Abstract
The magnetic excitation spectrum of the singlet-doublet magnet double-hexagonal close-packed praseodymium, which has been measured by means of inelastic neutron scattering, is analyzed using a diagrammatic high-density expansion technique. The lowest-order random-phase-approximation diagrams give a detailed description of the wave-vector and temperature dependence of the four modes in terms of a Hamiltonian including isotropic Heisenberg interatomic exchange interactions and anisotropic, dipolarlike exchange interactions. In general, the modes are linearly or elliptically polarized. The leading contributions to the line shapes of the excitations are obtained by extending the 1Z expansion of the generalized susceptibility propagators one order beyond the random-phase approximation. This damping corresponds to spin-wave scattering on single-site fluctuations. The propagators are calculated self-consistently by including internally renormalized Green's functions. The theoretical spectral functions are in detailed agreement with experiment.