Study On Structure,Magnetism And Magneto-optical Properties Of New Nanocrystalline Magneto-optical Glasses

In the era of rapid development of information transmission,photonic glass and optical fiber play an important role in large capacity information transmission.As one of the important branches of photonic glass,nanocrystalline magneto-optical glass has the advantages of both glass and crystal:magneto-optical glass has low/no temperature-dependent Faraday rotation,high magnetization,and good thermal/mechanical stability,and is required for a variety of magneto-optical sensing and photonic applications.Glass containing magnetic nanocrystals has very unique optical and magnetic properties,which has become one of the hot research directions.LaAlO₃-doped diamagnetic glass,during heat treatment,the crystal precipitated will undergo phase transition induced oxygen vacancy,which helps to enhance the magneto-optical properties.Nanocrystals formed by transition metal ions have an unfilled electron shell due to the unpaired electrons in the d orbital.Under the action of crystal field,energy level splitting will occur and enhance the magneto-optical properties of glass.Moreover,nanocrystals have smaller optical band gap than conventional materials,and their doping magneto-optical glass can enhance the magneto-optical properties of the glass.Diamagnetic glass with Pb²⁺,Bi³⁺,and Te²⁺plasmas containing outer electron structure that is easily polarized and large ionic radius also has strong magneto-optical properties.Therefore,it is of great significance to study the structure and magneto-optical properties of nanocrystalline magneto-optical glass,which can provide theoretical support for the development and application of this kind of materials.In this thesis,three kinds of nanomaterials were prepared by hydrothermal method and sol-gel method.All three materials have the advantages of smaller optical band gap,higher purity and smaller size than conventional materials.The materials were successfully prepared by X-ray diffraction（XRD）,electron microscopy（SEM）,Fourier transform spectroscopy（FT-IR）and UV-Vis spectra.Nano-LaAlO₃（LAO）,CoCr₂O_4（）CCOand KBiFe₂O₅（KBFO）doped with Pb O-TeO₂-B₂O₃（PTB）,Pb O-Bi₂O₃-B₂O₃（PBB）and Pbo-Bi₂O₃-B₂O₃-Al₂O₃（PBBA）glass systems have been studied.The nanocrystalline glass is formed by heat treatment at a certain temperature.The structure,magnetism and magneto-optical properties of the nanocrystalline glass were studied by a series of properties.To explore the improvement of magnetic and magneto-optical properties of glass,the following conclusions are mainly obtained:（1）LaAlO₃ was doped into PTB glass at the concentration of 1,5,10,15 mol%,respectively.By means of nuclear magnetic resonance（NMR）,X-ray neutron diffraction,transmission electron microscopy,X-ray photoelectron microscopy,vibrating sample magnetometer,electron paramagnetic resonance（EPR）and other techniques,the phase transition of LaAlO₃in lead-oxide tellurate glass and its effects on its structure,magnetism and Faraday rotation properties were obtained and verified.The cubic phase of LaAlO₃ nanocrystals in annealed glass is 17 nm,belonging to Pm3m space group.The cubic LaAlO₃ phase changed to a 14-27nm-rhomboid phase due to temperature induced viscosity and strain in the melt after 2 h crystallization treatment of 360℃.Through the phase transition,the transformation of TeO₄→TeO₃,AlO₆→AlO₄,BO₄→BO₃ occurs,and non-bridging oxygen is produced,resulting in a great change in the glass structure.The doping of LaAlO₃ enhanced the diamagnetism of the glass,but the weak diamagnetism changed to strong ferromagnetism after the phase transition,and the magnetization of the glass increased significantly with the increase of LaAlO₃ doping amount（M_s=55 emu/g）.The magnetic enhancement mechanism of glass was investigated by ²⁷AlNMR and EPR.The Verdet constant of 10%LaAlO₃ doped glass at 633 nm is increased to 0.5853 min/G·cm,which is higher than that reported in the literature.（2）CoCr₂O₄ exhibits ferromagnetism and a small optical band gap.PBB glass was doped with 0.1,0.2,0.3 and 0.4 mol%,respectively.It was crystallized at 350℃ for 2 h.The crystal is proved to be tetragonal Bi_7.53Co_0.47O₁₂ by XRD.SEM shows that the grain size is 50±5nm.FT-IR has proved that CoCr₂O₄ doping causes BO₄→BO₃ transition in nanocrystalline glass.UV-Vis showed that CoCr₂O₄ doping reduced the optical band gap of nanocrystalline glass,and the electron transition occurred for ⁴T_1g（F）→²T_1g（H）octahedral Co²⁺ions,⁴A_2g（F）→⁴T_1g（P）tetrahedral Co²⁺to ⁵T_2g→⁵E_1g octahedral Co³⁺ions.The hysteresis loop shows that the transformation from diamagnetism to ferromagnetism occurs when the CoCr₂O₄ doping amount is 0.4 mol%.The maximum Verdet constant is 0.206 min/G·cm@633 nm.（3）The new KBiFe₂O₅ nanocrystalline E_g is relative narrow（1.6 e V）and has strong magnetic and photoelectric activity.It was doped with 0.5,1,1.5 and 2 mol%PBBA glass and annealed at 340℃for 2 h to precipitate crystals in the glass.XRD results show that the crystal is Bi₂Fe₄O₉.The grain size on the SEM surface is in the range of 30-40 nm.FT-IR demonstrated the conversion of BO₄ to BO₃ and the evolution ofAlO₄ toAlO₆ in glass.UV-VIS shows that the optical band gap of nanocrystalline glass decreases with increasing doping amount of KBiFe₂O₅.It can be seen from the hysteresis curve that the glass doped with 1.5mol%and 2 mol%has changed from diamagnetism to ferromagnetism.The maximum Verdet constant is 0.1589 min/G·cm@633 nm.