Correlation between structural, magnetic and electrical transport properties of barium vacancies in the La 0.67 Ba 0.33− x □ x MnO 3 ( x =0, 0.05, and 0.1) manganite

We investigate the effects of barium vacancies on the physical properties of the La0.67Ba0.33-x square xMnO3 (x = 0, 0.05, and 0.1) manganite. The X-ray diffraction studies show that all samples crystallize in the rhombohedral structure within the space group R-3c (No. 167). Rietveld refinement shows that the barium vacancies modify the structural parameters such as the volume, the Mn-O bond length and the Mn-O-Mn angles. An increase of the Mn-O bond length with increasing barium deficiencies leads to the decrease of the one-electron bandwidth W, which results in a decrease of the Curie temperature (T-C) and of the metal-semiconductor transition temperature (TM-SC). Magnetic measurements showed a paramagnetic to ferromagnetic transition at T = T-C. The temperature dependence of the resistivity shows that all samples undergo a sharp metal-semiconductor transition at TM-SC accompanying the ferromagnetic-paramagnetic transition. Metallic resistivity rho(T) = rho 0 + rho 2T(2) + rho 4.5T(4.5) is observed below TM-SC. Above TM-SC the electrical conductivity is dominated by adiabatic small polaron hopping model (ASPH), giving the electrical resistivity in the paramagnetic region as rho = BT exp (E-a/k(B)T). Based on the idea that the doped manganites consist of ferromagnetic-metallic and paramagnetic-semiconducting regions coexisting in the same specimen, a good fit of rho(T) with a phenomenological percolation model may be obtained by combining the contributions of the resistivity above and below TM-SC by a single expression in the temperature region between 15 and 390 K. We found that the estimated results are in good agreement with the experimental data. Our results show that the transition to a metallic state occurs if the volume fraction of the ferromagnetic phase f reaches a percolation threshold. We also found that the volume fraction of the ferromagnetic phase f has the same physical meaning as the reduced magnetization M/M-S (M-S, saturation moment). (C) 2013 Elsevier B. V. All rights reserved.