Spin waves and central modes in crystal field systems

For magnetically ordered intermetallic compounds such as PrAl2, Pr3Tl and TbSb neutron inelastic scattering experiments show that the spin-wave spectrum consists of several branches of excitations. These correspond at T = 0 to allowed transitions between the ground state and excited states of the lowest multiplet of the rare-earth ion in the exchange field existing in the material. A good description of the spin-wave spectrum can be obtained within the random phase approximation with crystal field terms and interionic interactions. At elevated temperatures excited-state spin waves are observed which correspond to transitions between excited states of the multiplet. Close to the ordering temperature and ordering wavevector the spectrum is dominated by elastic scattering with less important contributions from inelastic transitions. Generalization of the spin-wave theory to finite temperatures leads to a qualitatively correct description of these effects and insight into the nature of the ''central'' mode