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

Magnetization, electrical resistivity and magnetoresistance of Pr0.50Ba0.50 MnO3-delta manganites with perovskite structure have been investigated as a function of preparation conditions. It was found that the as-prepared samp les (prepared in air) show T-C = 110 K (first order phase transition), wher eas for those annealed in flowing argon the T-C value increases up to 340 K without change of cubic symmetry (second order phase transition). Ferromag net-paramagnet transition is accompanied by both a metal-insulator transiti on and a magneto resistance peak. The X-ray study has revealed that the sam ples annealed in argon have broad peaks apparently due to microstrains and crystal structure defects. The argon-treated samples improve the magnetizat ion after subsequent annealing in air at T less than or equal to 1000 degre esC. T-C of argon-treated samples is stable with respect to annealing in ai r up to 1300 degreesC where it becomes again 110 K. In contrast, a treatmen t in vacuum destroys the ferromagnetic order. Auger-spectroscopy has not re vealed any additional ions except Pr, Ba, Mn and O for all the samples. Ext ernal 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-delta, x g reater than or equal to0.30, exhibited the above-mentioned effect of increa sing T-C after treatment in flowing argon without changes of the phase stat e, whereas the samples x <0.30 decomposed into different phases. The dramat ic increase of T-C after argon treatment is supposed to result from microst rains and crystal structure defects in the sample. (C) 2001 Elsevier Scienc e B.V. All rights reserved.