SCALING BEHAVIOR OF THE HOMOGENEOUS MAGNETIZATION DYNAMICS IN THE FERROMAGNETIC STATE OF EUS

Using a broadband vectorial microwave reflectometer, we measured the l ongitudinal dynamic susceptibility of a EuS sphere between 0.1 and 20 GHz in the temperature interval 4.2 K less-than-or-equal-to T less-tha n-or-equal-to T(c)=16.5 K. For low internal fields, the shape correspo nds to a heavily overdamped Lorentzian resonance, chi(z)-1 (omega) = c hi(z)-1(0)iomega/L(z) -(omega/OMEGA(z))2 where L(z) less-than-or-equal -to OMEGA(z). The field and temperature dependence of the intrinsic ki netic coefficient and resonance is fully explained by that of the stat ic susceptibility chi(z)(0). Surprisingly, the resulting scaling funct ion, L(z)[chi(z)(0)], agrees exactly with that observed earlier above T(c), where L(z)[chi(z)(0)] could quantitatively be explained by a cro ssover from a van Hove behavior for large chi(z)(0), due to dipolar an isotropic fluctuations, to a critical speeding-up for chi(z)(0)<1, whe re isotropic spin diffusion prevails. This finding suggests that the d ipolar anisotropy dominates the magnetization dynamics also below T, o f EuS, which seems to be supported by the fact that the correlation fr equency of the torques acting on the magnetization, omega(c) = OMEGA(z )2/L(z), agrees with the relaxation rate of the longitudinal magnetiza tion fluctuations, deltaM(q) parallel-to q.