Colossal magnetoresistance in manganites: Role of the electronic phase segregation

The huge intrinsic magnetoresistance found in mixed-valence manganites has as origin the existence of a metal-insulator transition. Usually, this transition is accompanied by a para-ferromagnetic transition. In the paramagnetic phase the formation of dynamic phase segregation in the form of magnetic polarons provides the mechanism for carrier localisation. The insulator state can also be achieved in the antiferromagnetic state or when the long-range ferromagnetic interaction is not extended across the whole sample, giving rise to the existence of static phase segregation in which metallic ferromagnetic clusters coexist with insulating paramagnetic, antiferromagnetic or spin-glass regions. In such insulating regions, charge ordering can also be present. The application of a magnetic field can increase the size of the ferromagnetic clusters up to a critical field at which they start to percolate and, consequently, the carrier mobility is strongly increased producing giant magnetoresistance. In this communication we report on experiments of magnetovolume effects and neutron diffraction including SANS (small angle neutron scattering) giving evidence for such effects. They can be explained by the interplay of magnetic long-range interaction and the large electron-phonon interaction observed in these compounds. The relevance of this interaction for the electronic phase segregation has been proved by means of the oxygen isotopic effect.