STRUCTURAL DIELECTRIC AND MAGNETOELECTRIC PROPERTIES OF DOPED BiFeO3

Abstract

Multiferroics represent a class of multifunctional materials exhibiting several ferroic orders like (anti)ferroelectricity, (anti)ferromagnetism and ferroelasticity simultaneously. Nowadays, the multiferroic term is used even to a ferrotoroid order. The possibility of the existence of cross coupling, known as Magnetoelectric (ME) coupling between the order parameters leading to the two phenomena, namely the magnetism and the ferroelectricity is the most fascinating characteristic exhibited by these materials. From the early days of multiferroic era (in the early 1960’s) to our present time, BiFeO3 has remained prototypical example of a multiferroic exhibiting diversified uncommon properties. Multiferroics properties in BiFeO3 ceramics have not been realized except under high magnetic fields, in spite of spanning nearly 55 years of investigations. The reason for this disappointment may be due to the fact that the linear magnetoelectric interaction and magnetic vector get trapped within the spin cycloid and hence can be destabilized at high fields. Several issues have to be resolved before realization of BiFeO3 in devices such as leakage current density, large coercive field, inhomogeneous magnetic spin structure, chemical fluctuation and weak magnetoelectric coupling. Since perovskite structure can tolerate enormous amount of compositional modification, including many of the elements on the periodic table, enabling the tailoring of corresponding properties; many attempts have been made via A-site and B-site ion substitution, by making its composites and its solid solutions in order to enhance the multiferroic properties of BiFeO3 as it is having rhombohedral perovskite structure. Moreover magnetoelectric composites comprising of piezoelectric and magnetostrictive phase, has been studied to get strong ME coupling coefficient. Various phenomena concerned with structural, electrical, magnetic and magnetoelectric properties of multiferroic materials has to be understood for the specific use of these materials in designing of novel multifunctional devices. To achieve this, it requires many experimental data acquired through various techniques. Outlining the progress made so far on the study of multiferroics properties of BiFeO3, in this thesis work, the structural, dielectric, magnetic, optical and magnetoelectric properties of BiFeO3; composites of MFe2O4 (M = Mg and Ni) and BiFeO3; and solid solution of BiFeO3 with BaTiO3 have been investigated. The present thesis is divided into seven chapters followed by a bibliography. A brief summary of the work presented in each of the chapter is as under: Abstract ii The Chapter 1 contains introductory aspects and literature survey on multiferroic materials with a specific mention of BiFeO3 and briefly describes the ferroelectricity, magnetism, and causes of multiferroism in single phase and composite materials. The Chapter 2 describes the characterization techniques for structural, electrical and magnetic study employed in the present investigation. In chapter 3, we present the effect of partial substitution of high valence Nb5+ ions for Fe3+ ions on structural, dielectric, magnetic and magnetoelectric properties of BiFeO3 and La doped BiFeO3 synthesized by standard solid state reaction method. The appearances of impurity peaks for x > 0.05 in BiFe1-xNbxO3 ceramics suggested the limit of solubility of Nb5+ ions in BiFeO3. The lowering of dielectric loss tangent by Nb5+ addition in the measured temperature range implies the control of oxygen vacancy and improved resistivity in modified BiFeO3 ceramics. The highest value of magnetocapacitance has been observed to be 1.48% for the x = 0.03 composition. Later we studied the codoped Bi0.8La0.2Fe1-xNbxO3 ceramics with varying composition upto the solubility limit of Nb. It is observed that the dielectric loss tangent decrease significantly in the measured temperature range. The magnetoelectric coupling in codoped samples was evidenced by the variation of dielectric constant and conductance with the magnetic field. The influence of rare earth ions (Sm3+, Ho3+ and Pr3+) towards structural, dielectric, magnetic and optical properties of BiFeO3 has been studied and presented in Chapter 4. From the XRD and Rietveld refinement, it is observed that Bi1-xRExFeO3, (where RE = Sm3+, Ho3+ and Pr3+/Pr4+), ceramics crystallizes with rhombohedral R3c structure, which is also supported by Raman spectroscopy studies. With the increase in the rare earth ion doping content, significant enhancement is observed in dielectric and magnetic properties of doped BiFeO3. The highest value of magnetization has been observed for Ho-doped samples and is 1.71 emu/g at 300 K and 10.45 emu/g at 5K for x = 0.10 composition. The observed optical properties for Bi1-xRExFeO3, (where RE = Sm3+, Ho3+ and Pr3+/Pr4+), ceramics from PL spectra lying in the visible region show their potential application in optoelectronic devices. In Chapter 5 we have studied the synthesis and characterization of magnetoelectric nanocomposites of xMgFe2O4–(1−x)BiFeO3 and xNiFe2O4–(1−x)BiFeO3 and their structural evolution, dielectric, magnetic and magnetoelectric properties. The phase identification of sol-gel derived nanocomposites was examined by X-ray diffractograms. TEM analysis was employed to get the crystallite size and revealed that the average particle size to lie between 30-70 nm. We observed increase in dielectric constant with the Abstract iii increasing ferrite content. With the increase in ferrite content (MgFe2O4 and NiFe2O4), the saturation magnetization was found to increase. The highest value of magnetocapacitance ( 3.39%) is observed for 10 mol% of MgFe2O4 content and (4.5%) for 40 mol% addition of NiFe2O4 content. The value of magnetoelectric coupling coefficient (γ) is calculated using linear fitting of the fractal change of magnetic field induced dielectric constant (Δε ~ γM2) and is found to be 1.03 X 10-2 (emu/g)-2 for 10 mol% addition of MgFe2O4 content and 2.85 X 10-4 (emu/g)-2 for 40 mol % addition of NiFe2O4 content. In Chapter 6, we presented the structural, dielectric, magnetic, magnetodielectric and impedance properties of (1-x)BiFeO3- xBaTiO3 solid solutions. The XRD analysis revealed that BaTiO3 addition does not affect the rhombohedral structure upto x = 0.30. We observed improved dielectric and magnetic properties for the prepared samples. We observed maximum remnant magnetization for x = 0.20 sample and with further increase in BaTiO3 content decreases the magnetization due to the non magnetic contribution of BaTiO3. The magnetoelectric coupling effect between electric and magnetic dipoles was evaluated by measuring the change of the dielectric constant (Δεr) with an applied magnetic field (H) at room temperature. The electrical behavior of the samples, studied over a wide range of temperature and frequency using the technique of complex impedance spectroscopy (CIS) and the temperature dependent complex impedance spectrum between Z' vs. Z'' (Nyquist Plots) showed the increase in the impedance of the BiFeO3- BaTiO3 solid solution with the addition of BaTiO3. Finally Chapter 7 presents a detailed and overall summary and conclusions on the work presented in the thesis, mainly concerned with the synthesis and studies of structural, dielectric, magnetic and magnetoelectric properties of the doped bismuth ferrite compounds. Systematic and exhaustive studies of electrical (dielectric constant and dielectric loss tangent), magnetic and magnetic field induced dielectric constant (magnetocapacitance) properties of the above compounds have provided important information about the magnetoelectric coupling.