We address the problem of the percolative phase separation in polycrystalline samples of (formula presented) for (formula presented) (hole doping n between (formula presented) and (formula presented) We perform measurements of x-ray diffraction, dc magnetization, electron spin resonance (ESR), and electrical resistivity. These samples show a paramagnetic (PM) to ferromagnetic (FM) transition at the Curie temperature (formula presented) However, for (formula presented) we found that in a wide temperature range below (formula presented) an important PM volume still survives, coexisting with the FM phase. On lowering T below the charge-ordering (CO) temperature (formula presented) all the samples exhibit a coexistence of the FM metallic and CO (antiferromagnetic) phases. In the whole T range the FM phase fraction (X) decreases with increasing n. Furthermore, we show that only for (formula presented) the metallic fraction is above the critical percolation threshold (formula presented) As a consequence, these samples show very different magnetoresistance properties. In addition, for (formula presented) we observe a percolative metal-insulator transition at (formula presented) and for (formula presented) the insulating-like behavior generated by the enlargement of X with increasing T is well described by the percolation law (formula presented) where t is a critical exponent. On the basis of the values obtained for this exponent we discuss different possible percolation mechanisms, and suggest that a more deep understanding of geometric and dimensionality effects is needed in phase separated manganites. We present a complete T vs n phase diagram showing the magnetic and electric properties of the studied compound around half doping.
|Número de páginas
|Physical Review B - Condensed Matter and Materials Physics
|Publicada - 2002
|Publicado de forma externa