STRUCTURE, ELECTRON-TRANSPORT PROPERTIES, AND GIANT MAGNETORESISTANCEOF HOLE-DOPED LAMNO3 SYSTEMS

Results of a detailed investigation of the structure and electron-tran sport 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 containin g different percentages of Mn4+. The electrical resistivity (rho) and magnetoresistance (MR) of polycrystalline pellets have been measured i n the 4.2-400 K range in magnetic fields up to 6 T and the Seebeck coe fficient (S) from 100 to 400 K. The electrical measurements were suppl emented by ac susceptibility and magnetization measurements. MR is lar ge and negative over a substantial range of compositions and peaks aro und 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 magnetizati on although rho in the metallic state is generally much larger than th e Mott's maximum metallic resistivity. The occurrence of giant magneto resistance is linked to the presence of an optimal proportion of Mn4ions and is found in the rhombohedral and the cubic structures where t he Mn-O distance is less than 1.97 Angstrom and the Mn-O-Mn angle is 1 70 degrees+/-10 degrees. The field dependence of MR shows the presence of two distinct regimes. The thermopower S shows a positive peak in t he composition range at a temperature where MR also peaks; S becomes m ore negative with increase in Mn4+.