On the influence of the particle packing fraction on the after-effect function of ferrofluids

Results of the decay of magnetization or after-effect function, b(t) are presented, for three ferrofluids of saturation magnetizations 0.09, 0.06 and 0.04 T and corresponding approximate packing fractions of, 0.22, 0.15 and 0.1, respectively, which show b(t) to he little affected by the packing fraction. b(t) was similar for all three samples with an initial rapid decay up to an approximate time of 0.4 x 10(-9) s and a time constant of 1.4 x 10(-9) s. Data from this time region are realized by the transformation of the frequency-dependent complex magnetic susceptibility data of the samples, chi(omega) = chi'(omega) = i chi"(omega), measured in the gigahertz frequency range. Because the complex susceptibility data indicate the presence both of relaxational and of resonance mechanisms, chi(omega) is modelled in terms of its parallel, chi(parallel to)(omega), and perpendicular, chi(perpendicular to)(omega), susceptibility components with corresponding relaxation times, tau(parallel to) and tau(perpendicular to), respectively. chi(omega) is suitably modified to include a distribution of particle sizes, r, and anisotropy constant, K. The decay of magnetization has essentially two components, that due to magnetic moments which relax by the Neel relaxation mechanism corresponding to tau(parallel to) and that due to the precession of magnetic moments which relax by means of the transverse time, tau(perpendicular to). By application of a varying polarizing magnetic field to the samples, which effectively reduces the contribution of the chi(parallel to)(omega) component, the variation in tau(perpendicular to) is shown to be in accord with that predicted by the equations of Coffey et al.