Exchange coupling in Eu monochalcogenides from first principles

Using a density functional method with explicit account for strong Coulomb repulsion within the 4f shell, we calculate effective exchange parameters and the corresponding ordering temperatures of the (ferro)magnetic insulating Eu monochalcogenides (EuX; X = O, S, Se, Te) at ambient and elevated pressure conditions. Our results provide quantitative account of the many-fold increase of the Curie temperatures with applied pressure and reproduce well the decrease of ferromagnetic coupling across the EuO-EuTe series. The first J(1) and second J(2) neighbor effective exchange are found to follow different pressure dependencies. Finally, our calculations show explicitly that the mixing of Eu 4f orbitals with the ligand states is necessary for the ferromagnetic ordering to take place at any pressure.