Ferromagnetism and infrared conductivity of the homogeneous hexaboride alloy Eu_1−xCa_xB₆

We have studied the magnetic and optical properties of hexaboride Eu1-xCaxB6 for x = 0, 0.13, 0.25, 0.35, 0.54 and 1.0 from measurements of dc magnetization and wide- range absolute reflectivity. The ferromagnetic transition temperature T-c decreases with Ca- doping and is completely suppressed when x exceeds 0.35. The Drude plasma frequency omega(2)(P) also decreases with x and disappears at the same composition. We have analysed this correlation between T-c and omega(2)(P) in terms of the RKKY theory. We have also simulated optical spectra of Eu1-xCaxB6 using the effective medium theory and compare them with the measured data. The results show that Eu1-xCaxB6 is not a composite of EuB6 and CaB6 but forms a homogeneous magnetic alloy.