CONTRIBUTION OF PARTICLE INERTIAL EFFECTS TO RESONANCE IN FERROFLUIDS

The effect of the moment of inertia of single domain ferromagnetic par ticles on the frequency-dependent complex susceptibility chi(omega) = chi'(omega)-i chi ''(omega) of ferrofluids is reported. It is demonstr ated that particle inertial effects that arise from rotational Brownia n motion can give rise to a resonant behavior, which is indicated by t he real component chi'(omega) becoming negative at a frequency substan tially lower than the Larmor frequency. This provides a possible expla nation for previously published data that display such an effect in th e 10 to 100 MHz region. The Langevin treatment of Brownian motion is u sed to incorporate thermal agitation into a model which represents, fo r the purpose of analysis, a typical ferroparticle, P, as a composite particle comprising a magnetic particle, P-m (assumed to be spherical) , which may rotate inside and in contact with a concentric rigid spher e, P-s, representing the surfactant, so that P-m and P-s may have diff erent angular velocities about a common center. This leads to a three- dimensional form of the itinerant oscillator model in the small oscill ation approximation. The model predicts inertia corrected Debye relaxa tion in the form of the Rocard equation that arises for P-m and P-s ro tating as a unit, and resonance behavior arising from the relative mot ion of P-m and P-s.