Abstract
A nanoscale phase is known to coexist with colossal magnetoresistance (CMR) in
manganites, but its volume fraction is believed to be too small for a causal
connection. Here we provide nanodiffraction real-space images of nanoclusters that
form and evolve with temperature in La1-xCaxMnO3, x = 0.45. The nanoclusters are
not doping inhomogeneities; their structure corresponds to x = 0.60, at which the
bulk material at low temperatures is antiferromagnetic, �charge-ordered�, and
insulating; and their volume ratio exhibits a large peak at the CMR critical
temperature at finite magnetic fields, suggesting a zero-field volume fraction of ~ 35
%. These features are sufficient to account for the CMR peak. We propose that the
phenomenon arises from an interplay of magnetic and structural entropies.