Spin dynamics in the cubic Heisenberg ferromagnet EuS

Abstract

Using the inelastic-neutron-scattering technique, we have undertaken an extensive study of the spin dynamics in EuS. The experiments were performed on a $^{153}\mathrm{Eu}$S single crystal with the scattering vector $\stackrel{\ensuremath{\rightarrow}}{\mathrm{q}}$ along the [100] direction. The $q$ values extended over the range between 0.11 and 1.06 ${\mathrm{\AA{}}}^{\ensuremath{-}1}$, and the temperatures ranged from $1.2 \mathrm{to} 50 \mathrm{K}(=3{T}_{c})$. It was found that spin-wave renormalization theory holds well up to $0.8{T}_{c}$. For temperatures above ${T}_{c}$ and up to the highest temperatures of our experiments, the line shapes are well described by the recently developed correlation theories. From the $q$ dependence of the linewidth of the dynamical response at $T={T}_{c}$, a dynamical scaling exponent $z=2.09(6)$ was deduced. The deviation from the pure Heisenberg value of $z=\frac{5}{2}$ is attributed to the dipolar interactions.