Multiple-structure optical spectral weight transfer in ferromagnetic EuB₆

We have obtained the optical conductivity of EuBe over a wide range from 2 meV to 5.5 eV from reflectivity and ellipsometry measurements. At the ferromagnetic transition (T_c = 15.5 K) (1) σ1 (w) was found to decrease at three different energies w = 0.5, 1.4 and 2.75 eV and (2) the lost spectral weight was found to be recovered in the Drude conductivity. We show that these unusual multiple-structure optical conductivity changes can be explained by using the spin-dependent splitting of the electronic bands at the ferromagnetic phase.