Percolative phase separation induced by nonuniformly distributed excess oxygen in low-doped La_1-xCa_xMnO_3+δ (x≤0.2)

Zero-field ¹³⁹La and ⁵⁵Mn nuclear magnetic resonances were studied in La_0.8Ca_0.2MnO_3+δ with different oxygen stoichiometries δ. The signal intensity, peak frequency, and line broadening of the ¹³⁹La NMR spectrum show that excess oxygen atoms have a tendency to concentrate and establish local ferromagnetic ordering around themselves. These local orderings connect ferromagnetic clusters embedded in the antiferromagnetic host to form percolative ferromagnetic conduction paths. This phase separation is not a charge segregation type, but an electroneutral type. The magnetoresistance peak at the temperature where percolative paths start to form provides direct evidence that phase separation is one source of the colossal magnetoresistance effect.