Faraday rotation of bismuth substituted terbium iron garnets

The spontaneous magnetization and the Faraday rotation FR, or [110]-oriented single crystal of composition Bi_xTb_3-x Fe₅O₁₂ with x=0.60 have been investigated as a function of temperature and compared with bismuth-free terbium iron-garnet. The experimental data, obtained at 1150 nm wavelength in the 10-300 K temperature range, are discussed in terms of electric and magnetic dipole transitions within the three magnetic sublattices. Assuming that the contribution of the Fe³⁺ ions to FR is the same as bismuth-substituted yttrium iron garnet (for identical Bi content), the contribution to FR of the Tb dodecahedral ion exhibits a strong reduction induced by the bismuth incorporation. A strong temperature variation of the 1st order electric dipole coefficient for the rare earth is found; in a first approximation, this coefficient presents a linear temperature dependence in the 100-300 K temperature range as previously observed in the pure TbIG