Incommensurate magnetism in UAu2

Abstract

The aim of this thesis was to identify a candidate material for spin-triplet superconductivity with a two-component order parameter. This unconventional superconducting state is thought to allow for exotic quantum states such as Majorana fermions. A wide survey was taken into potential candidate materials, and UAu2 was chosen for in-depth investigation. This little-studied hexagonal heavy fermion compound's unusual resistivity behaviour, combined with a series of interesting features in magnetisation and heat capacity, make it an extremely interesting material to study.

The phase diagram of UAu2 was determined with measurements of heat capacity, resistivity, magnetisation and magnetoresistance on the first single crystalline samples of this material. No superconductivity was detected. Instead, a range of magnetic phase transitions were observed, which were further investigated with muon-spin relaxation experiments and time-of- ight neutron powder diffraction.

UAu2 was found to undergo a transition to an incommensurate antiferromagnetic state (q1 = (1=3; 1=3; δ)) below TN = 43:5 K, but then develops signatures of weak ferromagnetism below T = 20 K. The ferromagnetism coincides with a 2q magnetic structure, with a coexistence of q1 and q2=(1/3,1/3,0). The magnetic structures of both phases were found to be most likely amplitude-modulated, with moments aligned along the crystallographic c-axis. A transition to a ferromagnetic state was observed in magnetic fields applied parallel to the c-axis.

TN was found to remain almost constant in applied magnetic fields up to 9 T, while hydrostatic pressures of up to 6 kbar weakly suppress the antiferromagnetic transition temperature. The field-induced transition was found to be strongly pressure-dependent, shifting to higher applied fields with increasing pressure. The residual resistivity of UAu2 samples prepared by both the Czochralski method and quenching from the melt is relatively large, which may inhibit Cooper pairing and hence may be the reason for the absence of superconductivity in the samples investigated. Solid-state electrotransport (SSE) equipment was developed, which can induce the motion of a crystal's constituents and thereby lead to vastly improved sample quality. Refinement of UAu2 samples with SSE could be a further step in the search for spin-triplet superconductivity in this material.