Electrical and Magnetic Studies on Bi-based Perovskite AFe2O4 (A=Co, Ni) Composite Structures

Abstract

Multiferroic materials have recently drawn increasing interest and provoked a great number of research activities due to their potential applications for multifunctional devices. Multiferroism refer to any combination of ferroelectric and magnetic order parameters in single phase as well as in artificial composites. The coexistence of several interactions, particularly existence of a cross-coupling between ferroelectric and magnetic orders, known as “magnetoelectric (ME) coupling”, brings about novel physical phenomena and offer possibilities for new multifunctional devices such as sensors, transducers, memories and spintronics. Observation of ME coupling in a material implies that the electric polarization/magnetization can be induced by application of magnetic/electric field; are termed as direct and converse ME effect respectively. Bismuth ferrite BiFeO3 (BFO) is one of the most extensively studied single phase multiferroic having magnetic order (magnetic transition temperature TN = 643 K) and ferroelectric order (ferroelectric transition temperature Tc FE = 1103 K) at room temperature. However, magnetic properties of BFO is weak and limits its applications in electronic devices. Therefore, many approaches have been developed to improve the multiferroic properties by combining the ferroelectric and ferromagnetic orders named as artificial composite type materials. Among various non lead based multiferroic composites, Bi based multiferroic composites are most extensively studied. Recently, there is a surge in research on Bi based material e.g., BFO and (Bi1/2Na1/2TiO3) BNT to be used as a ferroelectric/piezoelectric component for preparation of multiferroic composites. Therefore thesis focuses on the development of non lead Bi based multiferroic composites. In this work, studies based on sol-gel driven BiFeO3- AFe2O4(A=Ni, Co) nanocomposite thin films and Bi1/2Na1/2TiO3(BNT)-Bi1/2K1/2TiO3(BKT)- BiMg1/2Ti1/2O3(BMgT)-AFe2O4 (A = Co, Ni) particulate composites are presented. The thesis is organized into five chapters Chapter I (Introduction) gives a brief introduction about the multiferroic materials and need or advantages of the Multiferroic materials. The appropriate definitions of various terms related to the field (e.g, definitions of multiferroics, magnetoelectric coupling coefficient, types of multiferroicity) are given. Multiferroic materials are classified on the basis of single phase as well as composite structures. Further, composites are classified on 2 the basis of lead and non lead based composites. Finally the different connectivity schemes of composites have been discussed. Chapter II (Literature Review) gives historical development in this field of multiferroic, followed by an extensive summary of the research carried out in single phase multiferroic as well as composite structures. Chapter III (Experimental and characterization techniques) This chapter give details the processing methods employed for synthesizing the samples of different compositions. A brief details of the various characterization techniques used to study structural, microstructural, dielectric, ferroelectric, magnetic and magnetoelectric properties are provided. Chapter IV (Results and discussion) In this chapter, the experimental results of the synthesized samples are presented. This chapter is organized in two sections. In the first section, preparation and characterization of BFO is discussed, followed by development of BiFeO3-AFe2O4 (A = Co, Ni) composites are given. The effect of addition of ferrites in BFO matrix has been studied on their structural, dielectric, magnetic and ferroelectric properties. In the second section, studies based sol-gel assisted BNT-BKT-BMgT-AFe2O4 (A = Co, Ni) particulate magnetoelectric composites have been discussed. The effect of weight ratio of ferrites on their structural, dielectric, ferroelectric, magnetic and magnetoelectric properties has been systematically studied. Brief summary of the results are give below: Studies on BFO thin films This study details the work on BFO thin films annealed at different temperature. BFO thin films were synthesized by the sol-gel spin-coating technique followed by annealing at different temperatures. The effect of annealing temperature on structural, dielectric and ferroelectric properties of BFO thin film has been investigated. Single phase polycrystalline BFO films are confirmed by X-ray diffraction patterns. A strong dependence of grain size and shape on annealing temperature was observed. The film annealed at 500 ºC showed a well saturated polarization-electric field loops. The remnant polarization and saturation polarization values are decreased with increasing annealing temperature. The anomaly in temperature dependent dielectric permittivity was observed at magnetic transition point. 3 Studies on BiFeO3-CoFe2O4 composite thin films In this study (1-x)BFO–xCFO (x = 0, 0.1, 0.2, 0.3) nanocomposite thin films were deposited using sol-gel spin coating technique. The annealing of the films were carried out at 600ºC and 650 ºC. X-ray diffraction, transmission electron microscopy examinations confirm the coexistence of both perovskite BFO and spinel CFO phases. The magnetic and dielectric properties are improved by incorporation of CFO grains in matrix of BFO. The saturation magnetization (Ms) increases as x varies from 0 to 0.3. For x = 0.3, Ms is found as high as ~ 115 emu/cm3 and 158 emu/cm3 for the films annealed at 600ºC and 650 ºC respectively. Coercivity is increased with CFO concentration and found maximum (Hc ~ 107 Oe) for x = 0.30. However for x=0.3, ferroelectric behaviour degraded for the films annealed at 600 oC and 650 oC. Higher magnetic content as well as bismuth loss with increasing annealing temperature is responsible for decrease the ferroelectric behaviour. Studies on BiFeO3–NiFe2O4 composite thin films In this part of the thesis (1-x)BFO–xNFO (x = 0, 0.1, 0.2, 0.3) nanocomposite thin films were prepared by sol-gel technique and their structural, electrical and magnetic properties were studied. X-ray diffraction and transmission electron microscopy examinations confirmed that NFO nanoparticles were well distributed in BFO matrix. The magnetic and dielectric properties were significantly improved by incorporation of NFO nanoparticle in matrix of BFO. The saturation magnetization (Ms) and remnant magnetization (Mr) increased as high as ~34 emu/cm3 and ~7 emu/cm3 respectively for x = 0.1. The dielectric constant of the films increased from 160 (for x = 0) to 280 (for x = 0.3). However, the values of ferroelectric polarization were decreased with increasing x. Importantly, the sample with (x = 0.1),the best sample in this study demonstrates improved ferroelectric as well as magnetic properties. Studies on BNT-BKT-BMgT/NFO Particulate composite This section discusses the details of modified BNT based magnetoelectric composites. In first part of this section the results on BNT-BKT-BMgT/NFO (where x=0, 0.1, 0.2, 0.3) ME composite have been discussed. The effect of NFO weight ratio on the structural, dielectric, ferroelectric, magnetic and magnetoelectric properties has been systematically studied. Lead free ternary solid solution 72.5BNT-22.5BKT-5BMgT) with effective piezoelectric 4 coefficient (d33) of 180 pC/N has been prepared. Further, particulate composite consisting of (1−x)[72.5(BNT)-22.5(BKT)-5(BMgT)]−xNFO (x = 0, 0.1, 0.2, 0.3) were synthesized and its magnetoelectric (ME), magnetic, ferroelectric and dielectric properties were studied. The composite with x = 0.2, showed the large value of ME voltage coefficient (αE ) ~73 mV/cmOe accompanied by higher d33 (~125 pC/N). The temperature dependent dielectric spectra of samples indicated two anomalies at 200 °C and 320 °C are ascribed to ferroelectric to antiferroelectric, and anti-ferroelectric to paraelectric phase transitions respectively. Studies on BNT-BKT-BMgT/CFO Particulate composite In this study the results on BNT-BKT-BMgT/CFO (where x = 0, 0.1, 0.2, 0.3) particulate ME composite are discussed. As prepared lead free ternary solid solution 72.5(BNT)- 22.5(BKT)-5(BMgT) was used to synthesis of (1−x)[72.5(BNT)-22.5(BKT)- 5(BMgT)]−xCoFe2O4 (CFO) (x = 0, 0.1, 0.2, 0.3) particulate composite. Further its structural, dielectric, ferroelectric and magnetoelectric properties were studied. The composite showed that the ferroelectric as well as ferromagnetic properties are sustained in the composite. The composite with x = 0.2, is the best sample of our series having large value of ME voltage coefficient (αE ) ~112 mV/cmOe. The temperature dependent dielectric anomalies are found similar to NFO based composites. Chapter V (Conclusions and future scope) gives an important conclusions and future scope in this field of research.