Magnetoconductivity of the La_1–xSr_xMnO₃@TiO₂ Nanocomposite

This paper deals with tunneling magnetoresistance in a composite system of the so-called 0-3 connectivity, in which ferromagnetic (FM) metallic nanoparticles La _1-x SrxMnO ₃ (0-D objects) are embedded in the (3-D) insulating matrix of TiO ₂ . The sample fabrication included the sol-gel preparation of manganite particles of the x = 0.35 composition and 25 nm mean crystallite size, their coating by TiO ₂ , and compacting the products by spark plasma sintering (SPS). A comparative nanogranular sample was prepared by SPS of bare manganite particles. The resistivities of the composite and comparative samples are 100 000 and 100 times higher compared to those of bulk metallic La _1-x SrxMnO ₃ . Otherwise, the temperature dependence observed in the nanogranular La _1-x SrxMnO ₃ sample is similar to single crystal data, and marked localization is absent also in the La _1-x SrxMnO ₃ @TiO ₂ nanocomposite. The data taken in applied fields up to 4 T reveal effects typical for grain-boundary tunneling in manganites, namely, the coexistence of the low-field magnetoconductance (LFMC), reflecting the field-induced alignment of FM cores, and high-field linear magnetoconductance (HFMC) that is generally ascribed to the effect of spin canting at localized Mn ⁴⁺ sites in the interface. This is considered as a signature for resonant tunneling of spin-polarized carriers, theoretically treated by Lee et al [1]. The present results show that the total extent of LFMC makes 45% in the La _1-x SrxMnO ₃ @TiO ₂ nanocomposite and 21% in the La _1-x SrxMnO ₃ nanogranular sample. The slope of HFMC has been determined to 5.4% and 4.9% per Tesla, respectively. The large LFMC effect observed in the nanocomposite exceeds the theoretical prediction of 33% for the second-order tunneling, which might suggest for higher order tunneling via resonant states.