SYNTHESIS AND MAGNETOELECTRIC STUDY OF MODIFIED BISMUTH FERRITE MULTIFERROICS

Abstract

Electricity and magnetism are closely linked to each other and this forms the basis of Maxwell equations. In a solid, a similar coupling was first considered by Pierre Curie between the magnetization and electric polarization in 1894. This magnetoelectric (ME) effect was recently understood to be potentially important for applications in information technology, because it would allow magnetic information to be written electrically (with low energy consumption) and to be read magnetically. ME coupling is a broader phenomena, but is mostly exhibited by materials which posses ferroelectric/antiferroelectric and ferromagnetic/antiferromagnetic orders. These materials are termed as multiferroic materials. This is quite a rare class of materials, since ferroelectricity and ferromagnetism make a mutually exclusive group. To be potentially important for applications, magnetoelectric materials should have the temperatures of magnetic and electric ordering above room temperature. This requirement sharply decreases the range of possible materials. Among high-temperature magnetoelectric materials, the BiFeO3 and GaFeO3 compounds and ferrite—garnet films can be selected. BiFeO3 possesses antiferromagnetic Néel temperature (TN) of 370 °C and ferroelectric temperature (Tc) of. 830 °C. The-ferroelectric polarization measurement in BiFeO3 is-always hampered by low resistivity and high coercive field. Also, the inhomogeneous spin structure of BiFeO3 leads to the cancellation of macroscopic magnetization which prohibits the linear magnetoelectric effect from being observed. This incommensurate spiral spin structure can be suppressed by strain, high magnetic field and by various doping. However, so far the magnitude and operating temperatures of any observed magnetoelectric (ME) coupling has been too low for applications. So to obtain the room temperature magnetoelectric materials is still an open area of research. To obtain high quality multiferroics for a specific use it is necessary to understand various phenomena concerning structural and electrical properties of the materials. For this purpose it is desired to make available as many experimental data as possible by various techniques. In this thesis the effect of doping of rare earth ions, Pr codoped with La, Ti codoped with Dy on the structural, dielectric, magnetic and ferroelectric properties of iii BiFeO3 has been addressed. We have also synthesized solid solution of Gd doped BiFeO3 with BaTiO3 and studied its multiferroic properties. Nanocomposites of BiFeO3 and ZnFe2O4 have also been synthesized with an aim of improving the multiferroic properties. The present thesis is divided into six chapters. The first chapter contains introductory aspects and literature survey on multiferroic materials with a specific mention of Bismuth ferrite, their structure, multiferroic properties and their importance. 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. 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. The effect of rare earth ions (Gd, Eu, Dy and Ho) doping on the structural, dielectric, multiferroic and magnetoelectric properties of BiFeO3 were studied and it is presented in Chapter 3. With the substitution of rare earth ions magnetic properties may be exhibited over a large temperature range with some spin reorientation transition. In addition to this, ferroelectric tendencies can also be altered by modification in Bi-O and Fe-Fe coupling by this substitution. Thus, in order to get enhanced magnetoelectric interactions in the synthesized materials based on BiFeO3 host crystals, the rare earth ions used as substituent were chosen with decreasing ionic radii, as Eu3+ (1.066 A), Gd3+ (1.053 A), Dy3+ (1.027 A) and Ho3+ (1.015 A) for giving rise to distortion and strain in the crystal lattice of the solid solution with variable degree to affect the magnetic anisotropy and dielectric properties over a temperature range. With the increase in rare earth ion concentration, the impurity peaks (which are usually obtained in BiFeO3) were suppressed and we obtained pure phase compounds for —10 mol % rare earth ion iv concentration. The temperature dependence of dielectric properties, show anomalous behavior around 370 °C (which is ascribed to the antiferromagnetic transition temperature) which is a signature of magnetoelectric coupling. This Neel temperature (TN) was further confirmed by high temperature VSM measurements. The dielectric constant corresponding to Neel temperature was found to increase with the rare earth ion substitution. The nature of the peak in dielectric measurements with temperature were found to be broad and diffused, which reflects the relaxor type of behavior shown by the disordered ferroelectrics. All the rare earth ions doping show enhancement in magnetization, which suggests the substitution induced suppression of spiral spin of BiFeO3. The room temperature ferroelectric hysteresis loops were measured and apart from Gadolinium doped BiFeO3, all the rare earth doped BiFeO3 showed saturated P-E loops. Then we measured the effect of external magnetic field on the P-E loops and hence established the room temperature magnetoelectric coupling for Dy, Eu and Ho ion doping. Chapter 4 describes the effect of substitution of supervalent ions in BiFeO3. The effect of Pr substitution at A site along with La on the magnetoelectric properties of BiFeO3 was studied. The antiferromagnetic Néel temperature was found to decrease to 310 °C for La and Pr doped BiFeO3. These ceramics show considerable decrease in dielectric loss. Hence we obtained saturated P-E loops for Pr doped BiFeO3. Though the magnetization loops had no tendency to saturate upto 7 Tesla but enhancement in magnetization was observed. For 20 mol % Pr substituted BiFeO3, the remnant magnetization was 0.1687 emu/g and magnetic coercivity (He) obtained was 7266 Oe. The low temperature magnetic study showed that coercive field of the sample at 5K was half of its room temperature value which indicates magnetic anisotropy change at low temperatures. In the other multiferroic system, Dy was substituted at A site with concentration of 0.1 (fixed on the basis of work described in Chapter 3) and Ti was substituted at B site with concentration varying from 0.1 to 0.3. Then the effect of Ti substitution on the multiferroic properties of Bi0.8La3.11Dy0.1Fe1_,Ti,03 was studied. The antiferromagnetic Neel temperature was decreased to 349 °C for Dy and Ti cosubstituted BiFeO3. As compared to Dy doped BiFeO3, the Dy and Ti doped BiFeO3 shows 7 times enhancement in magnetization at 1 Tesla magnetic field whereas 18% increase in remnant magnetization was observed. The substitution with higher valence dopants has led to decrease in dielectric loss (tano) values. The fifth chapter embodies the synthesis, dielectric and multiferroic characterization of solid solution of Gd doped BiFeO3 with BaTiO3, prepared by solid state reaction method and nanocomposites of BiFeO3-ZnFe204, prepared by sol gel method. The (1 - y)Bi0.9Gdo.iFe03—yBaTiO3ceramics showed rhombohedral structure and lattice volume was found to increase with increase in BaTiO3 content. An enormous increase in remnant magnetization value from 0.0212 emu/g for Gd doped BiFeO3 without BaTiO3 to 0.1565 emu/g for samples with 0.1 mol % BaTiO3 (with coercive field nearly same (2000e)) was observed and then the magnetization was found to decrease with further increase in BaTiO3 content because of its paramagnetic contribution. The ferroelectric hysteresis loops were also found to improve with addition of BaTiO3. We synthesized yZnFe2O4—(1-y) BiFeO3 nanocomposites at low temperatures by sol gel technique. The phase identification of the composites was done by XRD and TEM. The effect of annealing temperature and ZnFe2O4 content on the multiferroic properties of nanocomposites has been studied. The magnetization was found to increase with the addition of ferrite concentration. The composites with y = 0.3 and 0.4 show improved magnetic hysteresis loops with very small coercivity. Both dielectric constant and dielectric loss showed dispersion in the low frequency region. An anomaly around TN and strong dependence of dielectric constant on magnetic field suggest the magnetodielectric nature of the composite. The sixth chapter contains the brief summary and conclusions on the work presented in the thesis through chapters three to five. The overall comments and suggestions for future work have also been added in this chapter. vi