Role of stoichiometry and structure in colossal magnetoresistive La_1-xSr_xMn_1-yRu_yO_3+δ

We have systematically investigated the effect of stoichiometry [divalent doping (x), controlled oxygen content (δ) by vacuum annealing at elevated temperatures, and substitution (y) of Ru] and structure (substrate induced strain and its relaxation on annealing) on the magnetic/transport properties of the colossal magnetoresistive La_1-xSr_xMn_1-yRu_yO_3+δ bulk and thin films prepared by both sol-gel and pulsed laser deposition techniques. The following results have been found: (1) Oxygen-reduced La_0.7Sr_0.3MnO_z show a larger resistivity and lower Tc than the corresponding bulk materials. Moreover, their resistivity and MR behavior can be precisely controlled by vacuum annealing and, in fact, they duplicate all the salient features observed in divalent-doped manganites. (2) The metal-insulator transitions of thin films grown on lattice-matched substrates is observed to be a function of thickness due to the accommodation of epitaxial strain and the associated Mn-O-Mn bond-distortions. (3) Ru doped La_0.7Sr_0.3Mn_1-yRu_yO₃, 0 < -y < -0.2 samples show a surprisingly small decrease in T_c. This is attributed to the exceptional ability of Ru, unlike other substitutions in Mn sites, to stabilize magnetic ordering at elevated temperatures. (4) Oxygen K-edge (core level excitation of oxygen 1s electrons into empty p-like states) electron-energy-loss spectra of divalent-doped La_1-xSr_xMnO₃ (0 < x < 0.7) and oxygen-reduced La_0.7Sr_0.3MnO_z thin films conclusively show that these materials are charge-transfer-type insulators with carriers having significant oxygen 2p hole character. We discuss the implications of these results on the magnetic and transport properties of manganites and address the implications of our electron-energy-loss spectroscopy measurements on the double exchange mechanism.