Magnetic Anisotropy of Rare Earth Sulphates and the Asymmetry of their Crystalline Fields

Abstract

IN an important paper in the Physical Review for 1932 Penney and Schlapp¹ have discussed theoretically the Stark-splitting of the energy levels of the rare earth ions in crystals under the influence of the crystalline electric fields, and its influence on the magnetic behaviour of the ions. Experimentally, Spedding and his co-workers² have studied the absorption spectra of rare earth salts of the type M₂(S0₄)₃.8H₂O, where M = Pr, Nd, Er at different temperatures, and thence deduced the low-lying energy levels of the M+ + + ions in the crystals. They find that (1) the number of low-lying energy levels and their relative separations are the same as predicted by the theory for a field of cubic symmetry acting on the M+ + + ions ; (2) the intensity of the cubic field required to produce the observed separations is the same in all the three crystals, as should be expected from their isomorphism ; (3) the observed separation of the levels is not inconsistent with the available magnetic data for the mean susceptibilities of the crystals. From these and other results, it has been concluded that the fields acting on the M+ + + ions in these crystals should be almost rigorously cubic in symmetry.