A study of the effect of an external magnetic field on the resonant frequency of magnetic fluids

The complex magnetic susceptibility, chi(omega)=chi'(omega)-i chi ''(omega), of two magnetic fluids has been measured over the frequency range 0.1 to 6 GHz, and ferromagnetic resonance observed, indicated by the real component of the susceptibility going negative at a frequency, f(res). In this study the dependence of f(res) on the presence of an external magnetic field, H, in the approximate range 0 to 68 kA/m has been examined for colloidal suspensions of magnetite particles and cobalt particles in isopar M. In both cases, plots of this dependence are found to be linear with slopes having values of the magnetogyric ratio, gamma, within experimental error. These plots enable the average Value of the internal anisotropy field, <(H)over bar (A)>, to be measured, from which mean values of the magnetic anisotropy constant of the particles, (K) over bar, are derived. The experimental profiles of chi'(omega) and chi ''(omega), for the magnetite particle system as a function of H, have been fitted to modified equations of Raikher and Shliomis suitably adapted to include a normal distribution of particle energy barriers, KeffV, to the rotation of the magnetic moments. It is found that K,,V varies linearly with increasing H, whilst the standard deviation of the distribution of KeffV decreases with increase in H. The main contribution to the resonant linewidth, Delta omega, for the larger Values of H, arises from the mean random spacial distribution of H-A, and is given by <gamma(H)over bar (A)>,. This is consistent with the experimental observations.