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It was recently proposed that colossal magnetoresistive (CMR) properties of manganites could be explained by an electronic phase separation. An elegant manner to interpret the CMR properties is a percolation of a metallic ferromagnetic phase in an insulating antiferromagnetic matrix. A small change of the fraction or of the arrangement of the ferrodomains can induce the percolation. Among the manganites, the Pr1-xCaxMnO3 series is widely studied: CMR compounds are obtained in the range 0.3<x<0.4. Small angle neutron scattering (SANS) is a powerful technique to study the magnetic phase separation, since it allows observing the scattering by nanometric magnetic inhomogeneities in the range 1-50nm. We have recently shown that a compound with x=0.33 had a nanometric magnetic phase separation. We are studying single crystal samples of compositions between x=0.15 and x=0.4. The aim is to compare the evolution of the magnetic scattering in these different samples, and try to bring some responses to the CMR question. The study versus temperature gives the critical correlation lengths in all samples and shows the apparition at low temperature nanometric phase separation in CMR samples x=0.33 and x=0.37. By increasing the magnetic field across the isolator to metal transition, we have measured the evolutions of the topology of the nanometric magnetic objects concomitant with this CMR transition. We will discuss the result obtained by SANS in the light of more classical magneto-resistance data.
C. Yaicle et al., Phys. Rev. B 68, 094422 (2003) S. Mercone et al., Phys. Rev. B 68, 224412 (2003) D. Saurel et al., Physica B 350, 51 (2004) |