Oxygen-deficient $({\mathrm{La}}_{0.6}{\mathrm{Pr}}_{0.4}{)}_{0.7}{\mathrm{Ca}}_{0.3}{\mathrm{MnO}}_{3-\delta }$ thin films: Towards a first-order metal-insulator transition

Thin films of $({\mathrm{La}}_{0.6}{\mathrm{Pr}}_{0.4}{)}_{0.7}{\mathrm{Ca}}_{0.3}{\mathrm{MnO}}_3$ were grown on ${\mathrm{NdGaO}}_3\mathrm{}\left(110\right)\mathrm{}$ and ${\mathrm{LaAlO}}_3\mathrm{}\left(001\right)\mathrm{}$ substrates by pulsed laser deposition. The substrate temperature was held at 820 °C, and the oxygen pressure during deposition and cooling was controlled from a few mTorr to 400 mTorr. X-ray diffraction data show an interesting structural evolution with oxygen deficiency, which was estimated by non-Rutherford elastic resonance scattering of ${\mathrm{He}}^{+}$ ions at 3.045 MeV. With increase in oxygen deficiency the insulator-metal transition temperature ${(T}_p)$ shows a significant shift to lower temperature and the transition becomes much sharper and hysteretic, with a greatly enhanced temperature coefficient of resistance (TCR). Low field magnetotransport also shows peculiar characteristics. These results bring out the tendency towards increasingly first order character with oxygen deficiency, and are in clear contrast to those in oxygen deficient ${\mathrm{La}}_{0.7}{\mathrm{Ca}}_{0.3}{\mathrm{MnO}}_3$ films, wherein the TCR is seen to decrease with decreasing $T_p.$ The observations are discussed within the framework of the phase separation picture.