This manuscript presents the study by small angles neutron scattering of the magnetic inhomogeneities from nanometric to mesoscopic scales at low temperature in CMR manganite compounds of the series Pr1-xCaxMnO3, x close to 1/3, and its evolution under applied magnetic field.These systems show a large scale phase separation between a ferromagnetic insulating (FI) phase and an antiferromagnetic insulating (AFI) one, corresponding to two different crystalline structures. They are transformed into a third crystalline phase, ferromagnetic metallic (FM), under an external magnetic field. We have tried to understand the mechanism of this transformation.We have shown the existence of magnetic nanometric inhomogeneities in both FI and AFI phases. Our study under field reveals the apparition of a strong SANS signal due to a nucleation of mesoscopic clusters (several hundreds of nanometers) of FM phase and a consequent removing of the SANS intensity of the nanometric objects. Consequently we can assert that the CMR effect is not due to a nucleation and growth of nanometric conducting clusters, but mesoscopic ones.
