Phase transitions and magnetic-transport phenomena in the system La2/3Ba1/3(Mn1-xCox)O-3

A study is performed of the crystalline structure, magnetization, and magne totransport properties of the system La2/3Ba1/3(Mn1-xCox)O-3 with perovskit e structure. It is shown that cubic solid solutions exist over the entire r ange of cobalt concentrations 0 less than or equal to x less than or equal to 1. Compositions with x less than or equal to 0.2 are ferromagnets with m aximum resistance near T-C. Compositions with 0.2 < x < 0.4 manifest proper ties of inhomogeneous ferromagnets. Measurements of magnetic properties ind icate the absence of long-range magnetic order in compositions with 0.5 les s than or equal to x less than or equal to 0.9, which are probably spin gla sses. The spontaneous magnetization of cobaltate (2 mu(B) per formula unit) corresponds to ferromagnetic ordering of the moments of the Co3+ and Co4ions found in the intermediate spin state. It is conjectured that the magne toresistance consists of an extrinsic and an intrinsic contribution. The fi rst arises as a result of intergrain transport of spin-polarized charge car riers, and the second, as a result of magnetic ordering near T-C. The magne toresistance is essentially independent of the spontaneous magnetization an d decreases abruptly as the cobalt concentration is increased with a corres ponding transition from long-range to short-range magnetic order. (C) 1999 American Institute of Physics. [S1063- 7761(99)01708-4].