HOMOGENEOUS MAGNETIZATION DYNAMICS BELOW T-C EUO - EFFECTS OF SPIN-WAVES AND ANISOTROPY

Using a broadband reflectometer, the dynamic susceptibility chi(omega) of a EuO sphere has been measured from 0.3 to 20 GHz at temperatures ranging between 4.2 K and T-c = 69.5 K. For fields applied parallel to the hard [100] direction one heavily damped Lorentzian is observed wh ich for internal fields H larger and smaller than the anisotropy field arises from the dynamics of the homogeneous and the wall magnetizatio n, respectively. For fields near H, applied along one of the easy [111 ] axes, both processes contribute to chi(omega) but with increased and decreased values of the resonances and damping frequencies of the hom ogeneous and wall processes, respectively. The homogeneous process is identified by the static susceptibility chi(x)(omega) which obeys the H--1/2 divergence due to spin waves. Its line shape is explained by a frequency variation of the kinetic coefficient of the relaxation dampi ng, L(z)(omega) = L(z)(0)/[1 + (i omega/omega(c))]. The characteristic frequency, omega(c)/2 pi = 18 GHz, turns out to be independent of tem perature and magnetic field, and is related to the relaxation rate of the long-wavelength longitudinal magnetization fluctuations. The held and temperature variation of the kinetic coefficient of the relaxation can be described by the same scaling function, L(z)(chi(2)(T,H)), as previously observed above T-c. Based on results of numerical mode-coup ling work, this behavior is associated with the dipolar anisotropy of the magnetic fluctuations. (C) 1996 American Institute of Physics.