Anomalous Transport in Ca$_{1-y}$Sr$_y$MnO$_3$ ($0\leq y\leq 0.75$)~$^{\ast}$

Electron-doped manganites such as Ca$_{1-x}$La$_x$MnO$_3$ have attracted considerable interest in recent years due to their inhomogeneous magnetic ground state, consisting of nanoscale ferromagnetic (FM) droplets and/or spin canted clusters within a G-type antiferromagnetic matrix.$^1$ The nominally undoped (Mn$^{4+}$) Ca$_{1-y}$Sr$_y$MnO$_3$ compounds [for which $T_N$ increases from 125 K (y=0) to 200 K (y=0.75)] have a small electron concentration, $n\sim 10^{18} {\rm cm}^{-3}$, associated with native defects. We report transport measurements on these materials that reveal an unusual transition at a characteristic temperature, $T^{\ast}\sim (0.5-0.8)T_N$. For $T