Abstract:
Results of a detailed investigation of the structure and electron-transport properties of La(1-x)A(x)MnO(3) (A=Ca, Sr) over a wide range of compositions are presented along with those of parent LaMnO3 containing different percentages of Mn4+. The electrical resistivity (rho) and magnetoresistance (MR) of polycrystalline pellets have been measured in the 4.2-400 K range in magnetic fields up to 6 T and the Seebeck coefficient (S) from 100 to 400 K. The electrical measurements were supplemented by ac susceptibility and magnetization measurements. MR is large and negative over a substantial range of compositions and peaks around temperatures close to the ferromagnetic transition temperatures (T-c). An insulator to metal-like transition occurs near the T-c and the temperature dependence of rho below T-c is related to the magnetization although rho in the metallic state is generally much larger than the Mott's maximum metallic resistivity. The occurrence of giant magnetoresistance is linked to the presence of an optimal proportion of Mn4+ ions and is found in the rhombohedral and the cubic structures where the Mn-O distance is less than 1.97 Angstrom and the Mn-O-Mn angle is 170 degrees+/-10 degrees. The field dependence of MR shows the presence of two distinct regimes. The thermopower S shows a positive peak in the composition range at a temperature where MR also peaks; S becomes more negative with increase in Mn4+.
