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A strong correlation between the Curie temperature and the metal-insulator transition is displayed in Ru doped La[sub 0.6]Pb[sub 0.4]Mn[sub 1-x]Ru[sub x]O[sub 3], where an equiatomic ratio of Ru and Mn is present in the Mn-O-Ru sublattice. The magnetic pair making effect of Mn with Ru is facilitated by the variable valency of Ru (IV)/Ru (v) which is similar in ionic size to the Zener pair Mn (III)/Mn (IV). Further Ru, being a 4d metal with itinerant t[sub 2g] electrons, facilitates the exchange coupling interaction. The spin-wave excitation and the hole carrier density dominate the magnetization and the electron transport for T < 0.5 T[sub C] for a higher Ru doped composition. [ABSTRACT FROM AUTHOR]

Double exchange mediated magnetism and transport properties are exemplified in two-dimensional layered manganites, La[sub 1.2]Ca[sub 1.8]Mn[sub 2]O[sub 7]. We demonstrate here the role of ruthenium in showing increased metal-insulator transition temperature, T[sub P] and Curie temperature T[sub c], by about 15 K when Ru is substituted up to 10% (x = 0.2) at the Mn site. We discuss these unique features based on the existence of a redox ionic pair involving Mn[sup 3+]/Mn[sup 4+] ↔ Ru[sup 4+]/Ru[sup 5+] up to x∼0.2. Beyond the critical level (x∼0.2), Ru exists mostly in Ru[sup 4+] oxidation state leading to a weak double exchange interaction. We document here, an enhanced T[sub c] and T[sup P] in the layered manganites despite the disorder induced at the B site. [ABSTRACT FROM AUTHOR]