Inelastic-neutron-scattering methods have been used to measure the spin-wave spectrum in EuO and EuS over the entire Brillouin zone. The samples were polycrystalline powders enriched in 153Eu. Defining the interaction between pairs of spins to be -2JnmS⃗n·S⃗m, we obtained for EuO, J1/kB=0.606±0.008 °K and J2/kB=0.119±0.015 °K; for EuS, J1/kB=0.236±0.009 °K and J2/kB=-0.118±0.011 °K. The measured Curie temperatures for EuO, 69.15 ± 0.05°K, and for EuS, 16.57 ± 0.01 °K, are in excellent agreement with calculated values obtained from series expansions relating Tc to the exchange constants. In EuO, the neutron scattering measurements determine J1 and J2 separately. Although separate values for the exchange constants were reported in earlier experiments, we show that in fact only the spin-wave stiffness constant of EuO, which is proportional to J1+J2, was actually determined. In EuS, our values of J1 and J2 agree with a reanalysis of the specific-heat data.
Passell, L., Dietrich, O. W., & Als-Nielsen, J. A. (1976). Neutron Scattering from the Heisenberg Ferromagnets EuO and EuS. I. The Exchange Interactions. Physical Review B, 14(11), 4897-4907. https://doi.org/10.1103/PhysRevB.14.4897