Results of a detailed investigation of the structure and electron-transport properties of ${\mathrm{La}}_{1\mathrm{-}\mathit{x}}$${\mathit{A}}_{\mathit{x}}$${\mathrm{MnO}}_3$ (A=Ca, Sr) over a wide range of compositions are presented along with those of parent ${\mathrm{LaMnO}}_3$ containing different percentages of ${\mathrm{Mn}}^{4+}$. 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 (${\mathit{T}}_{\mathit{c}}$). An insulator to metal-like transition occurs near the ${\mathit{T}}_{\mathit{c}}$ and the temperature dependence of ρ below ${\mathit{T}}_{\mathit{c}}$ 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 ${\mathrm{Mn}}^{4+}$ 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 ${\mathrm{Mn}}^{4+}$. © 1996 The American Physical Society.

• Received 5 September 1995

DOI:https://doi.org/10.1103/PhysRevB.53.3348