Mechanisms for the Anomalous Properties of Eu-Chalcogenides Alloys. I.

Various anomalous magneto-optic and magnetic properties of Eu 1- x Gd x Se are explained on the magnetic exciton and the magnetic impurity state model. It is shown that the optically active magnetic exciton with the lowest energy is of the type of (5 d –5 d ), that is, 5 d character on the central Eu 3+ and also 5 d on the nearest neighbour Eu 2+ . The origin of the anomalous magneto-optic properties is attributed to the d – f exchange interaction on the nearest neighbour Eu 2+ ions. While, for the magnetic impurity state, (6 s –6 s ) type state has the lowest energy. At low temperature, a giant spin molecule ( S =46) composed of the spin of an impurity electron and the 4 f spin on the central Gd 3+ ion and the n.n.Eu 2+ ion is constructed through the s – f exchage interaction among them. However, this giant spin molecule disappears rather quickly as temperature increases exceeding J 1/κ , where J 1 is the s – f exchange constant at n.n. Eu 2+ and is estimated as 25κ. The anomalous magnetic properties of the dilute alloy is deduced from this behavior of the magnetic impurity state.