Magnetic Properties of Rare-Earth Compounds

The magnetic properties of binary rare-earth compounds are reviewed. The influence of crystal field and exchange interaction on the magnetic properties is shown to vary systematically in the group of LnX compounds Ln=Lanthanide atom, X=N, P, As, Sb. Crystal-field parameters are calculated. The europium-chalcogenides are of particular interest. From measurements of the magnetic susceptibility at normal and under high pressure it is concluded that EuSe is antiferromagnetic above 3°K approximately with a Néel temperature of 4.6°K, but it assumes a ferro- or ferrimagnetic structure at lower temperatures. The optical absorption of EuO, EuS, EuSe, and EuTe was measured as a function of temperature. Energy gaps and their variation with temperature were determined. A giant decrease of the gaps was observed below the Curie and Néel temperatures, respectively. The change of the gaps is shown to be proportional to the magnetization of the specimen. This effect yields a new method to measure the spontaneous magnetization without a magnetic field. A long tail of the spontaneous magnetization above the Curie temperature is found.