Growth and Properties of Bi-Doped Terbium Iron Garnet Crystals Produced Using the Top-Seeded Solution Growth Method

Bi-doped rare-earth iron garnet (Bi:RIG) single crystals are the core of optical isolators, and demand for them is surging due to the development of artificial intelligence (AI) technology. In this work, bismuth-doped terbium iron garnet (Bi:TbIG) single crystals with a composition of Bi_0.86Tb_2.14Fe₅O₁₂ and a size of 37 mm were successfully grown by the top-seeded solution growth (TSSG) method using a lead-containing flux system. These crystals exhibited a regular rhombic dodecahedron morphology enclosed by the {110} plane, and a growth rate of 0.018 mm/h perpendicular to the {110} planes. Systematic characterizations revealed that the crystals exhibited good compositional homogeneity, with no obvious Fe, Tb and Bi segregation from center to edge. Rocking curve tests presented a full width at half maximum of 172 arcsec. X-ray photoelectron spectroscopy (XPS) results demonstrated that Fe exists exclusively in the +3 valence state without detectable Fe²⁺, whereas Tb is present in the +4 valence state. In addition, O was lattice O²⁻, without obvious defects. Magneto-optical tests indicated that the uncoated TSSG-grown Bi:TbIG crystals had 71% transmittance in the 1200~1600 nm waveband, and a Faraday rotation coefficient of 0.132°/μm at 1310 nm. The 11 × 11 mm samples exhibited an extinction ratio stably above 40 dB. The 349 μm thick samples meet the application requirements of miniaturized optical isolators. This study verifies the feasibility of TSSG for growing Bi:TbIG single crystals, offering a new technical route for Bi:TbIG growth with potential value for practical application.