Magnetic excitations in plutonium monoantimonide

Neutron inelastic scattering studies of uranium compounds have illustrated the complexity of the interactions in this part of periodic table. Recently, large crystals of plutonium compounds have been grown at the transuranium Institute and we report here the first neutron inelastic scattering on PuSb. A monodomain sample of ferromagnetic PuSb was prepared by cooling in a field with Hparallel[001]. We observe an almost dispersionsless magnetic transition, assigned to arise within the GAMMA8 ground state, with an energy of 4.3 THz. Surprisingly, the lowest frequency (3.5 THz) excitation occurs at the zone boundary with a polarization along the wavevector. Theoretical predictions give the spin-wave gap and GAMMA8-GAMMA7 crystal field splitting (>12 THz experimentally) but are presently unable to explain the lifting of the degeneracy of the longitudinal and transverse modes, and the minimum of the former at the zone boundary. (orig.)