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 (L YCMO) are used in this study. The two initial calcined in air powders have almost identical ferromagnetic properties with T-c approximate to 250 K, an d saturation magnetization of sigma approximate to 90 emu/gm corresponding to Mn4+(0.3)/Mn3+(0.7). Heating the calcined powders in argon to high tempe ratures lowers T-c to approximate to 120 K; however, subsequent oxygen anne aling brings back the high T-c form. The LYCMO remains ferromagnetic with s igma approximate to 90 emu/gm in both heat treatments but only has a coloss al magnetoresistance (MR) for T-c approximate to 120 K. In contrast the LMO with T-c approximate to 120 K has sigma reduced to one-half its ferromagne tic value and the MR is almost zero. Neutron diffraction shows only the fer romagnetic structure below T-c for LYCMO but the low magnetization of LMO i s best interpreted as a mixed ferro-antiferromagnetic structure rather than a canted one. (C) 1998 American Institute of Physics. [S0021-8979(98)20611 -6].