Structure, electron-transport properties, and giant magnetoresistance of hole-doped<mml:math xmlns:mml="http://www.w3.org/1998/Math/MathML" display="inline"><mml:mrow><mml:msub><mml:mrow><mml:mi mathvariant="normal">LaMnO</mml:mi></mml:mrow><mml:mrow><mml:mn>3</mml:mn></mml:mrow></mml:msub></mml:mrow></mml:math>systems

Results of a detailed investigation of the structure and electron-transport properties of ${\mathrm{La}}_{1\mathrm{\ensuremath{-}}\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 (\ensuremath{\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 (${\mathit{T}}_{\mathit{c}}$). An insulator to metal-like transition occurs near the ${\mathit{T}}_{\mathit{c}}$ and the temperature dependence of \ensuremath{\rho} below ${\mathit{T}}_{\mathit{c}}$ is related to the magnetization although \ensuremath{\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 ${\mathrm{Mn}}^{4+}$ ions and is found in the rhombohedral and the cubic structures where the Mn-O distance is less than 1.97 \AA{} and the Mn-O-Mn angle is 170\ifmmode^\circ\else\textdegree\fi{}\ifmmode\pm\else\textpm\fi{}10\ifmmode^\circ\else\textdegree\fi{}. 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 ${\mathrm{Mn}}^{4+}$. \textcopyright{} 1996 The American Physical Society.