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

Abstract

From inelastic neutron scattering the strongly anisotropic spin-wave dispersion curves of a $^{153}\mathrm{EuS}$ 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, ${J}_{i}$, up to fifth nearest neighbors ($i=5$) as follows: $\frac{{J}_{1}}{{k}_{B}}=0.221\ifmmode\pm\else\textpm\fi{}0.003$ K, $\frac{{J}_{2}}{{k}_{B}}=\ensuremath{-}0.100\ifmmode\pm\else\textpm\fi{}0.004$ K, $\frac{{J}_{3}}{{k}_{B}}=0.006\ifmmode\pm\else\textpm\fi{}0.002$ K, $\frac{{J}_{4}}{{k}_{B}}=\ensuremath{-}0.007\ifmmode\pm\else\textpm\fi{}0.002$ K, $\frac{{J}_{5}}{{k}_{B}}=\ensuremath{-}0.004\ifmmode\pm\else\textpm\fi{}0.002$ K which correspond to a paramagnetic Curie temperature of ${\ensuremath{\Theta}}_{P}=21.1\ifmmode\pm\else\textpm\fi{}0.2$ K. The results confirm previous assumptions that the range of the ${J}_{i}$ is essentially limited to next nearest neighbors, and thus agree fairly well with most of the previous experimental results on ${J}_{1}$ and ${J}_{2}$. The temperature dependence of the ${J}_{i}$ disagrees with the predictions of the spin-wave renormalization theory.