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

The study of Mn(IV)-rich manganites Ca_1-xSmₓMnO₃ 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 TC, the smallest room-temperature ρ=10^-3Ωcm being reached for $x=0.12.$ However increasing hopping energy with x suggests the creation of small polarons as eg 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 ρ(T) and $S(T)$ curves exhibit also a transition at a fixed temperature Tₚ~110K for $0.075<~x<~0.12.$ Tₚ is related to the appearance of a ferromagnetic component as shown from T-dependent magnetization. Nevertheless, the ac-{χ} measurements reveal a complex behavior. CaMnO₃ exhibits a weak ferromagnetic component (TC=122K) whereas for Ca_1-xSmₓMnO₃ $(0<x<~0.12)$ the χ^'(T) curves are characterized by large bumps at Tₘ with Tₘ<TC. Moreover, the shapes of the latter are found to be hac dependent and Tₘ is also frequency dependent as shown from χ^''(T) curves. The magnetization measurements indicate a cluster glass state below TC and demonstrate the lack of long-range ferromagnetism in these electron-doped manganites.