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We have investigated several topics in the area of manganites including oxygen disorder, grain boundaries, low field magnetoresistance, magnetoresistive anisotropy and magnetic properties. Studied materials were in the form of polycrystalline samples and epitaxial thin films. The studied compounds were Sm(1-x)Sr(x)MnO3 (SSMO) and La(1-x)Ca(x)MnO3 (LCMO). 1-We have studied the effects of oxygen disorder and grain boundary disorder in the SSMO system close to half hole doping level. The temperature dependencies of resistivity and magnetoresistance were measured as a function of the vacuum annealing time. We observed a logarithmic increase of the resistivity as a function of vacuum annealing time. We have shown that an increasing grain boundary disorder softens the magnetic phase transition from a first order phase transition into a second order transition. Furthermore, the peaks in the resistivity and specific heat are broadened and there is an increase in the charge-carrier scattering rates in the metallic state. On the other hand, the polaronic hopping activation energies in the insulating state changed slightly as a function of grain boundary disorder. The origin of these phenomena is discussed. Magnetoresistive anisotropy has been studied as a function of the grain size. Results showed a strong grain size dependence of anisotropic electrical transport in granular samples of manganites. 2-We investigated the anisotropic magnetoresistance (AMR) in ultrathin LCMO films grown on various substrates. It was found that depending on the strain state, the AMR in some of these systems exceeds 100% and can even change sign. These changes are dramatic when compared to the few percent change in AMR in conventional ferromagnets. The mechanism behind these changes in the AMR is discussed. We have also studied the effects of strain on resistive peak broadening with a simple percolation model. We have shown that strain associated with a lattice mismatched substrate in thin films can cause new electronic behavior, not found in bulk materials or thicker films of the same chemical composition. Resistivity of the ultra thin films exhibit strong relaxation effects when measured as a function of time in a constant magnetic field.

L'existence de différentes phases, polymorphes, sels, solvates et co-cristaux génère de nombreuses questions concernant la caractérisation des matériaux à l'état solide, en particulier pour l'industrie pharmaceutique. Les problématiques relatives à l'identification des phases et à la surveillance des processus de crist(allisation et de transitions de phase ne peuvent toutefois pas toujours être résolues à l'aide de techniques d'analyse conventionnelles. Dans ce travail, nous développons une approche analytique basée sur le phénomène d'optique non-linéaire de génération de seconde harmonique (GSH). La GSH est une technique sensible et précise pour détecter l'absence de centre d'inversion au sein d'une structure cristalline et pour suivre des modifications subtiles de symétrie. A travers plusieurs exemples, nous montrons comment les mesures de GSH résolues en température (TR-SHG) peuvent être utilisées pour étudier les diagrammes de phases et pour suivre les mécanismes et cinétiques des transitions de phase, y compris pour des transitions de type ordre-désordre. La combinaison de la TR-SHG avec les techniques classiques (XRPD, DSC et microscopie) permet ainsi de démontrer l'utilité et le potentiel de l'optique non-linéaire dans la caractérisation des propriétés physico-chimiques des matériaux. / The existence of different phases, including polymorphs, salts, solvates and co-crystals generates concerns in the characterization of solid-state materials, especially for the pharmaceutical industry. Issues related to the identification of phases and the monitoring of phase transitions and crystallisation processes cannot be always solved using conventionnal techniques. In this work, a complementary analytical approach based on the nonlinear optical phenomena of second harmonic generation (SHG) is developed. SHG is a sensitive and accurate technique to detect the absence of inversion center in the crystalline structure and to capture subtle symmetry changes. Herein, through several examples we show how Temperature-Resolved SHG (TR-SHG) measurements van be used to study phase diagrams and for tracking mechanisms and kinetics of phase transitions including order-disorder phase transitions. The combination of TR-SHG with classical techniques (XRPD, DSC and microscopy) reveals in this study the usefulness and the potentials of nonlinear optics in material characterization.