EPR in dysprosium aluminum garnet at far-infrared frequencies

Far-infrared transmission measurements have been made on spherical single crystals of dysprosium aluminum garnet (${\mathrm{Dy}}_3$ ${\mathrm{Al}}_5$ ${\mathrm{O}}_{12}$) and diluted dysprosium aluminum garnet [(${\mathrm{Dy}}_{0.1}$ ${\mathrm{Y}}_{0.9}$ ${)}_3$ ${\mathrm{Al}}_5$ ${\mathrm{O}}_{12}$], at frequencies between 157 and 1629 GHz in magnetic fields up to 7.3 T. The strongest resonance was consistent with previous results, corresponding to $g_z$=18.02±0.07 and a small internal field. A number of weaker resonances could be identified with sites at which one or more nearest neighbors were reversed. The corresponding interaction parameter $K_1$=(0.8±0.05)$k_B$ is consistent with earlier estimates. In addition, a number of unexpected resonances were observed. Several of these could be identified with ${\mathrm{Dy}}^{3+}$ ions on octahedral crystallographic a sites, having ${\mathit{g}}_{\mathrm{\parallel }}$≊10.8 and ${\mathit{g}}_{\mathrm{\perp }}$≊2.8 and axes along {111}. The intensities were consistent with estimates based on x-ray data, which indicate concentrations of the order of 1% of a sites occupied by ${\mathrm{Dy}}^{3+}$ ions. The presence of such substitutions may explain various small inconsistencies previously noted in detailed analyses of thermodynamic and neutron-scattering data.