Dipolar anisotropy of spinwaves in the isotropic ferromagnets EuS and Pd₂ MnSn

The spin excitations in the model Heisenberg ferromagnets EuS and Pd₂MnSn have been investigated below T_c using polarized-neutron scattering. In EuS the spin waves with transverse polarization with respect to q (δ S ⊥ q) renormalize near T_C as expected according to dynamical scaling theory, and the excitations have a gap with Δ {reversed tilde equals} 0.040 meV. In contrast, the longitudinal spin waves (δ Sq) are possibly damped. These observations are clearly related to the strong dipolar anisotropy that has already been observed above T_C. Similar investigations in Pd₂MnSn show, that the spin waves are independent of the polarization within the q-range investigated. This result confirms that the dipolar interactions in Pd₂MnSn are indeed small.