Far-infrared and dc magnetotransport of CaMnO${}_3$-CaRuO${}_3$ superlattices

We report temperature- and magnetic-field-dependent measurements of the dc resistivity and the far-infrared reflectivity (FIR) (photon energies $\hslash \omega =50–700$ cm${}^{-1}$) 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 Néel temperature of CaMnO${}_3$, the dc resistivity exhibits a logarithmic divergence upon cooling, which is associated with a large negative, isotropic magnetoresistance. The $\omega \to 0$ 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.

Figure 1

Schematic picture of a CaMnO${}_3$-CaRuO${}_3$ superlattice. The sketch on the left displays the nominal electron configuration of the constituents: CaMnO${}_3$ in the high-spin state with $S=3/2$, and CaRuO${}_3$ in low-spin state with $S=1$. Leakage of itinerant electrons from CaRuO${}_3$ into CaMnO${}_3$ induces canting of antiferromagnetically correlated Mn spins close to the interfaces.

Figure 2

Resistivity versus temperature of a CaRuO${}_3$ film with thickness $~$$100\hspace{0.5em}\AA$. The line shows the result of a power-law fit to the high-temperature range.

Figure 3

Arrhenius plot of the temperature-dependent resistivity of a $~$$100\hspace{0.5em}\AA$ thin film of CaMnO${}_3$. The lines show the results of fits to exponentials with activation energies listed in the legend. The inset shows the data in log $\rho$-versus-$T$ plot.

Figure 4

Resistivity versus temperature for CaRuO${}_3$-CaMnO${}_3$ superlattices with $N=4$ (upper panel) and $N=10$ (lower panel) consecutive CaRuO${}_3$ unit cells for $H=0$ and 8 T (orange and red, green and blue circles, respectively). Note the logarithmic temperature scale. The straight solid lines (guide to the eyes) indicate a leading logarithmic term in the temperature dependence of resistivity below $T~$ 90 K.

Figure 5

dc-Magnetoresistance defined as $[\rho (H)-\rho (0)]/\rho (0)$ versus magnetic field $H$ of both superlattices at temperature $T=6$ K.

Figure 6

Room-temperature reflectivity spectra of a (001) LaAlO${}_3$ substrate (marked as LAO), a CaRuO${}_3$ film (marked as CRO), and the $N=4$ and $N=10$ superlattices. The inset shows the experimental geometry.

Figure 7

Ratio of reflectivity spectra taken for magnetic fields $H=8$ and 0 T for $N=4$ (upper panel) and $N=10$ (lower panel) superlattices. The solid lines are the results of model calculations described in the text.

Figure 8

Temperature-dependent resistivity extracted from the extrapolation of the optical response in magnetic fields $H=0$ and 8 T for $N=4$ (upper panel) and $N=10$ superlattices (lower panel). The shaded line marks the Néel temperature.

Figure 9

Magnetoresistance defined as $[{\rho }_{\omega \to 0}(H)-{\rho }_{\omega \to 0}(0)]/{\rho }_{\omega \to 0}(0)$ extracted from the extrapolation of the FIR optical response to zero frequency, for both superlattices at temperature $T=3$ K.