Experimental and theoretical profiles of the frequency-dependent complex susceptibility of systems containing nanometer-sized magnetic particles

Measurements of the frequency-dependent complex susceptibility chi(omega)= chi'(omega) = i chi ''(omega) of ferrofluids and magnetic tape are presented and compared with susceptibility profiles generated by means of classical models. In the case of ferrofluids, fitting is realized by means of a combination of the equations of Raikher and Shliomis and of Debye. These equations are for monodispersed particles, and as ferrofluids consist of polydispersed particles, it is demonstrated that to achieve a good fit to the experimental data it is necessary to provide for both a distribution of particle radii r and anisotropy constant K. In terms of the parallel, chi(parallel to)(omega), and transverse, chi(perpendicular to)(omega), components of the susceptibility, it is shown that the incorporation of a distribution of particle radii is required to fit the parallel component chi(parallel to)(omega) of the susceptibility, while a distribution of anisotropy constants is required to fit the resonant chi(perpendicular to)(omega) component. Susceptibility data for magnetic tape are fitted to the equations of Landau and Lifshitz, suitably adapted to provide for a distribution of K alone. The usefulness of the fitting technique in ascertaining average values of the damping parameter alpha and anisotropy constant K is demonstrated.