Preparation and properties of epitaxial La0.7Ca0.3MnO3-δ films with reduced carrier density

Epitaxial La0.7Ca0.3MnO3-delta thin films have been grown on LaAlO3(100) substrates by magnetron sputtering. By vacuum annealing at temperatures below 600 degrees C, oxygen-deficient films with ferromagnetic ordering temperatures, T-C, between 130 K and 270 K have been prepared. Oxygen deficiency reduces the Mn4+ content, i.e. the charge carrier density of the films. Effective oxygen diffusion was observed at temperatures as low as T = 80 degrees C. Resistance, magnetization and magnetoresistance behaviours of the films are reported. Our results are consistent with a magnetically phase-separated ground state-a mixture of ferromagnetic metallic and antiferrornagnetic insulating regions-of La0.7Ca0.3MnO3-delta for an Mn4+ concentration below similar to 20%. The electrical transport in the oxygen-deficient films, e.g. the non-monotonic temperature dependence of the resistance, the large magnetoresistance at T < T-C and the low-field magnetoresistance observed for some of the investigated samples, can be explained by a percolation mechanism in the phase-separated state.