Phase transitions and magnetic-transport phenomena in the system La_(2/3)Ba_(1/3)(Mn_(1-x)Co_x)O_3

摘要

A study is performed of the crystalline structure, magnetization, and magnetotransport properties of the system La_(2/3)Ba_(1/3)(Mn_(1-x)Co_x)O_3 with perovskite structure. It is shown that cubic solid solutions exist over the entire range of cobalt concentrations 0 < = x < = 1. Compositions with x < = 0.2 are ferromagnets with maximum resistance near T_C. Compositions with 0.2 < x < 0.4 manifest properties of inhomogeneous ferromagnets. Measurements of magnetic properties indicate the absence of long-range magnetic order in compositions with 0.5 < = x < = 0.9, which are probably spin glasses. The spontaneous magnetization of cobaltate (2#mu#_B per formula unit) corresponds to ferromagnetic ordering of the moments of the Co~(3+) and Co~(4+) ions found in the intermediate spin state. It is conjectured that the magnetoresistance consists of an extrinsic and an intrinsic contribution. The first arises as a result of intergrain transport of spin-polarized charge carriers, and the second, as a result of magnetic ordering near T_C. The magnetoresistance is essentially independent of the spontaneous magnetization and decreases abruptly as the cobalt concentration is increased with a corresponding transition from long-range to short-range magnetic order.