Neutron scattering from the Heisenberg ferromagnets EuO and EuS. I. The exchange interactions

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 -2Jₙₘ→Sₙ·→Sₘ, we obtained for EuO, J₁kB=0.606±0.008 ^∘{}K and J₂kB=0.119±0.015 ^∘{}K; for EuS, J₁kB=0.236±0.009 ^∘{}K and J₂kB=-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.