Investigação das propriedades de magneto-transporte das perovskitas La2-xCaxMnRuO6 com 1.0 ≤ x ≤ 2.0

Abstract

In this thesis we describe the synthesis and characterization of the structural, electronic, magnetic, and electrical transport properties of polycrystalline samples of the La2-xCaxMnRuO6 perovskites, at concentrations x = 1.0, 1.5, 1.75 and 2.0. The main objective of this work was to investigate the effects of substituting La with Ca, with particular interest in the magnetoresistance effect, characterized by the change in electrical resistance in response to the application of a magnetic field. The polycrystalline samples investigated in this study were synthesized by conventional solid-state reaction and studied using experimental characterization techniques. These include x ray powder diffraction, magnetization as a function of temperature [M(T)] and applied magnetic field [M(H)], electrical resistivity as a function of temperature [ρ(T)] and as a function of field [ρ(H)], x ray absorption spectroscopy (XAS), and x ray magnetic circular dichroism (XMCD). All compounds crystallize with orthorhombic symmetry in the Pnma space group. M(T) measurements indicate that the x = 1.5 sample exhibits the highest magnetic ordering temperature, at approximately 291 K, very close to room temperature. The M(H) curves of the x = 1.0, 1.5, 1.75 and 2.0 samples show characteristic behavior of ferrimagnetic systems. The ρ(T) measurements indicate negative magnetoresistance along the whole temperature interval investigated. Compounds with x = 1.5, 1.75 and 2.0 exhibits ρ(T) curves typical of half-metal systems. XAS measurements at the Mn L2,3 edges indicate Mn3+/Mn4+ mixed valence in intermediate concentrations of the series, and the opposite XMCD signals observed for Mn and Ru suggest antiferromagnetic coupling between the transition metal ions.