Investigation of structural, electrical, magnetic and optical properties of isovalent and aliovalent substituted BiFeO3

Abstract

BiFeO3 (BFO) is an interesting material as it exhibits multiferroic properties at room temperature and is a promising candidate for magnetoelectric applications. However, it is difficult to synthesize single phase BFO and control leakage current, which restrict its use in commercial applications. Presence of free charge carriers in the system formed due to volatile nature of Bi, also suppresses intrinsic ferroelectric response of the system. BFO exhibit complex magnetic structure i.e., presence of the spin cycloid structure results in the net cancellation of the magnetic moments . The main theme of this thesis is to present a comprehensive study on BFO by cationic modification and its effect on the structural, electrical, optical and magnetic properties. As BFO is a complex system, therefore, for the sake of clarity, the thesis is divided into different parts. Firstly, detailed study of the structural phase transformation triggered by the cationic substitution and co-substitution is discussed. Chemical pressure exerted by these substituents and its effect on bond lengths, microstrain, bond angles, octahedral tilting, ionic radii mismatch, and phase transformation is systematically investigated in detail. Correlation between these parameters have been drawn and represented in the form of a phase diagram. Change in peak position and intensity of Raman vibrational modes and transformation of structure from rhombohedral to orthorhombic phase are identified to be associated with the substituent and it`s concentration. Five electronic transitions, two d-d and three charge transfer (CT) transitions, are observed in optical measurement within the spectral range of 1–5 eV. Dependence of the direct band gap on the concentration of substituent ion is also studied. A-site and B-site cationic substitution with contrasting magnetic moment and dissimilar size, led to change in the local structure by modifying bond angles. Hence substitution effects spin structure through weakening of Dzyaloshinsky–Moriya (DM) interaction and resulted in destruction of spin cycloid configuration. Intriguing as well as anomalous magnetic response such as weak ferromagnetism with high coercivity, stair step like loops with significant drop in coercivity at low temperatures has also been observed as a function of dopant concentration. Weakening of DM interaction, field induced melting of antiferromagnetic clusters and modification in effective magnetic anisotropy including contribution of magnetoelectric coupling, at different dopant concentration are responsible for the observed results. DRSML QAU | xvi Detailed electrical and dielectric responses of BiFeO3 is also presented. Main goal in this section was to intentionally manipulate the formation of space charges via a variety of isovalent and aliovalent substitutions. Polydispersive inter- and intra-grain hopping mechanisms are traced as a function of substitution to highlight finer details associated with their origin. Additionally, for quantification purposes, equivalent circuit analysis of Nyquist and Cole-Cole plots is carried out to extract intra/inter grain parameters, such as resistivity and permittivity, and to establish the polydispersive nature of the conduction mechanisms. Equivalent circuit analysis is also utilized for estimation of the double Schottky barrier potential and width. The formation of double Schottky barriers at the grain boundaries is interpreted to be associated with the acceptor states formed at the grain boundary and responsible for the appearance of a positive temperature coefficient of resistance. In last section, thin films of BFO are prepared and characterized. Eu3+ being an isovalent ion is chosen specifically to avoid the formation of oxygen vacancies. Local structure of Eu3+ substituted BFO have been manipulated systematically and it’s correlation with the electronic structure has been presented in detail. Systematic variation of Eu3+ substitution on bond length, bond angles, octahedral rotation, and phase transformation is quantified and their effect on Raman modes has been traced as a function of Eu3+ concentration. Capacitance and dissipation spectrum as a function of temperature indicates the presence of two spin reorientation transitions in pure BFO whose origin is associated with the magnon-phonon interaction. In higher concentration of Eu3+ substituted samples, two additional transitions are also observed which is an indication of competing interactions between Eu3+–Fe3+ and Eu3+–Eu3+ ions. Optical response of all the prepared thin films have also been investigated in detail. An electronic energy-level diagram of polycrystalline Bi1-xEuxFeO3 (0<x<0.225) has been presented. An additional change is the emergence of the 4f levels of the rare earth substituent in the valence and conduction bands, Charge-transfer, and doubly degenerated d-d transitions (6A1g→4T2g and 6A1g→4T1g) have been traced as a function of local changes at the unit cell level. In centrosymmetric configuration, 6A1g→4T1g electronic transition is found to be forbidden by the Laporte Rule