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Hole-doping dependence of percolative phase separation in Pr<inf>0.5-δ</inf>Ca<inf>0.2+δ</inf>Sr<inf>0.3</inf> MnO<inf>3</inf> around half doping

Abstract:

We address the problem of the percolative phase separation in polycrystalline samples of Pr0.5-δCa0.2+δSr0.3 MnO3 for -0.04≤δ≤0.04 (hole doping n between 0.46 and 0.54). 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 TC. However, for n>0.50 we found that in a wide temperature range below TC an important PM volume still survives, coexisting with the FM phase. On lowering T below the charge-ordering (CO) temperature TCO 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 n≤0.50 the metallic fraction is above the critical percolation threshold Xc≃15.5%. As a consequence, these samples show very different magnetoresistance properties. In addition, for n≤0.50 we observe a percolative metal-insulator transition at TMI, and for TMI<T<TCO the insulating-like behavior generated by the enlargement of X with increasing T is well described by the percolation law p-1=δ∼(X-XC)t, 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.