the degree of isolation of the 2D layers through estimates of the effective inter-layer exchange coupling and in-layer correlation lengths at <var>T</var><sub>N</sub>. Likely metal-ion moment sizes and muon stopping sites in these materials are identified, based on probabilistic analysis of dipole-fields and of muon–fluorine dipole–dipole coupling in fluorinated materials.
| Identifer | oai:union.ndltd.org:bl.uk/oai:ethos.bl.uk:580910 |
| Date | January 2011 |
| Creators | Steele, Andrew J. |
| Contributors | Blundell, Stephen J. |
| Publisher | University of Oxford |
| Source Sets | Ethos UK |
| Detected Language | English |
| Type | Electronic Thesis or Dissertation |
| Source | http://ora.ox.ac.uk/objects/uuid:030d7e91-f38e-433f-9539-652b0f4996cc |
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