Magnetoresistance enhancement of (LBMO)1-x/(NiO)x composites based on Spin Valves

Master's Thesis from the year 2016 in the subject Physics - Nuclear Physics, , language: English, abstract: The objective of this thesis is to study the structural, electrical and magnetic properties of (La0.7Ba0.3MnO3)1-x/(NiO)x composites, where 0 ? x ? 0.20 step 0.05 wt.%, that show promising properties for spin- valve applications. The polycrystalline composites were prepared using the standard solid state reaction method. Besides to the as- prepared condition, the annealing process was taken into account as a variation in annealing temperature to see its influence on their properties of this system. An analysis of the structural properties was carried out by means of X-ray diffraction and the Rietveld technique. The doping process wasn't change the structural properties. The composites undergo ferromagnetic (FM) to paramagnetic (PM) transition at TC. Magnetization decreases with doping at x=0.05, 0.15 and 0.20 wt. %. M(T) relation in low temperature (x>0) show an obvious knee, which due to the presence of NiO. The temperature dependence of resistivity showed a metallic behavior below the transition temperature Tms and above this temperature, the behavior become semiconducting. There were various mechanisms that governed the conduction above and below Tms. The semiconducting region was characterized by two main conduction mechanisms, the small polaron hopping (SPH) and the variable range hopping (VRH). In the metallic region at T < Tms, there were different contribution of mechanisms that govern this region as grain boundaries, domains and temperature independent process. In addition to some interactions as electron-electron interaction, electron-phonon interaction and spin wave scattering process which is a temperature range, composition and annealing temperature dependent. The temperature dependence of resistivity was measured under the effect of 0.6T magnetic field for the as prepared and the annealed composites. The magnetoresistance was calculated and found to be affected by NiO content and annealing temperature. Thermoelectric power data (TEP) explains hole and electron contribution in conduction, and this also was found to be Ni amount and annealing temperature dependent. The TEP data analysis below temperature peak (Ts) confirmed the presence of some conduction mechanisms in electrical measurements as phonon drag, while the activation energy was determined from the region above Ts.