The magneto- and electro-transport properties of two series of nanocrystalline (1 − x)La0.7Ca0.3MnO3 + xA (A: Al2O3 and Ag) composites have been systematically and thoroughly studied. The observed electronic transport behavior over the whole temperature range (5–300 K), especially the change in metal–insulator transition temperature with increasing Al2O3 and Ag content while the ferromagnetic–paramagnetic transition remained unaffected, was explained by applying a two-component phenomenological model. We have attributed the unusual low-temperature resistivity upturn of composites to a change in charging energy. Most interestingly, magneto-transport measurements showed that the low-field magnetoresistance (LFMR), as well as the high-field magnetoresistance (HFMR), displayed a Curie–Weiss-like law behavior. Basing on the spin-polarized transport of conduction electrons at the grain boundaries, we have analyzed our experimental data and found that the temperature dependence of low- and high-field magnetoresistance is controlled predominantly by the nature of the temperature response of surface magnetization of particles. The competition between grain-boundary pinning strength (k), magnetic field and thermal energy (kBT ) created the temperature sensitive behavior of magnetoresistance as well as that of surface spin susceptibility (χb).

Keywords: manganite composites, low-field magnetoresistance, polarized tunneling

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