This thesis investigates the properties of colossal magnetoresistive perovskite manganites from a theoretical and experimental point of view. In light of the failings of the double-exchange mechanism which has traditionally been used to explain the physics of colossal magnetoresistive manganites, we investigate the properties of these materials using the current carrier density collapse (CCDC) theory. We show that the CCDC theory can adequately explain the resistivity of perovskite manganites when subjected to zero external magnetic field and when in the presence of an applied magnetic field, thus accounting for the large negative magnetoresistance in these materials. Also the theory in conjunction with a generic phase seperation model allows us to explain the resistivity and for the first time, specific heat in disordered manganites. Experimental investigations into La0.75 Sr0.25 MnO3 have been undertaken. Homemade samples of La0.75 Sr0.25 MnO3 have been produced. Different annealing treatments have allowed us
to produce samples with different crystallite sizes, as determined by Rietveld refinement. The effects of crystallite size upon the resistivity, magnetoresistance, magnetic properties and specific heat have been investigated. The resistivity, magnetoresistance and magnetic properties are in qualitative agreement with the CCDC theory, and the specific heat data are seen to be in quantitative agreement with the CCDC theory. We find no results that are in contradiction to the CCDC theory.
A Doctoral Thesis. Submitted in partial fulfillment of the requirements for the award of Doctor of Philosophy of Loughborough University.