Percolation and the Colossal Magnetoresistance of Eu-Based Hexaboride

Upon substituting Ca for Eu in the local-moment ferromagnet EuB₆, the Curie temperature TC decreases substantially with increasing dilution of the magnetic sublattice and is completely suppressed for x≤0.3. The Ca substitution leads to significant changes of the electronic properties across the EuₓCa_1-xB₆ series. Electron microscopy data for x≈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 xₚ is approximately 0.3. For x≈0.27, we observe colossal negative magnetoresistance effects at low temperatures, similar in magnitude as those reported for manganese oxides.