Muon-spin relaxation and its application in the study of molecular quantum magnets

Thesis

This thesis is concerned with the muon-spin relaxation (musr) technique and its application in the study of a number of molecular magnetic systems that may be driven through a quantum phase transition at low temperatures through the application of a magnetic field or hydrostatic pressure.

Musr is a highly sensitive probe of magnetism, but its utility can be severely limited by the lack of knowledge of the muon implantation site and the extent to which the muon perturbs its host. In a system of ionic fluorides, where partial information about the muon site is experimentally available, I demonstrate systematically that these problems can be addressed accurately using electronic-structure calculations. The F--$mu$--F complex formed by muons in many fluorides can be understood as an exotic molecule-in-a-crystal defect with a zero-point energy larger than that of any naturally-occurring triatomic molecule.

I demonstrate the interesting possibility of controlling the magnetic dimensionality in a molecular magnet using applied pressure. musr and high-field magnetisation experiments under applied pressure on the coordination polymer CuF$_2$(H$_2$O)$_2$(pyrazine) show a transition from a quasi-two-dimensional to a quasi-one-dimensional antiferromagnetic phase. Density-functional theory calculations and calculations of the dipolar anisotropy complement the experiments.

I describe how subtle differences in chemical composition can lead to starkly different structural and magnetic properties. [Cu(pyz)(H$_2$O)(gly)$_2$](ClO$_4$)$_2$ may be considered an antiferromagnetic chain that orders below 50 mK while the related compound [Cu(pyz)(gly)](ClO$_4$) is formed from Cu$^{2+}$ dimers and remains disordered down to 30 mK in zero field, but displays a field-temperature phase diagram consistent with the Bose-Einstein condensation of triplons. I also describe musr measurements on the strong-leg spin ladder DIMPY and on the molecular nanomagnets Cr$_8$Cd and Cr$_8$Mn which highlight some of the remaining challenges for longitudinal-field musr experiments.

Authors: Möller, JS

• Publication date: 2013
• DOI: 10.5287/ora-r5y1yke8a
• Type of award: DPhil
• Level of award: Doctoral
• Awarding institution: University of Oxford
• Language: English
• Keywords: Muon-spin relaxation; molecular magnetism; density-functional theory
• Subjects: Condensed Matter Physics