Doping Ru/Cr on B-site of La(BaCa)MnO thin films: driving insulator-to-metal transition temperature far apart from Curie temperature

Magnetic and transport properties of Ru/Cr doped La(Ba,Ca)MnO thin films fabricated by the pulsed laser deposition technique on (1 0 0) LaAlO3 substrates have been investigated. Ru-doping reduces both Curie temperature (TC) and insulator-to-metal temperature (TIM) in LaBaCaMnO thin films: under zero field, La0.7Ba0.1Ca0.2Mn0.9Ru0.1O3 thin films which have TC about 190 K show an insulator-to-metal transition at 75 K. It is likely that Ru-doping creates some antiferromagnetic insulating matrix which separates a basically ferromagnetic background into isolated ferromagnetic domains. In the case of partial substitution of Cr for Mn in La0.9Ba0.1MnO3 thin films, while TC remains almost the same as non-doped films, the magnitude of the magnetization is increased very much due to doping. However, this enhancement does not go along with the enhancement of metallicity: upon doping, TIM shifts to lower temperatures, and the resistivity becomes several orders higher as well. The nature of TIM being far apart from TC in Cr-doped samples are thought to be different from that of Ru-doped ones: it is supposed that the Cr substitution created inhomogeneities resulting in various bonds such as Cr3+OCr3+, Mn3+OCr3+ and MnOMn. The observation of TIM far below TC in B-site doping manganite thin films is very anomalous and cannot be explained by double-exchange theory.