Strain-dependent vacuum annealing effects in $La0.67Ca0.33MnO3−δ$ films

The effects of vacuum annealing on the structural and transport properties of the $La0.67Ca0.33MnO3−δ$ films grown on $SrTiO3$ (LCMO/STO) and $NdGaO3$ (LCMO/NGO) substrates have been studied. A lattice expansion due to oxygen release during the annealing is observed. Under the same condition, the change of the out-of-plane lattice parameter in LCMO/STO is two to three times larger than that in LCMO/NGO, indicating a strong tendency for the oxygen in the former to escape. Correspondingly, the metal-to-semiconductor transition shifts to lower temperatures, linearly with lattice constant until a critical value, $Δd=0.03$ Å for LCMO/STO and $Δd=0.05$ Å for LCMO/NGO, after which a sudden drop of the transition temperature to zero occurs. The different lattice strains in both films are presumably responsible for the different critical oxygen contents for the occurrence of the resistive transition.