Effect Of Tb And Pr Substitution On The Structural And Magnetic Properties Of BiFeO₃

Multiferroics are materials that display ferroelectricity （antiferroelectricity）,ferromagnetism （antiferromagnetism） and/or ferroelasticity （antiferroelasticity）simultaneously. In these functional materials, there exhibit not only the aboveproperties but also the coupling and controling effects among the properties, which isdifferent from the single ferroelectrics and ferromagnet. The coupling properties makemultiferroics possess of broad potiential applications in magnetoelectric storage,spintronic devices, magnetoelectric controller, and so on. Among all the multiferroics,BiFeO₃has attracted more attention due to the coexistence of ferroelectricity andferromagnetism at room temperature, ferroelectric transition temperature （TC¹100K）and antiferromagnetic transition temperature （T_N⁶40K）. However, BiFeO₃, theearliest researched multiferroic material, has not been applied in practice, which ismainly attributed to the difficulty on preparation of pure BiFeO₃, high leakage currentand weak magnetism.Many researches demonstrate that the physical properties of BiFeO₃can beimproved through the rare-earth ion substitution on Bi site. However, as the existenceof parasitical impurity phases in many reports, the mechanism of rare-earth ionsubstitution is explained differently, which is attributed to the existence of parasiticalimpurity phases in many reports. To overcome the obstacles mentioned above, mainresearches are listed in the following:1) The single phase Bi_1-xRe_xFeO₃（Re=Tb, Pr） powders were prepared bymodified sol-gel method. This provides a beneficial prerequisite to study thelattice structure and multiferroic properties of the Bi_1-xRe_xFeO₃system.2) The structural and multiferroic property evolutions of smaller Tb ionsubstituted on A site of BiFeO₃are studied by combining the X-ray diffraction,Raman scattering and physics property measurement system. It shows that thelattice structure of Bi_1-xTb_xFeO₃transformed from rhombohedral R3csymmetry to orthorhombic Pnma phase with increasing Tb concentration x to 0.125, accompanied with the ferroelectric to paraelectric phase transition. Themagnetometry measurement shows the magnetization is enhanced initiallywith Tb substitution, and then decreases gradually for x≥0.125after passingmaximum at the polar-nonpolar phase transition boundary. The spiralmagnetic structure was suppressed in R3c phase, resulting in the enhancedmagnetization, and then the spin formed well unparalled antiferromagnetismgradually in the Pnma phase. It follows that the obvious magnetoelectric effectshould be found in the compounds at the phase transition boundary.3) As the different radius of Pr and Tb ions, the substitution of the lagger Pr onBi site of BiFeO₃shows many differences from Tb substitution. The structureof Bi_1-xPr_xFeO₃powders changes firstly from rhombohedral R3c toorthorhombic Pbam at x0.15, and then to orthorhombic Pnma phase at x=0.3.The magnetization is improved with increasing Pr content x, and appearsobvious enhancement at the phase transitions x=0.15and0.25. Whereas theremnant magnetization begins to decrease as the lattice structure changes intoPnma phase at x0.3, which is similar to the magnetization evolution ofBi1-xTbxFeO3system. Interestingly, it is observed a―switching effect‖in thehysteresis loop of Bi0.5Pr0.5FeO3sample, indicating that the magnetism isperfect antiferromagnetic ordering rather than spiral structure.4) The relation between structure and superexchange interaction of Bi_1-xPr_xFeO₃（0.5≤x≤1.0） was studied by X-ray diffraction and DSC. With increasing Prsubstituted concentration, the lattice volume contracts gradually, and theinteraction between Fe3+ions in Fe-O-Fe linkage was enhanced, inducing theincrease on T_N. Additionally, it is satisfied the quantitative relationshipT_N∝-cosΘ=-（cosθ₁+2cosθ₂）/3between the T_Nand superexchange angle θ_....