Phonons and Crystal Field Levels in Dysprosium Garnets by Raman Scattering

Raman scattering from DyA1 and DyGa garnets at 80 K was investigated to determine the Raman-active phonons (3A1g+8Eg+14T2g) and the ⁶H15/2 and ⁶H13/2 crystal field levels. The frequencies and the symmetries of all the Raman-active phonons in DyGa garnets and all except one phonon in DyAl garnets were determined. The positions of the lowest four levels in the ⁶H15/2 manifold and all seven levels in the ⁶H13/2 manifold were measured in DyAl and DyGa garnets. For DyAl garnets, the ⁶H13/2 manifold has hitherto not been observed. A full crystal field calculation was carried out using our Raman data on the ⁶H15/2 and ⁶H13/2 manifolds along with the absorption data of Gr\"unberg et al. and of Aoyagi et al. on the other higher-energy crystal field levels. The crystal field Hamiltonian appropriate for the D₂ symmetry of the rare-earth sites was used, and the mixing of the different J states by the crystal field was included. The best-fit values for the nine crystal field parameters plus the free-ion energies were found. These values were similar to those obtained by Gr\"unberg et al. for the dilute garnets YAlG(Dy) YGaG(Dy). The relative electronic Raman intensities, measured for our selected combinations of incident and scattered polarizations, are listed. It is shown how these intensities are related to the Raman-scattering tensors, which in turn depend on the crystal field eigen-functions. In principle, the Raman intensity ratios can provide a good test of the crystal field analysis, analogous to the role of the g tensor.