Enhancement of the magnetoresistance at the Curie temperature of the ferromagnetic insulator ${\mathrm{La}}_{1.5}{\mathrm{Sr}}_{0.5}{\mathrm{MnRhO}}_6$

We report a study of the magnetic and electrical properties of the ferromagnetic insulator ${\mathrm{La}}_{1.5}{\mathrm{Sr}}_{0.5}{\mathrm{RhMnO}}_6.$ A significant magnetoresistance is found in this system which is largest at the Curie temperature ${(T}_{\mathrm{C}})$ even though there is no metal-insulator transition. The electrical transport is found to be activated above $T_{\mathrm{C}}$ and described by a variable range hopping law below $T_{\mathrm{C}}.$ Above $T_{\mathrm{C}}$ the carriers are magnetic polarons with a size which increases as the temperature approaches the magnetic transition. Rh substitution preserves ferromagnetic ordering, in contrast with the effect of other dopants on the B sites, but modifies the electrostatic potential leading to carrier localization. We attribute the peak in the magnetoresistance at $T_{\mathrm{C}}$ to the field-induced suppression of critical spin fluctuations which modulate the energetic barriers seen by the carriers.