| סיכום: | <p>The F–μ–F state occurs when a spin polarised muon implanted in a fluoride crystal becomes entangled with its nearest neighbour fluorine nuclei, resulting in a beating pattern observed in the muon decay asymmetry. Ever since these states were discovered in the 1980s, they have never been fully understood, as fitting the data always required a phenomenological relaxation function to account for the observed decay of the oscillations.</p>
<p>This thesis shows how this relaxation can be understood by considering it as a decohering process, achieved by taking into account the effect of all the nuclear spins beyond the nearest-neighbours. Starting with the simple cubic compounds CaF<sub>2</sub> and NaF, techniques are developed which allows one to calculate the way in which the quantum information of the muon gets irreversibly lost into the crystal. This thesis then expands on this to discuss the compounds PbF<sub>2</sub> and YF<sub>3</sub>, which have a more complicated crystal structure, and in the former compound it is possible to identify Frenkel defects with μSR for the first time.</p>
<p>This thesis then shows how it is possible to apply these techniques to magnets: starting with KPF<sub>6</sub> and KBF<sub>4</sub>, which contain the [PF<sub>6</sub>]− and [BF<sub>4</sub>]− molecular ions which are often found in molecular magnets, it shows how the location of the muon site in the compound considerably changes the observed muon polarisation, determining the muon stopping site by carefully analysing the muon’s environment. Finally, the antiferromagnetic material KAgF<sub>3</sub> is analysed with these methods, showing how a combination of DFT+μ, dipole field calculations and the techniques developed here can be used to determine the size of the electronic moments, confirm magnetic structures and narrow down the muon site. This final chapter also shows how the nearest-neighbour fluorine nuclear moment can affect the muon polarisation in magnets in a fascinating way.</p>
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