Percolative phase separation induced by nonuniformly distributed excess oxygen in low-doped La1-xCaxMnO3+delta (x <= 0.2)

Zero-field La-139 and Mn-55 nuclear magnetic resonances were studied in La0 .8Ca0.2MnO3+delta with different oxygen stoichiometries delta. The signal i ntensity, peak frequency, and line broadening of the La-139 NMR spectrum sh ow that excess oxygen atoms have a tendency to concentrate and establish lo cal ferromagnetic ordering around themselves. These local orderings connect ferromagnetic clusters embedded in the antiferromagnetic host to form perc olative ferromagnetic conduction paths. This phase separation is not a char ge segregation type, but an electroneutral type. The magnetoresistance peak at the temperature where percolative paths start to form provides direct e vidence that phase separation is one source of the colossal magnetoresistan ce effect.