Percolative phase separation induced by nonuniformly distributed excess oxygens

TL;DR

This study investigates how excess oxygen in LaCaMnO compounds causes local ferromagnetic regions to form and connect, leading to phase separation that contributes to colossal magnetoresistance.

Contribution

It reveals that oxygen excess induces electroneutral phase separation with ferromagnetic clustering, distinct from charge segregation, affecting magnetoresistance.

Findings

Excess oxygens concentrate and induce local ferromagnetic order.

Percolative conduction paths form due to phase separation.

Magnetoresistance peaks correlate with percolation onset.

Abstract

The zero-field $^{139}$La and $^{55}$Mn nuclear magnetic resonances were studied in $\rm La_{0.8}Ca_{0.2}MnO_{3+\delta}$ with different oxygen stoichiometry $\delta$. The signal intensity, peak frequency and line broadening of the $^{139}$La NMR spectrum show that excess oxygens have a tendency to concentrate and establish local ferromagnetic ordering around themselves. These connect the previously existed ferromagnetic clusters embedded in the antiferromagnetic host, resulting in percolative conduction paths. This phase separation is not a charge segregation type, but a electroneutral type. The magnetoresistance peak at the temperature where percolative paths start to form provides a direct evidence that phase separation is one source of colossal magnetoresistance effect.