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, E
1
and E
2
, which change with the magnetic order, indicating exchange splittings of the 5d states. Antiferromagnetic EuTe shows a different structure in the E
1
and E
2
peaks. 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.