Cobalt Effects on the Structure, Electrical and Magnetic Properties of La0.7Ca0.3Mn1-xCoxO3(x=0-0.05) Manganite Oxide

Manganite La_0.7Ca_0.3MnO₃, one of the CMR materials, is an insulator with a perovskite crystal structure and can be composed with various chemical compositions. When dope it with transition metal ions (TM = Fe, Co, Ni etc.), we obtain a LaCaMn_1-xTM_xO₃ system that shows a very large magnetoresistance. In the present paper, we report the structure, the magnetic and the electrical properties of Co-doped La_0.7Ca_0.3Mn_1-xCo_xO₃ CMR system. Sintering reactions proceeded rapidly in the order of La_0.7Ca_0.3MnO₃ > La_0.7Ca_0.3Mn_0.99Co_0.01O₃ > La_0.7Ca_0.3Mn_0.97Co_0.03O₃ > La_0.7Ca_0.3Mn_0.95Co_0.05O₃. It is assumed that cobalt may interrupt the sintering reaction of mangannite. Magnetic and electrical properties were evaluated with densified pellet. From M-T curve, we determined the Curie temperature, where the magnetization value decreases abruptly. Curie temperature were examined by superconducting quantum interference device (SQUID). Curie temperatures of 250K for La_0.7Ca_0.3MnO₃, 290K for La_0.7Ca_0.3Mn_0.99Co_0.01O₃, 280K for La_0.7Ca_0.3Mn_0.97Co_0.03O₃ and 260K for La_0.7Ca_0.3Mn_0.95Co_0.05O₃ were obtained. With very small amount of co-dopping, transition metal dominates overall property. However, the amount of cobalt increases, the Curie temperature decreases. It is considered that the decrement of magnetic order by decreasing of Mn3+ and lattice distortion. Temperature-dependent resistivity was measured by physical property measurement system (PPMS) between 0 and 300K at - a magnetic field of 10 kOe. In case of un-substituted pellet, maximum MR ratio and metal semiconductor transition temperature appears at 200K. MR ratio tends to increase with temperature up to Curie temperature and shows the maximum values around Curie temperature. In addition, La_0.7Ca_0.3Mn_0.99Co_0.01O₃ shows the highest MR ratio because it shows the highest Curie temperature.