Dynamics and local structure of colossal magnetoresistance manganites

We report Extended X-ray Absorption Fine Structure and anelastic spectrosco py measurements on on hole doped manganese oxides La1-xCaxMnO3 which presen t the colossal magnetoresistance effect. EXAFS measurements were realized b oth in the absence and presence of an applied magnetic field of 1.1 Tesla, in a wide temperature range (between 330 and 77 K) and at various dopings ( x = 0.25 and x = 0.33). The magnetic held orders the magnetic moments so fa vouring the electron mobility and the reduction of Mn-O octahedra distortio ns. We observe the presence of four short and two long Mn-O distances (1.93 and 2.05 Angstrom respectively) above and also below the metal-insulator p hase transition. The overall distortion decreases but does not completely d isappear in the metallic phase suggesting the possible coexistence of metal lic and insulating regions at low temperatures. The magnetic field reduces the lattice distortions showing evidence of a microscopic counterpart of th e macroscopic colossal magnetoresistance. We also present preliminary anela stic relaxation spectra in a wide temperature range from 900 K to 1 K on a sample with x = 0.40, in order to study the structural phase transitions an d the lattice dynamics. A double peak has been observed at the metal-insula tor transition in the imaginary part of Young's modulus. This double peak i ndicates that the metal-insulator transition could be a more complex phenom enon than a simple second order phase transition. In particular the peak at lower temperatures can be connected with the possible presence of inhomoge neous phase structures. Another intense dissipation peak has been observed corresponding to the structural orthorhombic-trigonal transition around 750 K.