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Glassy dielectric anomaly and negative magneto-capacitance effect in electron-doped Ca$_{1-x}$Sr$_x$Mn$_{0.85}$Sb$_{0.15}$O$_3$

Manganites exhibit various types of electronic phenomena, and these electronic characteristics can be controlled by carrier doping. Herein, we report the dielectric and magnetic properties of electron-doped manganite Ca$_{1-x}$Sr$_x$Mn$_{0.85}$Sb$_{0.15}$O$_3$ ($x$ = 0, 0.1, 0.2, and 0.3). The temperature dependence of the real part of the dielectric constant exhibits a broad and large peak just below the kink temperature of magnetization and a sharp decrease at lower temperatures, accompanied by an anomaly of the imaginary part. Furthermore, isovalent Sr substitution enhances the temperature of the dielectric peak by more than 50 K. Interestingly, the dielectric peak exhibits a negative magnetic-field effect. For all measured samples, the low-temperature variation of the dielectric constant can be qualitatively explained based on the Maxwell-Wagner (MW) model that describes a system composed of grain boundaries and semiconducting grains. However, the observed peak and its negative magneto-capacitance effect at high temperatures cannot be reproduced by a combination of the MW model and magnetoresistance effect. The dielectric peak strongly indicates polaronic relaxation in the present system. These results suggest that polarons form clusters with a dipole ordering and magneto-electric coupling, which might be consistently understood by the charge-ordering scenario.