Magnetic and structural phase transitions in some orthocobaltites doped by Ba or Sr ions

It has been found that reduction of La0.5Ba0.5CoO3 perovskite in vacuum leads to collapse of the ferromagnetic state without changes of cubic symmetry. R0.5Ba0.5CoO3-x (R=Nd, Tb) oxygen deficit orthorhombic phases undergo phase transitions approximately at temperatures Ti ∼ 240 K, TC ∼ 280 K and TM ∼ 350 K. The first order antiferro-ferromagnet transition at Ti is accompanied by a jump of conductivity, and the negative magnetoresistance reaches 25% in a field of 1 T. The magnetization data indicate a low-spin state of cobalt ions. Spontaneous magnetization develops at Ti and disappears at TC. The first order transition from a semiconductive to quasi-metallic state occurs at TM with a drop of resistivity of more than one order of magnitude. The transitions are most pronounced at x=0.25. It is supposed that the transition at TM results from oxygen vacancies ordering. The R0.5Sr0.5CoO3 perovskites are metallic ferromagnets with an intermediate-spin state of Co ions. The properties of R0.5Ba0.5CoO3-x compounds drastically differ from those of R0.5Sr0.5CoO3-x apparently due to rare-earth and barium ions ordering. (author)