STUDY OF MAGNETOELECTRIC RESPONSE IN FERROELECTRIC AND FERRITE BASED COMPOSITES

Abstract

Multiferroics represents a class of multifunctional materials which exhibits several ferroic orders like ferroelectricity/antiferroelectricity, ferromagnetism/antiferromagnetism and ferroelasticity simultaneously. The prospect of the existence of cross coupling, generally known as magentoelelctric (ME) coupling amongst the two order parameters, specifically ferroelectricity and magnetism is the most exciting attribute of the multiferroics. The presence of ME coupling in the multiferroics permits us to control induced polarization and magnetization, with the help of magnetic and electric field respectively. ME coupling in the multiferroics allows us to exploit their uses in various potential applications including spintronics, magnetic field sensors, data storage etc. Multiferroics are divided into two categories viz. single phase and composite multiferroics. Several issues have to be addressed before using the single phase multiferroics in device applications. One is tuning of their operational temperatures because most single phase multiferroics exhibit ME coupling below room temperature and other is very low value of ME coupling associated with them. In order to overcome the problems posed by single phase multiferroics, nowadays focus is shifted on synthesis and study of magnetoelectric composites comprising of a ferroelectric and ferrite phase simultaneously. The formidable progress in device miniaturization and their multi-functionality in late years have shifted the interest of science fraternity towards proliferation and development of magnetoelectric composites, owing to their large magnetoelectric coupling which can be exploited for use in diverse potential applications including data storage, high sensitivity magnetic field sensors, transducers etc. Given the importance of magnetoelectric composite systems from specific application point of view and to obtain higher magnetoelectric coupling, magentoelectric composites comprising of piezoelectric (ferroelectric) and magnetostrictive (ferrite) phase have been studied. Various phenomena associated with structural, dielectric, magnetic, ferroelectric and magentoelectric properties of the magnetoelectric composites has to be understood for exploiting their use in novel multifunctional devices. In order to achieve this, it requires many experimental data and characterizations acquired through various possible techniques. Outlining the progress made so far on the study of magnetoelectric composites, we present in this thesis the structural, microstructural, dielectric, magnetic, ferroelectric Abstract ii and magnetoelectric properties of lead-free magentoelectric composites including (K0.5Na0.5)NbO3-Ni0.2Co0.8Fe2O4, (K0.5Na0.5)NbO3-CoMn0.2Fe1.8O4, (Bi0.5Na0.5)TiO3- Co0.8Zn0.2Fe2O4, (Bi0.5Na0.5)TiO3-CoMn0.2Fe1.8O4, (Bi0.5Na0.5)TiO3-Ni0.2Co0.8Fe2O4 and (K0.5Na0.5)NbO3-BaFe12O19. The present thesis is divided into six chapters followed by a bibliography. A brief summary of the work presented in each of the chapter is as under: Chapter 1 contains the literature survey of the relevant work and brief introduction for magnetoelectric effect and its origin. It also includes brief discussion about ferroelectricity, magnetism, perovskite structure, spinel ferrites, classification of multiferroics, magnetoelectric composite systems and their applications. Chapter 2 contains a brief summary of experimental techniques and tools to be utilized in characterization of synthesized composites. Chapter 3 deals with the study of two magnetoelectric composites namely (x) Ni0.2Co0.8Fe2O4-(1–x) (K0.5Na0.5)NbO3 and (x) CoMn0.2Fe1.8O4-(1–x) K0.5Na0.5NbO3. In the first section of this chapter we have discussed about (x) Ni0.2Co0.8Fe2O4-(1–x) (K0.5Na0.5)NbO3 (for x = 0.0, 0.10, 0.20, 0.30, 0.40, 0.50 and 1.0) composites synthesized using solid state reaction method. Structural analysis has been carried out by using X-ray diffraction (XRD) which confirms the existence of both the constituent phases in the composites without any intermediate phase. Dielectric properties are studied as a function of temperature at three distinct frequencies (1, 5 and 10 kHz). P-E and M-H hysteresis loops are measured which confirms the presence of ferroelectric and magnetic ordering at room temperature. Magnetization vs. temperature studies provide a better insight of magnetic ordering in the composites. Zero field cooled curves for pure ferrite and composites indicate that they undergo charge ordering, metal insulator transition commonly known as Verwey transition around 140-145 K. Magnetoelectric coupling is observed in the composites and is confirmed by measuring ME voltage coefficient (αME) corresponding to different compositions. The highest αME is found to be 5.389 mV/cm-Oe for x = 0.20 composition. In the second section of this chapter, we have discussed the structural, dielectric, ferroelectric, magnetic and magnetoelectric properties of (x) CoMn0.2Fe1.8O4-(1 x) K0.5Na0.5NbO3 (for x = 0.0, 0.10, 0.20, 0.30, 0.40, 0.50 and 1.0) composites synthesized using solid state reaction method. Structural and microstructural analysis of the composites has been carried out using XRD and field emission scanning Abstract iii electron microscopy (FE-SEM). Dielectric properties including dielectric constant ( '  ) and dielectric loss (tan δ) are studied as a function of temperature and found to enhance with addition of ferrite. M-H hysteresis loops are obtained at 300 K and 5 K and indicate the presence of ferromagnetic ordering in the composites. Ferroelectric properties are found to decrease with addition of ferrite, unlike magnetic properties which improve with ferrite addition. ME voltage coefficient (αME) is measured which indicates the presence of ME coupling in the composites. We obtained maximum αME = 5.941 mV/cm-Oe for x = 0.10 composition. Chapter 4 deals with the study of ME composites based on Bi0.5Na0.5TiO3 namely (x) Co0.8Zn0.2Fe2O4-(1–x) Bi0.5Na0.5TiO3, (1–x) Bi0.5Na0.5TiO3-(x) CoMn0.2Fe1.8O4 and (x) Bi0.5Na0.5TiO3-(1–x) Ni0.2Co0.8Fe2O4. In the first section of this chapter we have discussed the (x) Co0.8Zn0.2Fe2O4-(1–x) Bi0.5Na0.5TiO3 (for x = 0.0, 0.10, 0.20, 0.30, 0.40, 0.50 and 1.0) composites synthesized using solid state reaction method. XRD analysis confirms the presence of both the constituent phases in the composites. Average grain size of the composites is determined from FESEM micrographs and found to decrease with addition of ferrite. Dielectric response of the composites is measured as a function of temperature and frequency and found to decrease with addition of ferrite. Ferroelectric properties are diluted with addition of ferrite unlike magnetic properties. Impedance analysis suggests the negative temperature coefficient of resistance (NTCR) behaviour of the composites and indicates bulk and grain boundary contribution to the overall electric properties. Maximum αME of 7.11 mV/cm-Oe is obtained for x = 0.10 composition. In the second section of this chapter we have discussed about the (1–x) Bi0.5Na0.5TiO3-(x) CoMn0.2Fe1.8O4 (x =0.0, 0.10, 0.20, 0.30, 0.40 and 0.50) ME composites. Dielectric response of the composites is measured as a function of temperature and frequency and is found to decrease with addition of ferrite. Addition of ferrite significantly improves the magnetization but lowers the polarization and coercive field (EC). Impedance analysis suggests the NTCR behaviour of the composites. The composites exhibit room temperature ME coupling which shows a decreasing trend with addition of ferrite. The third section of this chapter discusses the structural, dielectric, magnetic and magnetoelectric properties of (x) Bi0.5Na0.5TiO3-(1–x) Ni0.2Co0.8Fe2O4 (x = 0.30 – 0.80) composites. XRD analysis confirms the mixed spinelperovskite phase of the composites. Dielectric response of the composites is measured as a function of temperature and frequency which is found to enhance with addition of Bi0.5Na0.5TiO3 (BNT). The magnetic characteristics of the composites are diluted with Abstract iv addition of BNT as expected. ME coupling in the composites was found to enhance with BNT concentration up to x = 0.60 which exhibits highest αME of 7.538 mV/cm-Oe. Chapter 5 deals with the study of novel lead-free ME composite viz. (1–x) (K0.5Na0.5)NbO3-(x) BaFe12O19 (x = 0, 0.30, 0.40, 0.50 and 1.0) synthesized using hybrid processing route in which BaFe12O19 is synthesized using sol-gel method and (K0.5Na0.5)NbO3 is synthesized using solid state reaction method. FE-SEM micrograph asserts the existence of both the individual phases and the average grain size varies between 232 – 540 nm. Dielectric properties of the composites are studied as a function of temperature and the dielectric constant of the composites was found to decrease with addition of BaFe12O19. Magnetization of the composites is found to enhance with addition of BaFe12O19 unlike coercivity and anisotropy field which show a decreasing trend. Variation of magnetization with temperature is studied and gives a hint of spin glass behaviour in the lower temperature regime. Impedance and modulus studies of the composite signify the non-Debye type of relaxation in the composites and NTCR behaviour of the composites. ME coupling of the composites is determined by measuring αME and we obtained highest value of 4.08 mV/cm-Oe for composite with x = 0.30. Finally, Chapter 6 presents a detailed and overall summary of the thesis work, which is mainly concerned with the synthesis and studies of structural, dielectric, magnetic, ferroelectric and magnetoelectric properties of lead-free ME composites under consideration. Systematic and exhaustive studies of electrical (dielectric constant and dielectric loss tangent), ferroelectric, magnetic and magnetoelectric properties of the synthesized ME composites have provided important information about the magnetoelectric coupling which makes the studied composites quite promising from application point of view.