Neutron scattering study of spin waves and exchange interactions in ferromagnetic EuS

From inelastic neutron scattering the strongly anisotropic spin-wave dispersion curves of a ¹⁵³EuS single crystalline sample have been measured at 1.3 K for the main symmetry directions $〈100〉$, $〈110〉$, and $〈111〉$ over the entire first Brillouin zone. Least-squares fits to the Holstein-Primakoff spin-wave theory yield the isotropic exchange energy constants, Jᵢ, up to fifth nearest neighbors ($i=5$) as follows: J₁kB=0.221±0.003 K, J₂kB=-0.100±0.004 K, J₃kB=0.006±0.002 K, J₄kB=-0.007±0.002 K, J₅kB=-0.004±0.002 K which correspond to a paramagnetic Curie temperature of ΘP=21.1±0.2 K. The results confirm previous assumptions that the range of the Jᵢ is essentially limited to next nearest neighbors, and thus agree fairly well with most of the previous experimental results on J₁ and J₂. The temperature dependence of the Jᵢ disagrees with the predictions of the spin-wave renormalization theory.