Magnetic linear birefringence of light in Tb3Ga5O12

Abstract The spectra of the magnetic linear birefringence (MLB) in the terbium-gallium garnet TbGG (Tb3Ga5O12) have been studied within the visible spectral range for temperature T = 90 K. Analysis of the temperature and spectral dependences of MLB has made it possible to determine the “effective” frequencies of the allowed f → d electric dipolar transitions in rare-earth (RE) ions of Tb3+ ions responsible for the formation of quadratic magneto-optical effect in this garnet. The results of the performed magneto-optical experiments in TbGG have confirmed the Akulov's relations (for the quadratic magnetic effects) arising from the phenomenological consideration of MLB for the values of Δn measured at the orientation of the external field in the plane (110) along the main crystallographic directions of the garnet crystal. From analysis of the experimental data on MLB was obtained that for the TbGG MLB changes quadratically with the field within the temperature range 90–300 K. At the same time with a decrease in temperature, its value grows inversely proportional to the square of temperature. A certain difference was also found in the “effective" frequencies of the allowed transitions in Tb3+ ions of the garnet, which is determined from the optical absorption spectra of TbGG on the one hand, and from the analysis of the spectral dependence of MLD on the other one. It has been assumed that this difference can be due to significant distortions of the TbGG crystal lattice caused by this significant magnetostriction at low temperatures, leading to the "mixing" of the 9D term states with the "optically resolved" states of the 7D term.