Structure, magnetic, magnetocaloric and magnetoresistance properties of La1-xPbxMnO3 perovskite

La1-xPbxMnO3 (x = 0.1, 0.2, 0.3, 0.4, and 0.5) perovskites were prepared by a solid-state reaction. Except for x = 0.5 (cubic) and x = 0.4 (rhombohedral), the structure of the other compositions was pseudo-rhombohedral with P1 symmetry. The particle size of the grains is depending on the Pb content of the samples. The Curie temperature T, increases from 235 K for x = 0.1-310K for x = 0.2 and is almost constant (about 360K) for x greater than or equal to 0.3. The field-cooled and zero-field-cooled thermomagnetic curves measured at low field show a split below a so-called irreversibility temperature T-r, which is somewhat smaller than T-c except for x = 0.1, where it is 270 K. From a series of magnetic isotherms the magnetic entropy changes DeltaS(T) were determined for a field step of 500 Oe. The maximum value of DeltaS(max) increases with increasing x till x = 0.3 and then decreases with further increasing x. The conductivity of perovskites is metallic at low temperatures and semiconducting at high temperatures. Magnetoresistance measurements have been performed. (C) 2002 Elsevier Science B.V. All rights reserved.