Structure, electron-transport properties, and giant magnetoresistance of hole-doped LaMnO₃ systems

Abstract

Results of a detailed investigation of the structure and electron-transport properties of La_1−xA_xMnO₃ (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 (ρ) 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 ρ below T_c is related to the magnetization although ρ 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 Å and the Mn-O-Mn angle is 170°±10°. 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⁺.