Study of La-deficient La1-x square xMnO3-delta perovskites with giant magnetoresistance in the composition x=0.2

Magnetic, structural and electrical properties in self-doped La(1-x)square( x)MnO(3-delta) system are studied. X-ray diffraction shows that the materia ls with a composition range x = 0.0 - 0.33 are single-phase perovskite-like compounds. Depending on sintering temperature and cooling speed, La-defici ent phases undergo a complete transformation from orthorhombic to rhombohed ral symmetry at high Mn4+ content. These phases exhibit a ferromagnetic to paramagnetic transition with increasing temperature (except for LaMnO3 whic h is antiferromagnetic (M approximate to 2 emu/g) at 115 K). On annealing a t low temperature (800 degrees C), the structural transition is accompanied by an important increase in both magnetization M and Curie temperature T-c . These experimental facts show a close relation of oxygen stoichiometry an d Mn4+ stability with synthesis conditions. A sharp drop in resistivity and a magnetoresistance value (rho(0) - rho(5T))/rho(0) of 85% are observed cl ose to T-c = 268 K, for an annealed sample with x = 0.2.