Structure and transport properties of La1−xSrxMnO3 granular ceramics

Two granular ceramics were prepared by spark plasma sintering (SPS) at 600-800 °C and classical ceramic sintering (CCS) at 900 °C using molten salt synthesized nanoparticles of the composition La0.53Sr0.47MnO3 and 40 nm size. Extensive study of the structural, magnetic, and electric transport properties showed that the SPS and CCS products essentially retain the two-phase magnetic structure of the starting nanoparticles, which consist of a ferromagnetic (FM) core and an A-type antiferromagnetic (AFM) shell. After the sintering, the AFM phase forms a 10-15 nm thick spacer between neighbouring FM granules, which represents a barrier for the transmission of spin-polarized eg carriers. This assembly retains reasonable conductivity down to the lowest temperatures, without marked localization, and it still gives rise to a large negative magnetoresistance, which is treated theoretically in terms of low- and high-field positive magnetoconductance. In a detailed analysis of these low-field magnetoconductance (LFMC) and high-field magnetoconductance (HFMC) effects, which are related to the field-induced alignment of the FM granules and spin canting in the AFM matrix, respectively, we conclude that the bulk conductivity is governed by resonant tunnelling, i.e. the second-order transmission via Mn4+ sites in the intergranular space. The experimental data on the SPS product confirm the theoretically predicted scaling of the LFMC effect with squared reduced magnetization, and also provide also a quantitative comparison between the linear coefficient of the HFMC and the high-field paraprocess seen in the magnetization measurement.