Study of magnetic fluctuations and ordering in uranium compounds by heat capacity and neutron scattering measurements
Entwisle, Oliver John
URhGe is the first ferromagnet discovered that shows superconductivity at ambient pressure. It shows a rich temperature-magnetic field phase diagram with a re-emergence of superconductivity at high magnetic field where the moments rotate. This suggests that the quantum fluctuations associated with the moment rotation may provide the pairing interaction for superconductivity. The objective of this thesis was to study these critical fluctuations with inelastic neutron scattering and heat capacity measurements, using the latter to test the bulk nature of the superconductivity and determine the types of gap nodes to help test this hypothesis. To perform the heat capacity measurements, it was necessary to develop an apparatus that measures milligram samples in the temperature range 50-1000 mK, and magnetic field range 0-12 T. The field exerts a mechanical force upon the sample, which causes it to rotate, perturbing the system destructively. The apparatus developed in this thesis overcomes this diffculty by holding the sample with tensioned kevlar wires. Testing was done by making measurements on UPt3, a well characterised superconductor. It was then used to measure URhGe in zero magnetic field. The extension to measurements in high magnetic field were not performed however, due to the structural integrity of the apparatus being weak - this was in an attempt to reduce the thermodynamic signature of the background. After many iterations of apparatus design and build, the device was proved not appropriate for high fields. A discussion of the zero-field data, as well as the design and build process, is given. The Curie temperature of URhGe is suppressed with magnetic field (applied along the b-axis), reaching zero temperature at the moment rotation transition referred to above. Small angle neutron scattering (SANS) was measured at both zero and finite fields to detect the evolution and relaxation of the critical fluctuations. The scattering is inelastic and the SANS measurement integrates over energy. Nevertheless it was possible to compare models with different dynamical dependences for the magnetic relaxation. In field, however, the magnitude of the fluctuations was strongly reduced, falling below the detection limit at half the critical field. Comparing Landau damping to various forms of non-Landau damping, a result was found that agrees with that for the ferromagnetic superconductors UGe2 and UCoGe, but the lack of critical scattering at field is found to be in contradiction with NMR measurements, which is discussed. UAu2 is a new material on the heavy fermion landscape. The crystal structure found suggests some frustrated magnetism, culminating in a Neél temperature of 43 K and a further transition at 400 mK; this suggests some new quantum criticality not seen before, and so heat capacity measurements were performed with the already-tested apparatus to see if, as the resistivity measurements suggest, a Fermi-liquid state is found. Results revealed differences between annealed and non-annealed samples in their thermodynamic signature, and the behaviour expected for antiferromagnetic spin-fluctuations is found to continue to temperatures below 150 mK, suggesting the existence of a quantum critical point. The validity of these results along with implications are discussed.