The effect of A-site deficiency in simple perovskite manganites

The discovery of "colossal magnetoresistance" (CMR) around the ferromagnetic transition temperature TC has caused a renewed interest in the physics of manganite systems. The common and the most substantial characteristic of magnetoresistive materials is that their electrical resistance can be changed significantly by the application of an external magnetic field. The ratio Mn3+/Mn4+ is a key factor for the explanation of the magnetoresistance mechanism. Thus, in the last decades, the doping/substitution of elements at the A-site has been extensively investigated to alter this ratio. On the contrary, only a few studies reported the effect of deficiency in the lanthanum-calcium manganite system. In this thesis, the role of both lanthanum and calcium deficiencies upon the structural, microstructural, and magneto-electrical properties of the La0.7Ca0.3MnO3Fy (LCMOF) system were investigated. Lacunar perovskite manganites of the form La0.7-x■x Ca0.3MnO3Fy (x = 0.0, 0.1, and 0.2; ■:vacancy) and La0.7Ca0.3-x■xMnO3Fy (x = 0.1 and 0.2) were synthesized by the conventional solid-state reaction method in air with high sintering temperature process. It was found by X-ray diffraction (XRD) analysis that all samples were in orthorhombic structure and Pbnm space group. Cation deficiency caused a decrease in unit cell parameters. It was also seen in SEM micrographs that grain sizes decreased as lanthanum and calcium deficiency increased. All the deficient samples undergo a metallic to insulator phase transition (TMI) in the studied temperature range. A metallic behavior (i.e. positive dρ / dT) is observed in the region below the TMI, while a semiconductor behavior (i.e. negative dρ / dT) is observed in the region above the TMI. It has been found that as the A-site deficiency increased, the metal-insulator transition temperature is decreased from 265 to 59 K. At a magnetic field of 1 T, the non-lacunar sample (x=0.0) shows the highest negative magnetoresistance percentage (71 %), and the MR% value is found to be affected significantly with introducing deficiency in the LCMOF system. According to obtained temperature coefficient resistance (TCR) values, it was found that the obtained values for polycrystalline bulk samples were suitable for microbolometer applications. In addition, the A-site deficient samples may give satisfactory results for optimization applications as a thin film. KEYWORDS: Deficiency, Manganites, Magnetoresistance, Temperature Coefficient of Resistance