Ferromagnetic Europium Compounds

Abstract

Ferromagnetism has been found in several divalent europium compounds. Some of these materials are particularly simple in both crystal and magnetic structure and are ideal for experimental and theoretical study. The materials we have investigated can be divided into three groups, europium chalcogenides, europium halogens, and europium silicates. Magnetic information on these compounds are summarized and theoretical results reviewed. The europium chalcogenide series all have the rocksalt structure, the oxide, sulfide, and selenide being ferromagnetic, and it is on these materials that the most work has been done. This is especially true of EuS where detailed moment, specific heat, and nuclear magnetic resonance measurements are available. The oretical analysis indicates that there is positive exchange between nearest‐neighbor europium atoms which decreases with increasing atom spacings. There is also a weaker negative superexchange via the anions in these compounds which is responsible for antiferromagnetism in EuTe. The EuTe magnetic ordering has been studied by neutron diffraction at Brookhaven and the sublattice magnetization‐temperature behavior is found to be somewhat anomalous. EuSe is ferromagnetic but reveals low‐field magnetization curves that have an unusual shape. In view of this, EuS seems to be the simplest of the group so that fundamental parameters obtained from analyzing the data are probably the most reliable. In the halogen group, EuI₂ appears to be ferromagnetic while EuF₂ is apparently antiferromagnetic. The latter behavior is somewhat unexpected from interatomic distance considerations which hold for the chalcogenides. Of three silicates prepared, two were ferromagnetic. The most interesting was Eu₂SiO₄ which was obtained as transparent single crystals. Magneto‐optical studies show this compound to have a Verdet constant higher than any other previously reported.