EuS above T c: Spin wave excitations in magnetic fields

Inelastic neutron scattering is used to study the magnetic field and temperature dependence of the induced spin wave excitations in a Heisenberg ferromagnet in the paramagnetic phase. The measurements were performed on a ¹⁵³EuS single crystal in fields up to 50 kOe and for temperatures up to 30 K (1.9 T_c). The spin wave stiffness D depends upon magnetic field and temperature. Using a simple semiclassical description based on the magnetization change we explain the linear change of D( H, T) with field and the decrease of D( H, T) with increasing temperature.