Transport properties of La_(1-x)Ca_xMnO_3 (0.5 ≤ x < 1)

摘要

The transport properties of the La_(1-x)Ca_xMnO_3 (0.5 ≤ X < 1) system in magnetic fields up to 14 T were studied. We found that the relationship between the charge ordering temperature T_(CO) and Mn~(4+) content n_(Mn)4+) obeys the formula T_(CO)/T_(max) = 1 - a(n_(Mn~(4+))-n_0)~2, here n_0 and a are constants and T_(max) is the maximum of T_(CO). For x = 0.65, T_(CO) arrives at the maximum value of 249.5 K in zero magnetic field, while the charge ordered (CO) state is most stable around x = 0.75. For x = 0.5 when H < 6T the resistivity displays Mott's variable-range hopping (VRH) behavior, when 6 < H < 12 T it is suggested that two kinds of conduction mechanism, i.e., VRH and magnetic polarons, coexist in the material, and when H > 12 T the resistivity shows metallic-like behavior and the transport mechanism is attributed to coexistence of magnetic polarons and free carriers. For x = 0.95, the conduction mechanism accords with the coexistence of VRH and magnetic polarons.