Magnetic excitations in rare earth Al2 compounds

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Abstract
In the rare earth intermetallic compounds, the crystal field is often comparable to the exchange interaction between the magnetic ions. The magnetic excitations (excitons) in such systems are transitions between single‐ion atomic levels, propagating through the lattice via the exchange interaction. This article reviews recent experimental and theoretical work on rare earth Al2 compounds. The excitons in NdAl2 and TbAl2 have been studied by inelastic neutron scattering. The dispersion relations are analysed 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 5 K 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 4 K the magnetic excitations in TbAl2 are spin waves. At temperatures above 35 K the neutorn scattering measurements show a double peak structure. This splitting of the spin wave branch is the reuslt of an interaction between the spin‐waves and excitations associated with higher‐lying mean‐field states, a magnon‐exciton interaction.

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