Magnetic and electrical transport properties of orthocobaltites R0.5Ba0.5CoO3 (R = La, Pr, Nd, Sm, Eu, Gd, Tb, Dy)

Magnetization and electrical resistivity studies have established that La0.5Ba0.5CoO3-gamma (LaBaC) is a metallic ferromagnet whereas Pr0.5Ba0.5CoO3-gamma (PrBaC) is a magnetic semiconductor. The magnitudes of spontaneous magnetizations are in agreement with intermediate (LaBaC) and low-spin (PrBaC) cobalt states with itinerant 3d electrons. R0.5Ba0.5CoO3-gamma (RBaC, R = Sm, Eu, Gd, To, Dy) exhibit a sequence of a phase transitions at temperatures T-i, T-C, and T-M on heating. Below T-i these materials are antiferromagnets, whereas in the temperature interval between T-i and T-C, a ferromagnetic behavior has been revealed. For TbBaC the temperatures T-i and T-C are 245 and 285 K, respectively. The phase transition at T-i is accompanied by a jump of electrical resistivity and peak of magnetoresistance. For TbBaC an external magnetic field of 14 T leads to decreasing T-i from 245 to 85 K. At around T-M (337 K for TbBaC) the first-order phase transition from semiconductive to quasimetallic state (high-temperature phase) has been observed. The transition occurs without changes of orthorhombic symmetry, and the unit-cell volume of the high-temperature phase was shown to be larger as compared to the low-temperature one (approximately 0.2%). It is supposed that the transition at T-M results from oxygen vacancy ordering.