A Neutron Scattering Investigation of Uranium Selenide
US, USe, UTe form a series of rocksalt structure ferromagnets which exhibit unusual magnetic properties. At high temperatures all three compounds are paramagnetic, and exhibit local-moment-like behaviour. At low temperatures, however, only UTe exhibits an excitation spectrum which is characteristic of a local moment system. In US a broad continuum of magnetic response more akin to what is observed in an iinerent magnet is found. In this work, we focus on the intermediate member of the series, USe.
We have performed a detailed inelastic neutron scattering study of the excitations in the low temperature ferromagnetic phase of USe. A magnetic excitation branch is observed which arises from an extremely large zone centre anisotropy gap ( 10 THz) and disperses quadratically at small q. The spectrum is unusual in a number of respects. At the zone centre, the excitation appears as a well-defined peak in the inelastic spectrum but which an intrinsic width (3 THz). At larger q, the scattering weakens and broadens appreciably. Measurements on a single domain sample have shown that the scattering is unpolarized, i.e. it contains (approximoately) equal amounts of transverse and longitudinal response.
We have attempted to describe, as far as possible, the behaviour of the chalcogenides using localized electron model of the uranium ions. The Hund's rule ground state of the uranium ion is parturbed by a Hamiltonian which includes the effect of a cubic crystal field, exchange and quadrupolar interaction, and magnetoelastic coupling. With this model it is possible to reproduce a number of the magnetic and elastic observables in the UX compounds.
Finally, the ferromagnetic phase transition has been examined using critical neutron scattering techniques. The temperature dependence of the inverse correlation length and static susceptibility have been investigated and found to exhibit the characteristic power law behaviour of a critical phase transition. The critical exponents extracted from this measurement agree reasonably well with those of the three-dimensional Heisenberg model. The order parameter, however, varies in an unusual way: the onset of long range order occurs roughly 5 k above the critical temperature. This behaviour may be linked to the presence of large quadrupolar and magnetoelastic effects in the system.