Comparison of structural, magnetic and electrical transport behavior in bulk and nanocrystalline Nd-lacunar Nd0.67Sr0.33MnO3 manganites

Bulk and nanocrystalline Nd-lacunar Nd0.67Sr0.33MnO3 manganites were synthesized, and the structural, magnetic and electrical properties have been systematically studied. The Rietveld refinement confirms perovskite structure with orthorhombic crystal symmetry (Pbnm space group). The bulk compound shows a sharp second order transition at 275 K while the nanocrystalline compound exhibit a broadened transition with a TC of 242 K. Temperature dependence of electrical resistivity of both the compounds witness double peaks; a sharp peak near TC and a broad peak below TC, which is more prominent in the nanocrystalline compound whereas former peak is more prominent in the bulk compound. The combined effect of Kondo-like spin-dependent scattering and electron-electron interactions dominates the transport properties at low temperature in the nanocrystalline compound and results in upturn enhancement in resistivity, while the high-temperature insulating behavior of both compounds can be explained by the adiabatic small polaron hopping mechanism. The effective Mn ion valence of bulk and nanocrystalline compounds are found to be 3.56 and 3.25 and the observed crystal field splitting energy is slightly higher for the nanocrystalline compound. X-ray absorption spectra result quantitatively demonstrates that the broken symmetry at the surface of nanocrystalline compound aid to stabilize more Mn3+ ions that lead to interesting magnetic and electrical transport behavior.