Metal-insulator transition temperature enhancement in La_(0.7)Ca_(0.3)MnO_3 thin films

摘要

The effect of oxygen annealing on the structural and transport properties of La_(0.7)Ca_(0.3)MnO_3 thin films deposited on SrTiO_3 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 T_p=131 K. As the oxygen content is increased due to longer annealing times, significantly higher T_p 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 T_p dependence on the strain. The large increase in T_p is then interpreted in terms of a combined effect of the Mn~(4+)/Mn~(3+) ratio variation, the change in the carrier density in the a-b planes, and the increase in the hydrostatic strain with the oxygen annealing.