DIPOLAR ANISOTROPY OF SPINWAVES IN THE ISOTROPIC FERROMAGNETS EUS ANDPD(2)NSN

The spin excitations in the model Heisenberg ferromagnets EuS and Pd2M nSn have been investigated below T-c using polarized-neutron scatterin g. In EuS the spin waves with transverse polarization with respect to q(delta S perpendicular to q) renormalize near T-c as expected accordi ng to dynamical scaling theory, and the excitations have a gap with De lta similar or equal to 0.040 meV. In contrast, the longitudinal spin waves (delta S/q) are possibly damped. These observations are clearly related to the strong dipolar anisotropy that has already been observe d above T-c. Similar investigations in Pd2MnSn show, that the spin wav es are independent of the polarization within the q-range investigated . This result confirms that the dipolar interactions in Pd2MnSn are in deed small.