SYNTHESIS AND MAGNETOELECTRIC STUDY OF MODIFIED PEROVSKITE STRUCTURE ELECTROCERAMICS

Abstract

Magnetic and electric materials permeate every aspect of modern technology. The sensors industry relies heavily on a related class of materials known as ferroelectrics. Many ferroelectric are also ferroelastic. Such materials, which combine two or more ferroic properties in the same phase, are known multiferroics. Trends toward device miniaturization have lead to increased interest in combining electronic and magnetic properties into multifunctional materials, so that a single device can perform more than one task. Ferromagnetic ferroelectric multiferroics are particularly appealing not only because they have the properties of both parent compounds, but also because interactions between the magnetic and electric polarizations lead to additional functionalities. However, attempts to design multiferroics that combine ferromagnetism and ferroelectricity in the same phase have proved unexpectedly difficult. In absence of naturally occurring multiferroic materials, the current research-activities of multiferroics are limited to few representative perovskite oxides, namely, BiFeO3, BiMnO3, which combine several useful properties in the same material to device new functionalities in information storage and spintronics. Among these BiFeO3 is known to be the only material that possesses high ferroelectric Curie temperature (TO of —1103 K and high antiferromagnetic Neel temperature (TN) of —643 K, which makes it an excellent candidate for applications. In BiFeO3, the magnetic and polarization orders are driven by different causes, viz., the stereo chemical activity of the 652 lone electron pair of Bi3+ gives rise to ferroelectricity, while the partially filled 3d orbits of the Fe3+ ion cause G-type canted antiferromagnetic character. However, polarizations actually measured in a bulk single crystal were only —3.5 and —6.1 i.tC/cm2 along the (001) and (111) axes, respectively. Though BiFeO3 is promising but there are still difficulties, e.g. iii synthesis of single phase samples, high leakage current, small spontaneous polarization (Ps) and remnant polarizations (Pr), high coercive field, and weak ferromagnetism due to spin canting. The most serious problem of BiFe03 is the low electrical resistivity (high leakage current), possibly caused by oxygen nonstoichiometry and variable valence of Fe ions (Fe3+ to Fe2+), which bears strong correlation with processing conditions and the dopants. Natural multiferroic single-phase crystals are rare and exhibit relatively weak magnetic and electrical responses as well as weak coupling between the electric and magnetic fields. In contrast, two-phase ceramic composites, which incorporate ferroelectric and ferri-/ferromagnetic components, typically yield strong direct responses to electric or magnetic fields. One approach to obtain single phase BiFe03 is the synthesis followed by leaching in dilute nitric acid or rapid liquid phase sintering process. To improve the multiferroic properties of BiFe03, doping of +4 valance ions for Fe+3 was done. Doping of +4 valance ions requires charge compensation which can be achieved either by filling of oxygen vacancies or creation of Fe+2. Hence in the search of new magnetoelectric system, transition metal ions (such as IVIn+4, Ni+2 and Fe+3) doping in ferroelectric PbTiO3 was applied and for large magnetoelectric out put, nanocomposites of xCuFe204-(l -x)BiFe03 and xNi0.75Co0.25Fe204-(l-x)BiFe03 were synthesized by citrate sol-gel process. The present thesis is divided into seven chapters. The first chapter contains introductory aspects and literature survey on multiferroic materials and describes the ferroelectricity, magnetism, and causes of multiferroism in single phase and composite materials. The second chapter describes the characterization techniques for structural, electrical and magnetic study employed in the present investigation. These techniques include X-ray diffraction for phase identification, scanning electron microscopy using secondary electron imaging mode for investigating the surface morphology and energy iv dispersive X-ray spectroscopy for elemental analysis. Different techniques that have been used to prepare bulk samples such as solid state reaction and sol-gel method have also been described in detail in this chapter. Besides this, Dielectric and ferroelectric measurement, vibrating sample magnetometer and superconductive quantum interference device for magnetic property measurements have also been outlined. Chapter three describes the effect of annealing atmosphere in preparation of phase pure BiFeO3 by sol-gel technique followed by leaching in dilute nitric acid. Annealing under reducing atmosphere (Argon) is effective in minimizing the impurity phases as compared to air annealing. Ar annealing hampered the ferroelectric properties of BiFeO3 ceramics due to introduction of oxygen vacancies under reduced atmosphere. However it showed ferromagnetic behaviour due to the presence of Fe2+ ions. The most likely the origin of ferromagnetic ordering in Ar annealed BiFeO3 powder may be due to the presence of oxygen vacancies associated with Fe2+ ions because preparation under reducing conditions favoured formation of Fe2+. Another possibility is a ferrimagnetic arrangement in which the magnetic moments of the Fe2+ ions are aligned oppositely to -those of the Fe3+ ions, leading to a net magnetic moment. The detailed study of the influence of the annealing conditions on multiferroic properties is also presented. The fourth chapter embodies the synthesis, dielectric, magnetic and magnetoelectric characterization of BiFel_„Ti,,03 and BiFei_Xn,(03 ceramics. Single phase Mn doped ceramics without any trace of impurity phases were obtained by rapid liquid phase sintering process. In Ti doped ceramics prepared by solid state reaction method, few impurity peaks were observed. Ti doping increased the resistivity due to charge compensation mechanism and therefore, it improved the ferroelectric properties of the doped ceramics. An enhancement in magnetization was also observed in both Ti and Mn doped ceramics. An anomaly in dielectric measurement was observed in the vicinity of antiferromagnetic Neel temperature in both Ti and Mn doped BiFeO3 ceramics. No systematic variation in antiferromagnetic Neel temperature was observed in Ti doped BiFeO3 ceramics while in Mn doped BiFeO3 ceramics, Neel temperature decreased with increase in Mn content. The ferroelectric hysteresis loops of Ti doped BiFeO3 ceramics were not fully saturated and represent a partial reversal of the polarization. The spontaneous polarization (Ps), remnant polarization (Pr) and coercive field (Ec) values of Ti doped BiFeO3 ceramics was found to be encouraging. The increase in dielectric constant with increasing magnetic field also indicated the coupling between two order parameters. Finally, it is significant that the magnetic and electric ordering are present in Ti doped BiFeO3 at room temperature. The fifth chapter describes the effect of transition metal ions (Mn, Ni) doping in PbTiO3 and Ba doping for Pb in Pb(Fe0,5Tio,5)03. X-ray diffraction analysis revealed that lattice distortion and unit cell volume decreased with the increase in Mn, Ni and Ba contents. Temperature dependence of magnetization above room temperature and magnetization hysteresis (M-H) curves at room temperature indicate ferromagnetism in transition metal ions doped PbTiO3. It was observed that on increasing the. Mn and Ni concentration, the magnetization increases with a higher spontaneous moment. The increase in spontaneous magnetic moment with the increase in Mn content is expected due to the increase in unpaired d electrons. Mn and Ni have partially filled d-shell which contributes unpaired electrons to PbMn„Tii_x03 and Pbt,Ni.„TiO3 systems, respectively. The ferromagnetic Curie temperatures (TM) for PbMn,,Tii.,(03 ceramics were 340, 320 and 300°C for x = 0.1, 0.3 and 0.5, respectively, as observed from susceptibility versus temperature plots. We observed a decrease in ferroelectric Curie temperature of doped PbTiO3 as a consequence of decrease in lattice distortion (i.e. c/a ratio) and unit cell volume. In addition, the effect of Ba doping for Pb has been studied in Pb(Fe0.5Tio.5)03 compound. We were able to further decrease the ferroelectric Curie temperature of this compound by substituting Ba for Pb. All these three compounds showed coexistence of ferroelectric and magnetic ordering at room temperature along with the coupling between two order parameters. The sixth chapter describes Synthesis and characterization of magnetoelectric nanocomposites of xNioasC00.23Fe204-(1-x)BiFe03 (NCFBF) and xCuFe204-(1- x)BiFe03(CFBF) and their dielectric, magnetic and magnetoelectric properties. The phase identification of sol-gel derived nanocomposites was examined by XRD and TEM analysis which confirmed the crystallite size to be less than 100 nm. With the increase in annealing temperature the coercivity increased and then decreased above 500°C and 700°C for CFBF and NCFBF nanocomposites, respectively. The increase in coercivity up to certain temperature may be ascribed due to increase in crystallite size for both the nanocomposites. This can be expected as crystallites size increases with increase in annealing temperature. If the increase in coercivity depends only on the crystallite size then it would not decrease on further increase in temperature above 500°C and 700°C for 0.2CuFe204--0.8BiFe03 and 0.3Ni0.75C00.25Fe204-0.7BiFe03, respectively, which would give additional crystallite growth. This indicates that coercivity not only depend oir crystallite size but some other factors also affect the coercivity, otherwise coercivity would not have decreased on further increase in annealing temperature. The decrease in He may be attributed to the decrease in aspect ratio and the magnetization pinning defects incorporated into the nanocrysatllites, due to lattice mismatch between the intergrowing nanocrysatllites after the onset of significant crystallite growth at 500 °C and 700 °C for 0.2CuFe204-0.8BiFe03 and 0.3Ni0:75Coo.25Fe204-0.7BiFe03, respectively. These pinning defects would reduce on increasing the annealing temperature which results in decrease in coercivity (HO. It can also be seen that squareness increased with increasing annealing temperature up to 500°C and then decreased on further increase in temperature above 500°C for CFBF composites. Squareness also showed similar behavior as that of coercivity with annealing temperature for NCFBF nanocomposites. The nanocomposites vii exhibited strong magnetic properties and a large magnetoelectric effect. The large magnetoelectric output in the nanocomposites exhibits strong dependence on magnetic bias and magnetic field frequency. The seventh chapter contains the brief summary and conclusions on the work presented in the thesis through chapters three to six. The overall comments and suggestions for future work have also been added in this chapter. viii