Metal-insulating transition temperature enhancement in La0.7Ca0.3MnO3 Thin Films

The effect of oxygen annealing on the structural and transport properties of La0.7Ca0.3MnO3 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 temperature Tp=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 Tp dependence 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 the a-b planes, and the increase in the hydrostatic strain with the oxygen annealing.