Effect of the Anisotropic Exchange and the Crystalline Field on the Magnetic Susceptibility of Eu2O3

This paper invokes the combined action of the crystalline field and exchange in an attempt to explain theoretically why the susceptibility χ of cubic Eu2O3 is found by experimenters to be 30% higher than that for the free Eu3+ ion at temperatures below 50°K. The crystalline field is known approximately from optical studies, whereas the coefficient in the exchange coupling is treated as an empirical parameter. This parameter cannot be deduced from a comparison with Gd2O3 as we have shown elsewhere that there are contributions from anisotropic exchange which do not enter in the coupling of S-state ions such as Gd3+, but which do not necessarily vanish for Eu3+–Eu3+ interaction. The crystalline field is found to account for only 42% of the observed enhancement of χ, whereas the isotropic exchange of the type present in Gd2O3 in fact reduces χ by a small amount, thus demonstrating the importance of the anisotropic exchange. The latter needs to be opposite in sign and five times larger than the conventional isotropic exchange in order to explain the observed χ at T = 0°K. Then the temperature dependence of χ calculated with the molecular field approximation agrees with experiment very well.