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 ''(o mega), of two magnetic fluids has been measured over the frequency ran ge 0.1 to 6 GHz, and ferromagnetic resonance observed, indicated by th e 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 e xternal magnetic field, H, in the approximate range 0 to 68 kA/m has b een examined for colloidal suspensions of magnetite particles and coba lt 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 Val ue of the internal anisotropy field, <(H)over bar (A)>, to be measured , from which mean values of the magnetic anisotropy constant of the pa rticles, (K) over bar, are derived. The experimental profiles of chi'( omega) and chi ''(omega), for the magnetite particle system as a funct ion of H, have been fitted to modified equations of Raikher and Shliom is suitably adapted to include a normal distribution of particle energ y barriers, KeffV, to the rotation of the magnetic moments. It is foun d that K,,V varies linearly with increasing H, whilst the standard dev iation of the distribution of KeffV decreases with increase in H. The main contribution to the resonant linewidth, Delta omega, for the larg er 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 th e experimental observations.