Exchange mechanism in europium compounds

The magnetic interaction in EuO, EuS, and EuSe is characterized by a ferromagnetic nearestneighbor (NN) interaction and an antiferromagnetic next-nearest-neighbor (NNN) interaction. While the ferromagnetic interaction decreases in strength across the series, the antiferromagnetic interaction increases in magnitude. We show that the basic mechanism for this interaction is the indirect exchange mediated by electrons occupying the anion valence bands. The band structure for these materials is first calculated by the linear combination of atomic orbitals method and then used in a perturbative calculation of the indirect exchange interaction between europium ions. The calculated results agree with the experimental values of the NN and the NNN interaction both in sign and in order of magnitude for all three compounds considered.