Far-infrared and dc magnetotransport of CaMnO₃-CaRuO₃ superlattices

We report temperature- and magnetic-field-dependent measurements of the dc resistivity and the far-infrared reflectivity (FIR) (photon energies ω=50-700 cm^-1) of superlattices comprising ten consecutive unit cells of the antiferromagnetic insulator CaMnO₃, and four to ten unit cells of the correlated paramagnetic metal CaRuO₃. Below the Néel temperature of CaMnO₃, the dc resistivity exhibits a logarithmic divergence upon cooling, which is associated with a large negative, isotropic magnetoresistance. The ω→0 extrapolation of the resistivity extracted from the FIR reflectivity, on the other hand, shows a much weaker temperature and field dependence. We attribute this behavior to scattering of itinerant charge carriers in CaRuO₃ from sparse, spatially isolated magnetic defects at the CaMnO₃-CaRuO₃ interfaces. This field-tunable "transport bottleneck" effect may prove useful for functional metal-oxide devices.