SYNTHESIS AND CHARACTERIZATION OF PEROVSKITE MANGANITES

Abstract

In the first chapter we provide an overview of Manganite Perovskites. A brief history of the Manganites Perovskites in which evolution of Manganites Perovskites as a potential material is explained and how it is gradually becoming a choice of material for the research in recent days is explained. The crystal structure for the Manganite Perovskites is AB03 is shown and their position at unit cell is described. The study of Manganite Perovskites is governed by various mechanisms like double exchange, Jahn - Teller distortion, charge ordering etc. Double exchange model is proposed by Zener which explains the conductivity in manganites and ferromagnetism. Jahn - Teller effect is responsible for the distortion in manganites which leads to structural changes and results in orthorhombic and tetragonal structures. Charge ordering is associated with the antiferromagnetism in the compound, double exchange mechanism stops at the charge ordering temperature. Transport phenomenon is also governed by various models like -s Band gap model, small polaron hopping model and variable range hopping model at different temperatures. Application part includes the use of manganites as MRAM (magnetoresistive random access memory), and magnetic sensors. Their negative temperature coefficient of resistance (TCR) makes its use possible in bolometers, a microwave device. In the second chapter we discuss the various synthesis and characterization techniques that are used in the dissertation. Synthesis techniques include the sol - gel method which makes it possible the synthesis of material at lower temperatures. Sol - gel method also makes it useful to control the particle size by varying the sintering temperature. The single phase nature and crystallinity is determined by X-ray diffraction technique. X-Ray diffraction data was obtained with the help of X - Ray diffractomter (BRUKER AXS D - 8). XRD data can be analyzed to determine the lattice parameters, particle size and strain. Field emission scanning electron Iv microscopy (FESEM) Model FEI Quanta 200F is used to determine the surface morphology of the samples. It uses the electric field effect in order to produce the electrons. These electrons 01 eject the secondary electrons from the object which are used in the image formation of the samples. Energy dispersive X - ray analyzer (EDX) technique confirms the stoichiornetry of the sample. Four probe method because of its advantage of rectifying contact resistance is used for the measurement of electrical resistance. Four probe method uses the procedure in which the current is provided from the outer probes and voltage is measured from the inner two probes. SQUID magnetometer (MPMS XL Quantum) is used for plotting the M - T curve, M - H loop at 5 K and M - H loop at 300K. SQUID comprises superconductors separated by the insulating layer which creates the Josephson junction. In the third chapter we present our work in dissertation. Material was synthesized by the sol - gel method in order to control the particle size because we like to see the changes in electrical and magnetic property with the particle size. XRD pattern characterize the lattice structure and it was determined that samples exhibit orthorhombic structure (a :A b :~ c)and particle size of the samples are obtained from the "Debye - Scherrer" formula. Particle sizes of 25 nm, 28 nm, 72 nm, and 279 nrn were obtained by varying the sintering temperature. "Williamson - I-JaIl" equation suggests the effect of particle size on strain. FESEM technique is used to provide the surface morphology of the samples and grain size and connectivity. EDX results for all the samples confirms the desired stoichiometry within experimental error. Magnetic data suggests that samples show spin glass behavior at lower temperatures, may be competition arises between the ferromagnetic DE and antiferromagnetic superexchange interactions. Charge ordering is found to be suppressed on reducing the particle size due to the surface effects which leads to an enhancement of ferroniagnetism and supression of antiferromagnetism. Magnetization also increases with reduced particle size which also confirms LVA an enhancement in ferromagnetism. In the pararnagnetic region curve is fitted with "curie weiss" law and magnetic interactions were observed in the region. Unsaturated magnetization at higher fields also indicate the presence of ferromagnetic and antiferromagnetic phases present in the samples. Transport properties are studied by electrical resistivity temperature dependence and observed that resistivity decreases with increase in particle size due to the grain boundary effects. At higher temperatures the resistivity curve is fitted with small polaron hopping (SPH) conduction model to determine the effect on localization of polarons of particle size.