Contrasting magnetic and structural properties of two La manganites with the same doping levels

Powder and sintered samples of La0.9MnO3 (LMO) and La0.62Y0.07Ca0.31MnO3 (LYCMO) are used in this study. The two initial calcined in air powders have almost identical ferromagnetic properties with Tc≈250 K, and saturation magnetization of σ≈90 emu/gm corresponding to Mn4+(0.3)/Mn3+(0.7). Heating the calcined powders in argon to high temperatures lowers Tc to ≈120 K; however, subsequent oxygen annealing brings back the high Tc form. The LYCMO remains ferromagnetic with σ≈90 emu/gm in both heat treatments but only has a colossal magnetoresistance (MR) for Tc≈120 K. In contrast the LMO with Tc≈120 K has σ reduced to one-half its ferromagnetic value and the MR is almost zero. Neutron diffraction shows only the ferromagnetic structure below Tc for LYCMO but the low magnetization of LMO is best interpreted as a mixed ferro–antiferromagnetic structure rather than a canted one.