Transport properties of La _{1 - x} Ca _x MnO ₃ ( 0.5 ? x < 1)

Abstract:

The transport properties of the La_1-xCa_xMnO₃ ( 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 Mn⁴⁺ content n _{Mn4 +} obeys the formula T _CO / T _max = 1 - a ( n _{Mn4 +} - n ₀ ) ², here n₀ 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 < 6 T 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.