Growth And Characterization Of Bi³⁺ Doped Yttrium Iron Garnet Crystals

Yttrium iron garnet crystal（YIG）is a very important magneto-optical crystal.Its transmittance is 70-80%in the near infrared band and the Verdet constant is-0.26～-0.38min/（OE·cm）.Due to its excellent properties,YIG crystals are widely used in laser,electronic devices,satellite gyroscope and other fields.A series of garnet magneto-optical crystals can be formed by rare earth substitution,which can meet the performance requirements of different magneto-optical devices.However,the growth of pure and doped YIG crystals is still a technical problem for practical applications.So far,pure and doped YIG crystals were grown by liquid phase epitaxy（LPE）for optical communication.By this method,the wafer thickness of the obtained crystal is less than 0.5mm.The bulk crystal growth is still a challenge although YIG crystal has been grown by the traditional flux method,Czochralski method and optical floating-zone method.In the present work,we tried to combine the tranditional flux method and the vertical Bridgman method to grow YIG crystal.The aim of this thesis is to explore a novel approach by optimizing the flux compositions,the ratio of YIG and flux,and the growth parameters.In the experiment,mixed fluxes were designed based on PbO flux and Bi³⁺doped YIG crystal was grown in the home-made Bridgman furnace.The Y_3-xBi_xFe₅O₁₂ polycrystalline powders were pre-synthesized by solid-state reaction with x=0.1,0.3,0.5,0.7,0.9 and1.1.The pure phase powders were obtained by optimizing conditions.In the complex flux,the composition of the main flux PbO is more than 80mol%,and the others including Bi₂O₃ and B₂O₃ act as a modifier.Considering the limitation of time,we focused on the growth of Y_2.3Bi_0.7Fe₅O₁₂ crystal.The furnace temperature was set to 1200-1400℃,and the raw materials were melted.After the high-temperature solution reached a thermodynamic equilibrium,the solution was cooled down at a rate of 1℃/h～3℃/h and crucible was lowered down at a rate of 0.05mm/h.After the growth process finished,the ingot was taken off from Pt crucible.The as-grown crystals were obtianed by manual removal of solidified flux and soaking in acid.Several crystals were obtained and the largest one is up to20×25×15mm.The as-grown crystals have clear crystallographic facets and were determined as Y_2.2Bi_0.8Fe₅O₁₂ By X ray diffraction.The transmittance is measured above 75%in the range of 1300 nm～3000 nm.