Transition from a paramagnetic metallic to a cluster glass metallic state in electron-doped perovskite manganites

Abstract

The study of Mn(IV)-rich manganites ${\mathrm{Ca}}_{1\ensuremath{-}x}{\mathrm{Sm}}_{x}{\mathrm{MnO}}_{3}$ with low electron content corresponding to $0l~xl~0.12$ demonstrates the large difference of their electronic and magnetic properties with that of Mn(III)-rich manganites. In particular, a metalliclike temperature dependence of the resistivity (\ensuremath{\rho}) is observed above ${T}_{C},$ the smallest room-temperature $\ensuremath{\rho}{=10}^{\ensuremath{-}3}\ensuremath{\Omega}\mathrm{cm}$ being reached for $x=0.12.$ However increasing hopping energy with $x$ suggests the creation of small polarons as ${e}_{g}$ electrons are injected into the Mn(IV) matrix. The thermopower $(S)$ measurements confirm the increase of carriers with $x$ and can be described within a single-band metal model. The $\ensuremath{\rho}(T)$ and $S(T)$ curves exhibit also a transition at a fixed temperature ${T}_{p}\ensuremath{\sim}110\mathrm{K}$ for $0.075l~xl~0.12.$ ${T}_{p}$ is related to the appearance of a ferromagnetic component as shown from $T$-dependent magnetization. Nevertheless, the ac-\ensuremath{\chi} measurements reveal a complex behavior. ${\mathrm{CaMnO}}_{3}$ exhibits a weak ferromagnetic component ${(T}_{C}=122\mathrm{K})$ whereas for ${\mathrm{Ca}}_{1\ensuremath{-}x}{\mathrm{Sm}}_{x}{\mathrm{MnO}}_{3}$ $(0lxl~0.12)$ the ${\ensuremath{\chi}}^{\ensuremath{'}}(T)$ curves are characterized by large bumps at ${T}_{m}$ with ${T}_{m}l{T}_{C}.$ Moreover, the shapes of the latter are found to be ${h}_{\mathrm{ac}}$ dependent and ${T}_{m}$ is also frequency dependent as shown from ${\ensuremath{\chi}}^{\ensuremath{''}}(T)$ curves. The magnetization measurements indicate a cluster glass state below ${T}_{C}$ and demonstrate the lack of long-range ferromagnetism in these electron-doped manganites.