The short-circuit coaxial transmission line technique has been used to investigate ferromagnetic resonance in magnetic fluids over the frequency range 0.1-6 GHz. Measurements of the complex susceptibility, chi ( omega )= chi '( omega )-i chi "( omega ), of magnetic fluid samples of magnetite and cobalt particles, of approximate median diameters 10 and 5.8 nm are presented. The presence of loss peaks in the chi "( omega ) components at approximate frequencies of 1.2 and 2.5 GHz, respectively, coupled with the transition of the chi '( omega ) components from positive to negative values at slightly higher frequencies, is indicative of ferromagnetic resonance. Appropriate equations for the calculation of the complex susceptibility are presented.
Measurements of the frequency-dependent complex susceptibility ()ϭЈ()ϪiЉ() 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, ʈ (), and transverse, Ќ (), components of the susceptibility, it is shown that the incorporation of a distribution of particle radii is required to fit the parallel component ʈ () of the susceptibility, while a distribution of anisotropy constants is required to fit the resonant Ќ () 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 ␣ and anisotropy constant K is demonstrated.
This paper reports on the use of the autocorrelation function of the magnetization decay of a ferrofluid to investigate the effects which the moment of inertia of single domain ferromagnetic particles have on the frequency dependent complex susceptibility, xi ( omega )= xi '( omega )-i chi "( omega ), of ferrofluids. The contribution of particle inertial effects, arising from rotational Brownian motion, to an apparent resonance, indicated by the real component, chi '( omega ), going negative at a frequency lower than that predicted by theory, is investigated. The Langevin treatment of Brownian motion is used to incorporate thermal agitation into the analysis.
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