SUSCEPTIBILITY RESONANCE AND MAGNETIC VISCOSITY

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-tau 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 Neel's theory of this tau is inadequate for estimating the location of such a resonance. However, an appropriate analysis of the measurements of magnetic viscosity in the same sample can give a very good idea of the value of the resonance frequency and an estimation of its line width.