Quantum magnetism probed with muon-spin relaxation

Thesis

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 T_c = 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 (˜ 0.2µ_B) 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 T_N , 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 T_N. 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.

Authors: Steele, A

• Publication date: 2011
• DOI: 10.5287/ora-de8rzdpwv
• Type of award: DPhil
• Level of award: Doctoral
• Awarding institution: Oxford University, UK
• Language: English
• Subjects: Condensed Matter Physics