Investigation of the Magnitude and Range of the Ruderman-Kittel Interaction in Sm${\mathrm{Rh}}_4$${\mathrm{B}}_4$ and Er${\mathrm{Rh}}_4$${\mathrm{B}}_4$

The superconductive and magnetic transition temperatures taken together are shown to provide a unique probe which separately determines both the magnitude and range of the Ruderman-Kittel interaction in the $R{\mathrm{Rh}}_4{\mathrm{B}}_4$ magnetic superconductors ($R=\mathrm{E}\mathrm{r},\mathrm{S}\mathrm{m}$). Experimentally, an unexpected peak is found in the antiferromagnetic-ordering temperature of Sm${\mathrm{Rh}}_4$ ${\mathrm{B}}_4$ versus electron mean free path, while for Er${\mathrm{Rh}}_4$ ${\mathrm{B}}_4$ the ferromagnetic-ordering temperature decreases monotonically. These qualitative features, as well as the quantitative differences between Sm${\mathrm{Rh}}_4$ ${\mathrm{B}}_4$ and Er${\mathrm{Rh}}_4$ ${\mathrm{B}}_4$, are in excellent agreement with calculations using a mean-free-path-dependent Ruderman-Kittel interaction.