Role of Stoichiometry and Structure in Colossal Magnetoresistive La1–xSr xMn1–yRuyO3+δ

We have systematically investigated the effect of stoichiometry [divalent doping ( x ), controlled oxygen content ( d ) 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– x Sr x Mn 1– y Ru y O 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 0.7 Sr 0.3 MnO z show a larger resistivity and lower T c 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 0.7 Sr 0.3 Mn 1– y Ru y 3 , 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– x Sr x MnO 3 (0 < x < 0.7) and oxygen-reduced La 0.7 Sr 0.3 MnO z thin films conclusively show that these materials are charge-transfer-type insulators with carriers having significant oxygen 2 p 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.