This thesis investigates a number of novel magnetic materials and high temperature superconductors using high-field magnetometry and muon spin spectroscopy (μSR). The main measurement techniques are briefly described and a study of the dimer material [Cu(pyrazine)(glycine)]ClO<sub>4</sub> is presented to demonstrate the use of the proximity detector oscillator as a susceptometer in high magnetic fields. μSR is a highly effective tool for probing magnetic order, spin freezing and spin dynamics. However, in some circumstances its performance may be impaired by the extent to which it perturbs the material under study. Using μSR, density functional theory and crystal field calculations, I identify an experimental situation in the family of candidate quantum spin ices Pr<sub>2</sub>B<sub>2</sub>O<sub>7</sub> (B = Sn, Zr, and Hf), in which the measured response is dominated by a muon-induced distortion of the local structure. This issue is also addressed in a study of the spin dynamics in the canonical spin ice Ho2Ti2O7. Although computational work indicates a similar muon-induced effect in both Ho<sub>2</sub>Ti<sub>2</sub>O<sub>7</sub> and Dy<sub>2</sub>Ti<sub>2</sub>O<sub>7</sub>, the μSR data is not dominated by this perturbation. The remainder of this thesis is concerned with studying the superconducting properties of a number of Fe-based materials, including LiFeP which is found to have an enhanced superfluid stiffness in relation to its transition temperature. Also reported is the effect of structural disorder on the superconducting state in recently discovered Sr<sub>0.3</sub>(NH<sub>2</sub>)<sub>y</sub>(NH<sub>3</sub>)<sub>1-y</sub>Fe<sub>2</sub>Se<sub>2</sub>. Pulsed magnetic field measurements are used to probe the temperature dependence of the upper critical field, giving a maximum value of μ<sub>0</sub>H<sub>c2</sub>(0)≈33(2) T. I also investigate the effect of intercalating additional ammonia, via reversible adsorption and desorption in the related superconductor Li<sub>x</sub>[(NH<sub>2</sub>)<sub>y</sub>(NH<sub>3</sub>)<sub>1-y</sub>]<sub>z</sub>Fe<sub>2</sub>Se<sub>2</sub> (z = 1, 2). These reactions were carried out in situ on the muon beamline so that the superfluid stiffness could be measured using transverse-field μSR on a single sample.
| Identifer | oai:union.ndltd.org:bl.uk/oai:ethos.bl.uk:729974 |
| Date | January 2016 |
| Creators | Foronda, Francesca |
| Contributors | Blundell, Stephen ; Goddard, Paul |
| Publisher | University of Oxford |
| Source Sets | Ethos UK |
| Detected Language | English |
| Type | Electronic Thesis or Dissertation |
| Source | https://ora.ox.ac.uk/objects/uuid:55c7ed6b-af31-4b7b-8b64-217dcc6846ac |
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