Growth And Characterization Of Tb³⁺ Doped Yttrium Iron Garnet Crystals

The rare earth garnet ferrite(R₃Fe₅O₁₂,RIG)crystals,represented by Yttrium iron garnet(Y₃Fe₅O₁₂,YIG),have been regarded as excellent magneto-optical medium from the near-infrared to mid-infrared band.It is characterized with advantages of high Verdet constant,high light transmittance and low absorption loss,with great potential for the applications in the fields of optical fiber communication,microwave and high power lasers.However,the growth of large size YIG crystal is rather challenging,and high quality bulk single crystals has not been successfully grown by the traditional methods.In addition,due to the exchange interactions between rare earth ions and iron ions,the regulation of rare earth ions on the magnetic and magneto-optical properties of the system has not been systematically studied.In this paper,a series of Tb³⁺doped yttrium iron garnet crystals have been grown by the flux-Bridgman method,with optimized growth parameters.The crystal structure,crystalline morphology,optical and magneto-optical properties of the crystals have been systematically studied.A series of Y_3-xTb_xFe₅O₁₂（x=0.0,0.5,1.0,1.5,3.0）crystals have been grown by flux-Bridgman method,with Pb O-Pb F₂-HBO₃solvents.Combined with the defect analysis and crystallization mechanism,and the growth parameters have also been optimized.The furnace temperature is controlled between 1200℃and 1280℃,the temperature gradient is controlled from 5℃to 6℃/mm,and the temperature gradient in the constant temperature area is less than 3℃/mm.The rapid nucleation process was designed in order to obtain large size single crystals with a cooling procedure,and the growth period was shortened to within five days.Tb:YIG crystals with size up to 10-25mm have been obtained,indicating that flux-Bridgman method shows great potential to grow large size and high-quality RIG magneto-optical crystals.Tb:YIG crystals maintain typical cubic garnet structure with spatial group of Ia3d.Their lattice constants range from 12.377（?）（x=0.0）to 12.448（?）（x=3.0）,and the volumes change from 1896.063（?）³（x=0.0）to 1928.966（?）³（x=3.0）.Both of them change approximately linearly with the increase of terbium ion concentration.The crystals were processed into wafers along the direction of[111].Morphology and composition analysis showed that the distribution of elements on the whole wafer was uniform and in line with the standard stoichiometric ratio.Moreover,there were also some obvious inclusions,which are mainly Pb O formed due to the rapid nucleation during the crystal growth.The refractive index increases with the increase of Tb³⁺doping concentration,and it increases from 2.17（YIG）to 2.25（TIG）at 1050 nm wavelength.The refractive index decreases with the increase of wavelength.In the wavelength range of 1150nm-1600nm,the transmittance decreases from 74%（YIG）to 67%（TIG）with the increase of the doping concentration of Tb³⁺ions,and the characteristic absorption peak of Tb³⁺appears at 1850nm.The measured transmittance of YIG and TIG at1550nm is～99%and～92%of the theoretical value,respectively,showing higher transmittance and lower optical loss.The optical band gaps of Y_3-xTb_xFe₅O₁₂（x=0,1.5,3）crystals are 1.281,1.263 and 1.278 e V,respectively,indicating that Tb ion doping has little effect on the optical band gaps of YIG crystals.The saturation magnetization of YIG decreased from 22.51 emu/g（YIG）to 5.75 emu/g（TIG）.The specific Faraday rotation angles of both YIG and TIG increase with the increase of wavelength.The Faraday rotation Angle of YIG and TIG reached 316.78°/cm and 406.02°/cm at 1050 nm,respectively.