Optical Reflectance Study of Magnetic Ordering Effects in EuO, EuS, EuSe and EuTe

The reflectivity of the ferromagnetic semiconductors EuO, EuS and EuSe has been measured as a function of temperature and light polarization in an orienting magnetic field. In the energy range from just above the absorption edges to about 5.0 eV, there are two prominent features, E1and E2, which change with the magnetic order, indicating exchange splittings of the 5d states. Antiferromagnetic EuTe shows a different structure in the E1and E2peaks. However, with a large magnetic field applied (>40 kOe) the spectrum becomes characteristic of the other ferromagnetic europium chalcogenides suggesting a phase transition to parallel spin alignment at H ≈ 80 kOe. The effect provides direct experimental confirmation of the band theory prediction that there are superlattice splittings in the band structure of an antiferromagnetic crystal.