Lattice effects in magnetoresistive La(1-x)Sr(x)MnO(3) manganites

In the perovskite manganites $\rm La\sb{1-x}Sr\sb{x}MnO\sb3,$ various transitions in the transport properties and magnetism as well as the structure can be induced by temperature, composition or pressure. Upon application of a magnetic field the colossal magnetoresistance (CMR) effect is observed. In this thesis, a detailed investigation of the local atomic structure of this system was carried out as a function of temperature and composition. With the use of pulsed neutron powder diffraction and the atomic Pair Density Function (PDF) analysis, a direct link between the atomic structure and the observed properties was established. The PDF technique provides an accurate representation of the local structure and of any atomic displacements that remain undetected by probes that assume structural periodicity. This proved to be the key to the understanding of the magnetoresistive properties of manganites. It was found that the lattice becomes locally distorted to form polarons when LaMnO$\sb3$ is doped with Sr. The strong interaction of the charge with the spin and lattice gives rise to an inhomogeneous charge distribution even in the metallic phase. The observation of such charge inhomogeneity is unexpected but is consistent with the relatively poor conductivity of this system. It is suggested that the transitions in transport properties and magnetism are achieved by the percolation of charges through a network made of polarons. The mere existence of the polaron network suggests the presence of an inhomogeneous filamental metal and provides a clear evidence of the lattice involvement in the properties of this system. This work serves as an example where the application of a local technique such as the PDF can provide valuable information with regard to the origin of the properties