Magnetic excitations in rare earth-Al2 compounds

Recent experimental and theoretical work on rare earth-Al2 compounds is reviewed. The excitons in NdAl2 and TbAl2 have been studied by inelastic neutron scattering. The dispersion relations are analyzed within the random phase approximation using a Hamiltonian including 4th- and 6th-order crystal field terms and an isotropic exchange interaction. The spectrum of NdAl2 at 50K comprises both transverse and longitudinal modes, corresponding to transitions between the ground state and excited mean-field states. The bulk magnetic properties of NdAl2 can be understood quantitatively using the crystal field parameters derived from the analysis of the exciton spectrum. At 40K the magnetic excitations in TbAl2 are spin waves. At temperatures greater than 350K the neutron scattering measurements show a double-peak structure. This splitting of the spin wave branch is the result of an interaction between the spin-waves and excitations associated with higher-lying mean-field states, a magnonexciton interaction