Optical evidence for a spin-filter effect in the charge transport of Eu_0.6Ca_0.4B₆

We have measured the optical reflectivity R(ω) of Eu_0.6Ca_0.4B₆ as a function of temperature (T) between 1.5 and 300 K and in external magnetic fields (H) up to 7 T. R(ω) increases with decreasing T and increasing H field, but the plasma edge feature does not exhibit the sharp onset and steep slope that is observed in EuB₆. The analysis of the H-field dependence of the low-T optical conductivity confirms the previously observed exponential decrease of the electrical resistivity upon increasing bulk magnetization at constant T. The individual exponential magnetization dependences of the plasma frequency and scattering rate are also extracted from the optical data.