Excitations and Magnetic Properties of Rare-Earth Al₂ Compounds

Magnetic excitation spectra, measured by inelastic neutron scattering, and single-crystal magnetization curves of rare-earth AI₂ compounds are analysed using a Hamiltonian including 4th and 6th order crystal field terms and an isotropic exchange interaction between the magnetic ions. The magnetic excitations - "excitons" - are transitions between single-ion. atomic levels propagating through the lattice via the exchange interaction. In NdAl₂ dispersion relations of excited mean-field states are described for the first time. A double peak structure in the neutron spectrum of TbAl₂ is interpreted as a new effect: the magnon- exciton interaction. To calculate the excitation energies a Green's function theory was evaluated. A detailed microscopic description of the magnetic anisotropy of NdAl₂. PrAl₂ TbAl₂, and ErAl₂ is given in terms of two temperature and field independent crystal field parameters by means of a two-dimensional mean-field theory. The crystal field parameters are shown to behave systematically throughout the entire rar e earth AU series, indicating that the effective charge distribution around the magnetic ions is independent of the particular rare earth constituent.