Magnetoresistance (MR) in bulk samples of LaMnO3 has been investigated by varying the Mn4+ content from 10 to 33 per cent by chemical means, without aliovalent doping. With the increase in Mn4+ content, the structure of LaMnO3 changes first from orthorhombic to rhombohedral and then to cubic and the material becomes increasingly ferromagnetic, exhibiting a resistivity maximum akin to an insulator-metal transition atTPeak, just below the ferromagneticTc. The magnitude of MR is highest in the cubic sample (with 33% Mn4+) around theTPeak, and negligible in the non-magnetic orthorhombic sample (12% Mn4+).
