Study On The Structure And Magnetic Properties Of BiFeO₃with Tb Substitution

Multiferroic materials are those who simultaneously have ferroelectricityand ferromagnetism, and expected to have broad prospect of application forinformation storage, spintronic devices and photoelectric sensors. BiFeO3hasattracted much attention recently due to its coexistent ferroelectric order andmagnetic order at room temperature. However, its practical applications were greatlyhampered by weak ferromagnetism which caused by impurities and special magneticstructure. In the paper, different methods for the synthesis of bismuth ferrite withperovskite structure were used. The physical and chemical characteristics, such asstructure, morphology, optical properties and magnetic properties of bismuth ferritewere investigated by XRD, SEM, UV-Vis and VSM, respectively. Factors affectingthe bismuth ferrite properties were explored. Cation substitution was adopted byrare-earth ion Tb3+to improve optical properties and magnetic properties of bismuthferrite. The mechanism of rare earth ion Tb3+on the improvment of optical propertiesand magnetic properties was studied. The main discoveries were as follows:(1)By controlling hydrothermal temperature and the concentration ofmineralizer, BiFeO3powders could be synthesised by a KOH concentration of6Mfor a reaction time of6h at200-240℃. The concentration of KOH, hydrothermaltemperature played an important role in the synthesis of BiFeO3powders. BiFeO3isobtained by hydrothermal reaction with a perovskite structure and a weakferromagnetic property. Besides, the magnetic of BiFeO3powders graduallyweakened with the increasement of hydrothermal temperature.(2) BiFeO3powders can be obtained when the heat treatment temperature at550℃by a sol-gel process. XRD and FT-IR studies confirmed the rhombohedralstructure of the synthesized nanopowder. Pure BiFeO3nanopowder is with meangrain size of80-120nm. BiFeO3nanopowder may respond to visible light, theabsorption edge of the synthesized nanopowder at550nm, in visible region, with energy bandgap of2.19eV. In addition, the nanopowder exhibited a typicalferromagnetic behavior at room temperature due to the size confinement effect of thenanostructure. Single-phase BiFeO3powders showed exchange bias phenomenon.(3) Bi1-xTbxFeO3(x=0，0.03，0.05，0.08，0.1) podwers can be obtained when theheat treatment temperature at550℃by a sol-gel process. The XRD patterns showedthat Tb3+ions may occupy the Bi site in BiFeO3, and the structural transformation ofBi1-xTbxFeO3from rhombohedrally distorted perovskite structure to orthorhombicsymmetry with increasing doping level of Tb3+. The changes in structure led to thechanges in the optical and magnetic properties of Bi1-xTbxFeO3podwers. Responserange of BiFeO3to visible light could be adjusted by different Tb3+dopingconcentration, and the bandgap of Bi1-xTbxFeO3powders reduced with increasingdoping level. The magnetization of the Bi1-xTbxFeO3powders increased withincreasing doping level at room temperature and showed exchange bias phenomenon.(4) On the basis of the sol-gel method, the Bi1-xTbxFeO3ceramics were obtainedby traditional solid state reaction method.Impurities of BiFeO3ceramics can bereduced by the treatment of dilute nitric acid. The structural transformation ofBi1-xTbxFeO3ceramics from rhombohedrally distorted perovskite structure toorthorhombic symmetry with increasing doping level of Tb3+. The grain size ofceramics decreased and became more compacted with Tb3+substitution.Improvement of the magnetization of Bi1-xTbxFeO3ceramics is achieved by Tb3+atdifferent doping levels.