Electronic structure and magnetism of europium chalcogenides in comparison with gadolinium nitride

Electronic structure calculations were carried out for the europium chalcogenides (EuO, EuS, EuSe, EuTe) using the 'LSDA + U' approach, in which orbital-dependent Coulomb and exchange effects are added to the local spin density approximation (LSDA) for the f electrons. The usual LSDA gap underestimate is also corrected by adding U-d terms, which shift up the empty d states. While both GdN and EuO are found to be magnetic semiconductors, there are significant differences in the electronic structure, in particular in the location of the filled f bands. The origin of these differences and their effect on various other aspects of the band structure are discussed. The exchange coupling between magnetic moments is studied by mapping the energy differences of different magnetic configurations to a Heisenberg Hamiltonian with first- and second-nearest-neighbour interactions. The exchange interactions are in fair agreement with experimental values and explain the trends in magnetic properties.