Study of La-deficient La_1-x□_xMnO_3-δ perovskites with giant magnetoresistance in the composition x = 0.2

Abstract

Magnetic, structural and electrical properties in self-doped La_1-x□_xMnO_3-δ system are studied. X-ray diffraction shows that the materials with a composition range x = 0.0 -0.33 are single-phase perovskite-like compounds. Depending on sintering temperature and cooling speed, La-deficient phases undergo a complete transformation from orthorhom-bic to rhombohedral symmetry at high Mn⁴⁺ content. These phases exhibit a ferromagnetic to paramagnetic transition with increasing temperature (except for LaMnO₃ which is antiferromagnetic (M ≈ 2 emu/g) at 115 K). On annealing at low temperature (800°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 and Mn⁴⁺ stability with synthesis conditions. A sharp drop in resistivity and a magnetoresistance value (p₀—P5T)/PO of 85% are observed close to T_c = 268 K, for an annealed sample with x = 0.2.

Key Words:

• Perovskite oxides
• Self-doped La_1-x□_xMnO_3-δ
• Giant magnetoresistance