Spin and phonon excitations in actinide systems

Abstract

Recent Chalk River neutron scattering experiments on antiferromagnetic UN, on ferromagnetic UTe and US, and on non-magnetic UPd 3, are reviewed. The spin excitations may be sharp or diffuse and the phonons exhibit anomalies reminiscent of mixed-valent compounds. A very detailed study of the dispersion of the sharp collective excitations seen in dhcp UPd 3 has shown that the uranium is in a stable f 2, J = 4, ground state subject to a conventional crystal field and exchange. Thus, UPd 3 is the canonical 5f system, analogous to dhcp Pr in the 4f series. Although UN is cubic, large anisotropies are found in its critical scattering, and large anisotropy gaps are found in the spin wave spectra of UN and UTe.