Electrical and Magnetotransport Properties of

This paper presents a detailed study on the Co-doping influence on the electrical and magnetotransport properties of La_0.7Ca_0.3Mn_1-xCo_xO₃(x = 0.09-0.17) prepared by solid-state reaction. Magnetic measurements versus temperature revealed gradual decrease of the magnetization (M) and Curie temperature (T_C) with increasing Co concentration (x). The T_C values vary from 194 to 159 K as changing x from 0.09 to 0.17, respectively. H/M versus M² performances around T_C prove the x = 0.09 sample undergoing a first-order magnetic phase transition (FOMT) while the samples with x ≥ 0.11 undergo a second-order magnetic phase transition (SOMT). The other with x = 0.10 is considered as a threshold concentration of the FOMT-SOMT transformation. Considering temperature dependences of resistivity, ρ(T), in the presence and absence of the magnetic field, the samples (excepting x = 0.17) exhibit a metal-insulator transition at T_P = 60-160 K, which shifts toward lower temperatures with increasing x. the metallic-ferromagnetic region, the ρ(T) data are well fitted to a power function ρ(T) = ρ₀ + ρ₂ T² + ρ_4.5 T^4.5. This indicates electron-electron and electron-magnon scattering processes are dominant at temperatures T <; T_P. In addition, the conduction data at temperatures T > θ_D/2 (θ_D is the Debye temperature) and T_P <; T <; θ_D/2 obey the small-polaron and variable-range hopping models, respectively. The values of activation energy E_p, and density of states at the Fermi level N(E_F) were accordingly determined. Here, N(E_F) increases while E_p decreases when an external magnetic field is applied. We also have found that N(E_F) - ncreases when materials transfer from the FOMT to the SOMT, and N(E_F) value becomes smallest for the sample having the coexistence of the FOMT and SOMT (i.e., x = 0.10).