We study the nonlinear current-voltage (I-V) characteristics and analyze the voltage-dependent tunneling conductance in nanoparticles of La(2/3)A(1/3)MnO(3) (A = Ca, Sr). The powders were prepared by different wet-chemical routes and low calcination temperatures were used to obtain an average particle size D approximate to 30 nm. The data are comprehensively explained in terms of the tunneling picture, which allows one to estimate the height of the grain boundary insulating barrier (phi) for each sample. For constant D, our results show that the sample preparation route is mainly responsible for the value of phi in nanoparticles, while the Coulomb gap in the Coulomb blockade regime is similar to3 times higher for Sr- than for Ca-doping. We also show that a small fraction of the barriers contribute to the nonlinear transport, and the current is mainly carried through low-resistive percolated paths. In addition, despite the different barrier strengths, the low-field magnetoresistance (LFMR) is similar for all samples, implying that phi is not the fundamental parameter determining the LFMR. (C) 2003 American Institute of Physics. References: 37
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