Fabrication, Characterization and Structural Study of Ferrites of Technical Importance

Abstract

Ferrites are widely used in power electronics applications where the frequency range is from KHz to MHz. No other alternative materials except ferrites are available at such high frequencies. The areas of magnetic nanoparticles and thin films lead to revolutionary new approaches in basic and advanced magnetism, and are more effective in the field of high density storage media. The main objective of the present study was to produce single phase ferrites in the form of bulk, nano and thin films with improved structural, electrical and magnetic properties. This thesis examines the issue encountered in the growth, structural, microstructural, electrical and magnetic properties of ferrites in the form of bulk, nanoparticles and thin films. Here the materials examined include Cu 0.5 Zn 0.5 Fe 2- x Al x O 4 (x=0.0 to 0.5) ferrites prepared with solid state reaction method, Co 0.5 Mn 0.5 Fe 2 O 4 (calcined at 500, 600, 700, 800, 900°C), Mn 0.5 Cu 0.5-x Zn x Fe 2 O 4 (x=0.0 to 0.5), Mn 0.5 Cu 0.5-x Ni x Fe 2 O 4 (x=0.0 to 0.5) ferrites prepared with sol-gel combustion method and Fe 3 O 4 thin films prepared with pulsed laser deposition technique. The effect of Al3+ on the structural, electrical and magnetic properties were investigated in Cu 0.5 Zn 0.5 Fe 2-x Al x O 4 (x=0.0 to 0.5) ferrites prepared with solid state reaction method. Single phase cubic spinel structure was revealed by X-ray diffraction analysis. For all the samples, crystallite size remained in the range of 25-30 nm. Lattice constants of all the samples decreased, whereas porosity increased with increasing Al+3 concentration due to the substitution of smaller Al3+ ion (0.51 Å) for large Fe3+ ion (0.64 Å). Due to non-magnetic trend of Al3+ concentrations for a magnetic element Fe3+ at the B-site gradually decreased the saturation magnetization. Al+3 has significant impact on the dielectric constant ( ε /), tangent of dielectric loss angle (tanδ) and dielectric loss factor ( ε //). The possible reason for the variation in dielectric properties has been understood on the basis of space charge polarization. Three series of ferrites Co 0.5 Mn 0.5 Fe 2 O 4 (calcined at 500, 600, 700, 800, 900°C), Mn 0.5 Cu 0.5-x Zn x Fe 2 O 4 (x=0.0 to 0.5), Mn 0.5 Cu 0.5-x Ni x Fe 2 O 4 (x=0.0 to 0.5) were prepared by sol-gel combustion method. In Co 0.5 Mn 0.5 Fe 2 O 4 ferrites, crystallite size was determined with Scherrer’s formula. Crystallite size increases with calcination temperature but coercivity decreases. The decrease in coercivity at larger crystallite size can be attributed to domain walls. Single phase nanocrystalline Mn 0.5 Cu 0.5-x Zn x Fe 2 O 4 (x=0.0 to 0.5) ferrites were successfully prepared at low temperature of 300°C using citric acid as a fuel and nitrates as oxidants by sol-gel method. X-ray diffraction (XRD) and room temperature vibrating sample magnetometer (VSM) studies have been carried out in order to understand the structural and magnetic properties as a function of zinc concentration. The variations of observed lattice parameter and crystallite size have been explained by considering the larger ionic radius of zinc. The coercivity decreases as the crystallite size increases, attaining a minimum value of 46.32 Oe. This decrease at larger crystallite size could be due to three reasons. First, the crossover of single domain to multiphase domain, second combined effect of surface and surface anisotropy, third migration of Fe+3 ions from A to B-site. Another series of single phase nano-crystalline Mn 0.5 Cu 0.5- x Zn x Fe 2 O 4 (x=0.0 to 0.5) ferrites were successfully synthesized by combustion method at a temperature as low as 300°C. The presence of Ni2+ ions did not show a consistent trend in diffraction peaks shifting to either lower or higher angles. It was observed that with increasing nickel concentration, saturation magnetization (M s ) increased but coercivity (H c ) decreased which could be attributed to the substitution of soft ferromagnetic Ni2+ ions in place of diamagnetic Cu2+ ions. The minimum value of coercivity (87.20 Oe) was observed for the composition Mn 0.5 Ni 0.5 Fe 2 O 4 . Fe 3 O 4 thin films were deposited on Si(100) substrates with pulsed laser deposition technique. First we studied the effect of annealing and deposition temperature, and second the effect of annealing time of 30, 60 and 90 minutes on the structural and magnetic properties of Fe 3 O 4 thin films. Scanning electron microscopy, X-ray diffractometery and vibrating sample magnetometry were used to find the film thickness, Fe 3 O 4 phase and magnetic properties respectively. We demonstrate optimized deposition and annealing condition for an enhanced magnetization of 854 emu/cc that is very high as compared to the bulk sample. Effect of annealing time on Fe 3 O 4 thin films were studied by X-ray diffractometer and vibrating sample magnetometer. Single phase [111] oriented Fe 3 O 4 thin films independent of substrate orientation was obtained after ninety minutes annealing. This preferred [111] oriented growth was explained on the basis of the achievement of a thermodynamic stable state.