Very large magnetoresistance and spin state transition in Ba-doped cobaltites

Structural, magnetization, and magnetotransport measurements have been performed for anion deficient La0.5Ba0.5Co1-xFexO3-delta (x <= 0.4) and La1-yBayCoO3-delta (0.5 <= y <= 0.6) perovskites. It has been found that the iron doped compositions with x <= 0: 23 are predominantly ferromagnetic. The Curie point and magnetization are slightly larger in insulating phase (x = 0.1) than in metallic one (x = 0.05). Magnetoresistance ratio rises with lowering temperature and increasing iron content reaching three orders of magnitude in predominantly antiferromagnetic state (x >= 0.25). The rise of the barium content in La1-yBayCoO3-delta series above y = 0.5 leads to stabilization of antiferromagnetic phase and strong enhancement of the magnetoresistance. Antiferromagnetic ordering is accompanied by increase in resistivity. The lack of the low field intergrain magnetoresistance is in agreement with a weak spin polarization of the charge carriers. Magnetoresistance is associated with antiferromagnet-ferromagnet (from mixed high/low into intermediate spin state) transition induced by magnetic field. It is suggested that ferromagnetism is originated from superexchange interaction via oxygen. (C) 2013 American Institute of Physics. [http://dx.doi.org/10.1063/1.4790316]