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 ≤0:3. The Ca substitution leads to significant changes of the electronic properties across the
Eu _x Ca_1-x B₆ 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 x _p 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.