Role of stoichiometry and structure in colossal magnetoresistive La1-xSrxMn1-yRuyO3+δ

We have systematically investigated the effect of stoichiometry: [divalent doping (x), controlled oxygen content (delta) 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 La1-xSrxMn1-yRuyO3+delta bulk and thin films prepared by both sol-gel and pulsed laser deposition techniques. The following results have been found: (1) Oxygen-reduced La0.7Sr0.3MnOz show a larger resistivity and lower T-c than the corresponding bulk materials. Moreover, their resistivity acid 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 La0.7Sr0.3Mn1-yRuyO3, 0 less than or equal to y less than or equal to 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 La1-xSrxMnO3 (0<x<0.7) and oxygen-reduced La0.7Sr0.3MnOz 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. [S0163-1829(99)11141-X].