Reinforcement of double-exchange ferromagnetic coupling by Ru in La1.24Sr1.76Mn2-yRuyO7 manganite system

The effect of Mn-site doping on magnetic and transport properties in the bilayer manganites La1.24Sr1.76Mn2-yRuyO7 (y = 0.0, 0.04, 0.08 and 0.15) has been studied. The undoped compound La1.24Sr1.76Mn2O7 exhibits a ferromagnetic metal to paramagnetic insulator transition at T-C = 130 K and the substitution of Ru shifts the transition temperatures to higher temperature values. The increased metal-insulator transition by Ru substitution, obtained from temperature dependence of resistivity measurements, indicates that the Ru substitution enhances the metallic state at low temperature regime and favours the Mn-Ru pairs in the Ru doped samples. Moreover, the activation energy values calculated from the temperature dependence of resistivity curves suggest that the Ru substitution weakens the formation of polarons. The increased magnetoresistance ratio from 108% to 136% by Ru substitution, measured at 5 K, points out that the Ru substitution also enhances the inter-grain tunneling magnetoresistance. Thus, the ferromagnetic order and metallic state in La1.24Sr1.76Mn2O7 system have been enhanced by the presence of Ru in the Mn-site. These reinforcements of ferromagnetic metallic state and magnetoresistance have been interpreted by the ferromagnetically coupled high spin states of Ru with Mn ions in the Mn-O-Ru network. (C) 2013 Elsevier Ltd. All rights reserved.