Tunneling magnetoresistance of hydrothermally sintered La1-Sr MnO3-silica nanocomposites

The study deals with tunneling magnetoresistance in a series of composite systems of so-called 0–3 connectivity, in which highly conducting manganite nanoparticles of ferromagnetic groundstate (0-dim objects) are embedded in the (3-dim) insulating matrix of silica. The sample fabrication included the sol-gel preparation of La1–xSrxMnO3 particles of the x = 0.35 composition and 25 nm mean crystallite size, their coating by silica of controlled thicknesses in the 1–5 nm range, and subsequent consolidation by hydrothermal sintering at 300 °C. Temperature dependent electric resistivities of the materials were measured in magnetic fields of 0 and 40 kOe down to 5 K, and field-dependent isothermal curves were taken by sweeping magnetic field up to 140 kOe. The field-dependent effects were interpreted in terms of the low- and high-field magnetoconductance. It is important that extents of these phenomena are markedly independent of the silica amount, despite the five orders change in absolute resistivity values. Microscopic mechanisms of the intergrain transport, applicable to polycrystalline manganites, nanogranular compacts and nanocomposites in general, are proposed

Magnetoconductivity of the La 1–xSrxMnO3@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 La1–xSrxMnO3 (0-D objects) are embedded in the (3-D) insulating matrix of TiO2. 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 TiO2, 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 La1–xSrxMnO3. Otherwise, the temperature dependence observed in the nanogranular La1–xSrxMnO3 sample is similar to single crystal data, and marked localization is absent also in the La1–xSrxMnO3@TiO2 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 Mn4+ 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 La1–xSrxMnO3@TiO2 nanocomposite and 21% in the La1–xSrxMnO3 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

Design of 0–3 type nanocomposites using hydrothermal sintering

We report here the successful design of 0–3 type nanocomposites where 30 nm ferromagnetic metallically conducting cores of manganite La0.66Sr0.34MnO3 (LSMO) are discretely distributed in an insulating silica matrix. Starting from LSMO@SiO2 core@shell nanoparticles, hydrothermal sintering process was used as a low temperature densification route (300 °C, 350 MPa, 90 min) in presence of 0.2 M aqueous sodium hydroxide solution. This process based on a pressure solution creep in the contact zones between nanoparticles allows the design of complex microstructures preventing the grain growth of the cores and the formation of interphases between the cores and the matrix