Susceptibility resonance and magnetic viscosity

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Abstract
It is suggested that the phenomenon of superparamagnetism in small ferromagnetic particles may manifest itself as a resonance in the magnetic susceptibility, measured at a frequency which corresponds to the average relaxation time τ of these particles. It is argued that this resonance has not been observed because it is difficult to know in advance at which frequency it should be looked for, and Ne´el’s theory of this τ is inadequate for estimating the location of such a resonance. However, an appropriate analysis of the measurements of magnetic viscosity in the same sample mple can give a very good idea of the value of the resonance frequency and an estimation of its line width.