Magnetic and crystal structure phase transitions in R1-xBaxCoO3-y(R = Nd, Gd)

Perovskites of the R 1- x Ba x CoO 3- y type where R is Nd or Gd have been prepared and characterized. The substitution of Nd by Ba ions up to x = 0.4 leads to a nonhomogeneous ferromagnetic state appearance similar to that found for the La 1- x Sr x CoO 3 system studied earlier. Structural refinement of Nd 0.5 Ba 0.5 CoO 3- y reveals the chemical ordering of Nd and Ba ions. The magnetic properties of Nd 0.5 Ba 0.5 CoO 3- y depend drastically on oxygen content. Reduced samples ( y >0.2) exhibit magnetic phase transitions at around T i ~200 K and T C 280 K as well as a crystal structure transition at T M 350 K which leads to a drop in the electrical conductivity. The transitions at T i and T M are more pronounced in the Gd 1- x Ba x CoO 3- y series. The decrease of Ba content below x = 0.5 leads to decreasing T i whereas the increase of Ba ions above x = 0.5 leads to a suppression of both spontaneous magnetization and phase transition at T M . It is supposed that the transition at T M results from the oxygen vacancy ordering whereas at T i the antiferromagnetic state (low temperature phase) transforms into a weak ferromagnetic one with spontaneous magnetization around 0.2 µ B per cobalt ion.