Structure and transport properties of La1−xSrxMnO3 granular ceramics

Abstract

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.