The LnBaCo 2 O 5+δ cobaltites with ordered A-site cations exhibit a very rich phase diagram as a function of the oxygen content, which determines the doping of CoO 2 planes. By means of thermogravimetry analysis (TGA), we study the appropriate way to tailor oxygen contents in PrBaCo 2 O 5+δ compounds. Starting from an air-synthesized sample, we study the evolution of δ during different heat treatments with "in situ" TGA measurements. We find that δ can be tailored from δ = 0.2 to δ = 0.9. Thermogravimetry measurements also show that oxygen ions can flow through (0 0 1) PrO δ planes at relatively low temperatures (200 °C, this temperature increases in compounds with smaller rare earths). By means of neutron powder diffraction at room temperature, we determine the structural details and study the eventual order of oxygen vacancies for different values of δ.

The magnetoresistance of La2/3Sr1/3MnO3/CeO2 composites is explored as a function of the metal/insulator composition, temperature, and magnetic field. An important enhancement of the low-field magnetoresistance is observed for samples close to metallic percolation threshold. The improved field sensitivity is still fairly large at room temperature. Manganese perovskites composites may constitute a new alternative for the development of magnetoresistive devices.