Comparative study of the magnetic and electrical properties of Pr1-xBaxMnO3-δ manganites depending on the preparation conditions

Magnetization, electrical resistivity and magnetoresistance of Pr0.50Ba0.50MnO3-δ manganites with perovskite structure have been investigated as a function of preparation conditions. It was found that the as-prepared samples (prepared in air) show TC=110 K (first order phase transition), whereas for those annealed in flowing argon the TC value increases up to 340 K without change of cubic symmetry (second order phase transition). Ferromagnet-paramagnet transition is accompanied by both a metal-insulator transition and a magnetoresistance peak. The X-ray study has revealed that the samples annealed in argon have broad peaks apparently due to microstrains and crystal structure defects. The argon-treated samples improve the magnetization after subsequent annealing in air at T≤1000 deg. C. TC of argon-treated samples is stable with respect to annealing in air up to 1300 deg. C where it becomes again 110 K. In contrast, a treatment in vacuum destroys the ferromagnetic order. Auger-spectroscopy has not revealed any additional ions except Pr, Ba, Mn and O for all the samples. External pressure enhances the Curie point of the sample prepared in air at a rate of 43 K/GPa. We have observed that the samples Pr1-xBaxMnO3-δ, x≥0.30, exhibited the above-mentioned effect of increasing TC after treatment in flowing argon without changes of the phase state, whereas the samples x<0.30 decomposed into different phases. The dramatic increase of TC after argon treatment is supposed to result from microstrains and crystal structure defects in the sample