Electronic transport in ferroelectric-ferromagnetic compositesLa5∕8(Ba,Ca)3∕8MnO3:LuMnO3

Abstract

An almost complete immiscibility between metallic ferromagnet ${\mathrm{La}}_{5∕8}{\mathrm{Ba}}_{3∕8}{\mathrm{MnO}}_{3}$ or ${\mathrm{La}}_{5∕8}{\mathrm{Ca}}_{3∕8}{\mathrm{MnO}}_{3}$ and insulating ferroelectric ${\mathrm{LuMnO}}_{3}$ has been established from structural, magnetic, and transport studies. Both $(x)$${\mathrm{La}}_{5∕8}{\mathrm{Ba}}_{3∕8}{\mathrm{MnO}}_{3}:(1\ensuremath{-}x){\mathrm{LuMnO}}_{3}$ and $(x)$${\mathrm{La}}_{5∕8}{\mathrm{Ca}}_{3∕8}{\mathrm{MnO}}_{3}:(1\ensuremath{-}x){\mathrm{LuMnO}}_{3}$ show a metal-insulator transition below a critical volume fraction ${x}_{vc}$ of the metallic component. Over the entire range of $x>{x}_{vc}$, electronic conduction follows a classical percolation model. The conductivity scaling exponent $t$ is the same as that of the universal value $(=2)$ for the three-dimensional (3D) system; ${x}_{vc}$ is also close to the theoretical prediction for the 3D continuum model. For $x<{x}_{vc}$, the transport phenomenon is dominated by the insulating ${\mathrm{LuMnO}}_{3}$. The temperature dependence of both resistivity and thermopower for $0\ensuremath{\le}x<{x}_{vc}$ shows that the conduction is due to the thermal activation of charge carriers with a band gap $\ensuremath{\sim}0.5\phantom{\rule{0.3em}{0ex}}\mathrm{eV}$.