Percolation and the colossal magnetoresistance of Eu-based hexaboride.

Upon substituting Ca for Eu in the local-moment ferromagnet EuB6, the Curie temperature T(C) decreases substantially with increasing dilution of the magnetic sublattice and is completely suppressed for x<or=0.3. The Ca substitution leads to significant changes of the electronic properties across the EuxCa1-xB6 series. Electron microscopy data for x approximately 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 approximately 0.27, we observe colossal negative magnetoresistance effects at low temperatures, similar in magnitude as those reported for manganese oxides.