Colossal magnetoresistance—an unusually large change of resistivity observed in certain materials following application of magnetic field—has been extensively researched in ferromagnetic perovskite manganites. But it remains unclear why the magneto-resistive response increases dramatically when the Curie temperature (T_C) is reduced. In these materials, T_C varies sensitively with changing chemical pressure; this can be achieved by introducing trivalent rare-earth ions of differing size into the perovskite structure, without affecting the valency of the Mn ions. The chemical pressure modifies local structural parameters such as the Mn-O bond distance and Mn-O-Mn bond angle, which directly influence the case of electron hopping between Mn ions (that is, the electronic bandwidth). But these effects cannot satisfactorily explain the dependence of magnetoresistance on T_C. Here we demonstrate, using electron microscopy data, that the prototypical (La,Pr,Ca)MnO_3 system is electronically phase-separated into a sub-micrometre-scale mixture of insulating regions (with a particular type of charge-ordering) and metallic, ferromagnetic domains. We find that the colossal magnetoresistive effect in low-T_C systems can be explained by percolative transport through the ferromagnetic domains; this depends sensitively on the rela-tive spin orientation of adjacent ferromagnetic domains which can be controlled by applied magnetic fields.
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