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|Title:||Composition-Driven Structural and Magnetic Phase Transitions in Bismuth Ferrite|
|Abstract:||The coupling among the different ferroic and/or antiferroic orders offers an additional degree of freedom for the device design. As an example, the coupling between the ferroelectric and magnetic order parameters gives rise to a magnetoelectric effect. It allows the design of new devices such as, electric field controlled magnetic memories. The potential applications span from frequency-dependent devices, i.e. filters, oscillators and phase filters to transducers and magnetic field sensors. Besides the technological interest, it provides the unique materials which offer the chance to understand the complex interactions between different ferroic orders. As a single phase material, perovskite BiFeO3 (BFO) has received rapidly increasing attention since the first report of enhanced multiferroic properties in its epitaxial thin film. It exhibits the coexistence of ferroelectric order and G-type antiferromagnetic order well above the room temperature (TC ~ 1103 K and TN ~ 643 K). The high ferroelectric and ferromagnetic polarization or large magetoelectric coupling constant at room temperature have been reported by modifying the crystal structure through Bi-site and/or Fe-site substitution/doping in BFO. Although there are several reports on the crystal structure of BFO and its derivative, the determination of oxygen octahedra tilting and quantitative crystal symmetries is still lacking in the literature which is essential to explain the composition-driven magnetic phase transitions. Also, the analysis based on ferromagnetic contributions in the magnetic hysteresis loops and Arrott plot technique for confirmation of ferromagnetic ordering is essential to explain the magnetic phase transition. The signature of crossover from antiferromagnetic to weak ferromagnetic and vice-versa can be clearly shown using Arrott plot analysis. The main aim of the present thesis is to calculate the percentage of different crystal symmetries and oxygen octahedra tilting angle using multiphase Rietveld analysis of XRD patterns to explain the magnetic properties of co-substituted BFO. The thesis has been organized in seven chapters namely: (1) Introduction, (2) Experimental Techniques, (3) Crystal Structure and Magnetic Properties of BiFeO3, (4) Crystal Structure and Magnetic Properties of Bi1-xCaxFe1-xTxO3 (T = Mn & Ti), (5) Crystal Structure and Magnetic Properties of Bi1-xLaxFe1-xTxO3 (T = Mn & Ti), (6) Crystal Structure and Magnetic Properties of Bi1-xAxFe1-xMnxO3 (A = Nd & Eu) and (7) Conclusions. A few of important conclusions of the thesis are, (1) the co-substitution suppresses the impurities phases in BFO; (2) composition-driven structural transition from rhombohedral (R3c space group) to orthorhombic symmetry (Pbnm space group) has been observed; (3) The co-substitution enhances the remnant magnetic moment (Mr) due to the destruction of cycloid spin structure and uncompensated surface spins; (4) Further increase in the percentage of co-substitution results in the reduction of remnant magnetization due to the appearance of collinear antiferromagnetic ordering in the Pbnm space group which becomes dominant for higher percentage; (5) Arrott plot analysis of magnetic hysteresis curve clearly indicates the composition-driven crossover from the antiferromagnetic to weak ferromagnetic ordering and vice-versa.|
|Appears in Collections:||05. PH|
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