Abstract
The low-field (LFMR) and high field (HFMR) magnetoresistance response of ceramic ${\mathrm{La}}_{2/3}{\mathrm{Sr}}_{1/3}{\mathrm{MnO}}_{3}$ (LSMO) samples having grain sizes diameters \ensuremath{\emptyset} ranging from 10 \ensuremath{\mu}m to 20 nm have been investigated. A limiting LFMR of about \ensuremath{\approx}30% is obtained for the \ensuremath{\emptyset} \ensuremath{\approx}0.5 \ensuremath{\mu}m but no larger values are obtained by further reduction of the grain size down to the nanometric range. On the contrary, the HFMR progressively rises when reducing the grain size. Magnetic and transport measurements suggest that HFMR originates from the existence of a noncollinear surface layer, having a thickness t that increases when reducing \ensuremath{\emptyset}. We have disclosed a clear relationship between the thickness, the intergranular resistance, and the height of the surface energy barrier. In addition, we show that in samples with submicronic grains, there is an intergranular Coulomb gap.
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