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

The study of Mn(IV)-rich manganites ${\mathrm{Ca}}_{1-x}{\mathrm{Sm}}_x{\mathrm{MnO}}_3$ with low electron content corresponding to $0<~x<~0.12$ 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 (ρ) is observed above $T_C,$ the smallest room-temperature $\rho {=10\mathrm{}}^{-3}\Omega \mathrm{cm}$ being reached for $x=\mathrm{0.12.}\mathrm{}$ 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 $\mathrm{}\left(S\right)\mathrm{}$ measurements confirm the increase of carriers with $x$ and can be described within a single-band metal model. The $\rho \mathrm{}\left(T\right)\mathrm{}$ and $S\left(T\right)\mathrm{}$ curves exhibit also a transition at a fixed temperature $T_p\sim 110\mathrm{K}$ for $0.075<~x<~\mathrm{0.12.}$ $T_p$ is related to the appearance of a ferromagnetic component as shown from $T$-dependent magnetization. Nevertheless, the ac-χ measurements reveal a complex behavior. ${\mathrm{CaMnO}}_3$ exhibits a weak ferromagnetic component ${(T}_C=122\mathrm{}\mathrm{K})$ whereas for ${\mathrm{Ca}}_{1-x}{\mathrm{Sm}}_x{\mathrm{MnO}}_3$ $(0<x<~0.12)$ the ${\chi }^{\prime }\mathrm{}\left(T\right)\mathrm{}$ curves are characterized by large bumps at $T_m$ with $T_m<T_C.$ Moreover, the shapes of the latter are found to be $h_{\mathrm{ac}}$ dependent and $T_m$ is also frequency dependent as shown from ${\chi }^{″}\mathrm{}\left(T\right)\mathrm{}$ 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.