MAGNETIZATION DYNAMICS BELOW T-C OF EUO AND EUS - EVIDENCES FOR EFFECTS OF DIPOLAR-ANISOTROPIC FLUCTUATIONS

Between 4.2 K and the Curie temperatures of the cubic Heisenberg ferro magnets EuS and EuO, their homogeneous dynamic susceptibilities chi(zz )(omega,T,H) have been investigated by means of a broadband reflectome ter operating from 0.1 GHz to 40 GHz. For internal magnetic fields lar ger than the anisotropy fields H-A(T) of both materials, their static susceptibilities chi(z)(T, H) exhibit a 1/root H-divergence, which rev eals quantitatively the dominance of dipolar-anisotropic spin-wave flu ctuations. chi(zz)(omega) displays a Lorentzian shape the damping freq uency of which obeys scaling in terms of chi(z)(T, H). The scaling fun ction agrees quantitatively with work by Frey and Schwabl [19] for dip olar Heisenberg ferromagnets at temperatures above T-c. Building upon their approach, the resonance frequency of the Lorentzian can be relat ed to a memory effect in the damping determined by the large value of the relaxation rate of the longitudinal magnetization fluctuations Gam ma(l). For EuS, this relation is substantiated directly by inelastic n eutron scattering. All these features reveal the hitherto uncovered im portance of the dipolar anisotropic fluctuations below T-c of ferromag nets.