Metal-insulator transition temperature enhancement in La0.7Ca0.3MnO3 thin films

The effect of oxygen annealing on the structural and transport properties ofLa0.7Ca0.3MnO3 thin films deposited on SrTiO3 substrates has been investigated by x-ray diffraction analysis and resistive measurements. The as-grown films are fully tensile strained on the substrates and show a depressed metal-insulator transition temperatureTp = 131 K. As the oxygen content is increased due to longer annealing times, significantly higher Tp are measured, up to 247 K. Correspondingly, an increase of the out-of-plane lattice parameter is observed while the in-plane lattice constants do not change with respect to the as-grown films, which prevents any interpretation of a Tpdependence on the strain. The large increase in Tp is then interpreted in terms of a combined effect of the Mn4+/Mn3+ ratio variation, the change in the carrier density in thea-b planes, and the increase in the hydrostatic strain with the oxygen annealing.