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Title: Quantum magnetism probed with muon-spin relaxation
Author: Steele, Andrew J.
ISNI:       0000 0004 2744 1121
Awarding Body: University of Oxford
Current Institution: University of Oxford
Date of Award: 2011
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This thesis presents the results of muon-spin relaxation (µ+SR) 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+SR—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 Ba2NaOsO6 and Ba2LiOsO6 are investigated with µ+SR. In Ba2NaOsO6 long-range magnetic order is detected via the onset of a spontaneous muon-spin precession signal below Tc = 7.2(2) K, while in Ba2LiOsO6 a static but spatially-disordered internal field is found below 8 K. Bayesian analysis is used to show that the magnetic ground state in Ba2NaOsO6 is most likely to be low-moment (˜ 0.2µB) ferromagnetism and not canted antiferromagnetism. Ba2LiOsO6 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+SR investigations concerning magnetic ordering in several families of layered, quasi–two-dimensional molecular antiferromagnets based on transition metal ions such as S = ½ Cu2+ bridged with organic ligands such as pyrazine. µ+SR allows us to identify ordering temperatures and study the critical behaviour close to TN , which is difficult using conventional probes. Combining this with measurements of in-plane magnetic exchange J 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 TN. 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.
Supervisor: Blundell, Stephen J. Sponsor: Engineering and Physical Sciences Research Council
Qualification Name: Thesis (Ph.D.) Qualification Level: Doctoral
EThOS ID:  DOI: Not available
Keywords: Condensed Matter Physics