Local probe studies on lattice distortions and electronic correlations in manganites

This thesis presents an experimental study on lattice distortions and electronic correlations in colossal magnetoresistive magnetic oxides. The Perturbed Angular Correlation local probe technique is used to study selected manganite systems in order to obtain relevant insight into microscopic phenomena responsible for their macroscopic pr operties. Complementary structural, magnetic and electric characterization was performed. The work is focused on the following aspects: \\Lattice distortions and polaron clusters in LaMnO$_{3+ \Delta}$ system. A study of the electric field gradi ent and magnetic hyperfine field was performed in representative samples of the LaMnO$_{3+ \Delta}$ system, and correlated with macroscopic information obtained in the same samples. Particular attention was given to the LaMnO$_{3.12}$ sample since this compound is a prototype of a ferromagnetic-insulat or manganite, presenting a rhombohedric- orthorhombic structural phase transition near room temperature. We found that random distributed polaron clusters su rvive in the undistorted Rhombohedric phase, up to temperatures as high as 776 K. These distortions are as strong as those observed in the orbital ordered LaMnO$_{3}$ . Lowering temperature, the clusters continuously expand until a micr oscopic transition takes place. Below the transition, the distortions are ac commodated into a weaker JT distorted phase. Additionally, the macroscopic stru ctural phase transition can be viewed as a percolation transition of the microscopic environments. \\Coexistence of electric and magnetic order in the Pr$_{1-x}$Ca$_{x}$MnO$_{3}$ system. The electrical field gradient (EFG) was studied for several compositions of the Pr$_{1-x}$Ca$_{x}$MnO$_{3}$ system. This local probe analysis was complemented with the study of the magnetic and structural pro perties. This allowed the determination of the electrical field gradient phase diag ram for this system. The problematic of the charge/orbital order was also st udied. Perturbed Angular Correlation studies were used to infer about atomic-scale distortions in a wide temperature range encompassing the charge/orbital and magnetic ordering transitions. The electrical-field gradient generated by the charge distribution around the probe shows strong anomalies when the system undergoes the charge-order (CO) transition. In particular, the principal component of the EFG presents a sharp discontinuity below the CO transition. The anomaly of EFG below T$\scriptstyle_\textrm{CO}$ was related with the displacements of the io ns causing a distortion of the local symmetry and thus connected with the exis tence of a local electric polarization.