Far-infrared and dc magnetotransport of CaMnO3-CaRuO3superlattices

Abstract

We report temperature- and magnetic-field-dependent measurements of the dc resistivity and the far-infrared reflectivity (FIR) (photon energies $\ensuremath{\hbar}\ensuremath{\omega}=50--700$ cm${}^{\ensuremath{-}1}$) of superlattices comprising ten consecutive unit cells of the antiferromagnetic insulator CaMnO${}_{3}$, and four to ten unit cells of the correlated paramagnetic metal CaRuO${}_{3}$. Below the N\'eel temperature of CaMnO${}_{3}$, the dc resistivity exhibits a logarithmic divergence upon cooling, which is associated with a large negative, isotropic magnetoresistance. The $\ensuremath{\omega}\ensuremath{\rightarrow}0$ 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 CaRuO${}_{3}$ from sparse, spatially isolated magnetic defects at the CaMnO${}_{3}$-CaRuO${}_{3}$ interfaces. This field-tunable ``transport bottleneck'' effect may prove useful for functional metal-oxide devices.