Upon substituting Ca for Eu in the local-moment ferromagnet ${\mathrm{E}\mathrm{u}\mathrm{B}}_6$, the Curie temperature $T_C$ decreases substantially with increasing dilution of the magnetic sublattice and is completely suppressed for $x\le 0.3$. The Ca substitution leads to significant changes of the electronic properties across the ${\mathrm{E}\mathrm{u}}_x{\mathrm{C}\mathrm{a}}_{1-x}{\mathrm{B}}_6$ series. Electron microscopy data for $x\approx 0.27$ indicate a phase separation into Eu- and Ca-rich clusters of 5 to 10 nm diameter, leading to percolation-type phenomena in the electrical transport properties. The related critical concentration $x_p$ is approximately 0.3. For $x\approx 0.27$, we observe colossal negative magnetoresistance effects at low temperatures, similar in magnitude as those reported for manganese oxides.

• Received 31 March 2004

DOI:https://doi.org/10.1103/PhysRevLett.93.147203