Cation disorder and size effects in magnetoresistive manganese oxide perovskites

Abstract

Large disorder effects due to size differences between $A$-site ${R}^{3+}$ ($R=\mathrm{L}\mathrm{a},\phantom{\rule{0ex}{0ex}}\mathrm{P}\mathrm{r},\phantom{\rule{0ex}{0ex}}\mathrm{N}\mathrm{d},\phantom{\rule{0ex}{0ex}}\mathrm{S}\mathrm{m}$) and ${M}^{2+} (M=\mathrm{C}\mathrm{a},\phantom{\rule{0ex}{0ex}}\mathrm{S}\mathrm{r},\phantom{\rule{0ex}{0ex}}\mathrm{B}\mathrm{a})$ cations have been found in magnetoresistive $({R}_{0.7}{M}_{0.3})\mathrm{Mn}{\mathrm{O}}_{3}$ perovskites. The ferromagnetic-metal-paramagnetic-insulator transition temperature ${T}_{m}$ varies as ${T}_{m}={T}_{m}(0)\ensuremath{-}p{Q}^{2}$ due to strain fields resulting from ordered or disordered oxygen displacements $Q$ that are parametrized by the statistical mean and variance of the $A$ cation radius, respectively. The value of $p$ is related to the Mn-O force constant showing that ${\mathrm{Mn}}^{3+}$ Jahn-Teller distortions assist electron localization at ${T}_{m}$. The maximum possible ${T}_{m}$ is estimated to be \ensuremath{\sim}530 K although experimentally observable values are \ensuremath{\le}360 K. A large suppression of magnetoresistance due to cation disorder is also evidenced.