On the theory of the direct excited exchange in Eu-chalcogenides

A mechanism of direct excited exchange between the nearest Eu²⁺ ions in Eu-chalcogenides caused by simultaneous electron 4f⁷ → 4f⁶5d¹ transitions on both ions is proposed. The wavefunctions are considered, the energy spectrum is found, and the form of effective spin-Hamiltonian is determined for the Eu²⁺ ? Eu²⁺ “molecule” in crystal field of D_2h symmetry. The change of effective exchange integral between nearest Eu²⁺ ions in a series of Eu-chalcogenides is discussed briefly.