This thesis presents the development of the first SQUID-based ac-magnetometer built in a dry dilution fridge. The possibility of expanding the frequency respond from tens of kHz of a commercial magnetometer to the bandwidth of muon-spin relaxation μSR (MHz), an indirect magnetic probe, which measures magnetic dynamics from the depolarisation of fundamental particles, is demonstrated. It opens a new scenario to investigate classical and quantum magnetic fluctuations by a direct probe in a range of frequencies that have an important role in exotic magnetic phases. Geometrical spin ices, frustrated magnets where magnetic excitations can be deconfined forming monopoles are good candidates to investigate. The study of magnetic fluctuations will help to understand the magnetic dynamics of these elementary excitations. Quantum spin liquids, frustrated systems without long-range order but with spins highly correlated that still fluctuate down to zero Kelvin are also interesting systems. The temperature dependence of their quantum fluctuations investigated by μSR have shown a dynamical plateau that might be corroborated by susceptibility measurements. The fabrication of the magnetometer has been combined with μSR investigations of the ground state of NbFe2 and CeRhIn5. The magnetic ground state of the ferromagnetic quantum critical point induced by growing around 1% Nbrich Nb1−yFe2+y is claimed to be reached in this kind of clean itinerant systems by a long–range spin density wave (SDW), although several attempts to identify the ground state by neutron scattering were unsuccessful. The μSR measurements prove that the ground state of the stoichiometric compound is governed by static and short-range correlations or by an incommensurate and helical SDW. The heavy fermion CeRhIn5 was investigated at ambient pressure due to the coexistence of antiferromagnetic and superconducting order in an intermediate region of pressures. The filamentary or bulk nature of the superconducting phase is still until debate and the onset of superconductivity at ambient pressure reported in some works may shed some light into the nature of the ground state. μSR and resistivity measurements were carried out to investigate the antiferromagnetic and helical phase without any signature of a superconducting transition.
| Identifer | oai:union.ndltd.org:bl.uk/oai:ethos.bl.uk:742833 |
| Date | January 2017 |
| Creators | Margineda, Daniel |
| Publisher | Cardiff University |
| Source Sets | Ethos UK |
| Detected Language | English |
| Type | Electronic Thesis or Dissertation |
| Source | http://orca.cf.ac.uk/111448/ |
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