Impact of deposition oxygen pressure on the thickness effects in epitaxial Nd_0.7 Sr_0.3 MnO₃ thin films

Nd0.7Sr0.3MnO3 films of various thicknesses were grown on (LaAlO3)(0.3)(Sr2AlTaO6)(0.7)(001) substrates by the pulsed-laser-deposition method, and their structure, metal-insulator (ferromagnetic-paramagnetic) transition temperature T-P (T-C) were examined by x-ray diffraction, resistivity and magnetization measurements. To clarify the effects of in situ deposition oxygen pressure on the thickness dependence of T-P (T-C), the films were grown at an oxygen pressure of 21 Pa and 35 Pa, respectively. X-ray reciprocal space mapping on the films showed that they were all grown coherently on the substrates. For films grown at low oxygen pressure the transition temperature decreases more rapidly with the reduction of film thickness, while for those deposited at high oxygen pressure, with the film thickness decreasing a strain-induced decrease in T-P (T-C) was observed. Our results indicate that to get higher T-P (T-C) especially for the ultra-thin films a higher deposition oxygen pressure is indispensable, and this is crucial for understanding the thickness effect in epitaxial manganite films. The ex-situ annealing effects on the thin and thick films were also discussed in terms of their different microstructure.