Neutron and X-ray Diffraction Studies of Magnetic Order in U-based Heavy Fermion Superconductors

Abstract

<p>UPt₃, URu₂Si₂, UNi₂Al₃ and UPd₂Al₃ form a special group among the uranium alloys because they exhibit heavy fermion character, magnetic order and superconductivity. This main interest in the study of this group of compounds resides in the simultaneous occurrence of magnetism and superconductivity at low temperature. Such a state could potentially involve an unconventional superconducting pairing mechanism, different from that contained in standard BCS theory.</p> <p>In this thesis, the magnetic states of three of these materials (URu₂Si₂, UNi₂Al₃ and UPd₂AI₃) is investigated with neutron and the relatively new resonant magnetic X-ray scattering techniques. The work presented here on URu₂Si₂ follows an earlier effort that demonstrated the applicability of the resonant X-ray technique to this weak magnetic system. Access to reciprocal space was extended in order to confirm the multipolar form of the resonant X-ray cross-section and to explore the limits of the technique compared to neutron scattering. The situation with the newly discovered UNi₂Al₃ and UPd₂Al₃ was different since their magnetic structure and phases needed first to be established. This task was achieved using two magnetic probes (neutron and X-ray scattering). Several magnetic order parameters in the normal and in the superconducting phase were also measured. The incommensurate magnetic order found in UNi₂Al₃ by neutron scattering constitutes the first observation of long range order in this compound. Other measurements on this compound provided some clues about the evolution of the magnetic structure in high magnetic fields.</p>