Frequency of ferromagnetic resonance in ferrofluids.

Abstract

In the measurement of the complex susceptibility, $\ensuremath{\chi}(\ensuremath{\omega})={\ensuremath{\chi}}^{\ensuremath{'}}(\ensuremath{\omega})\ensuremath{-}i{\ensuremath{\chi}}^{\ensuremath{'}\ensuremath{'}}(\ensuremath{\omega})$, of ferrofluids, resonance is indicated by a transition in the value of ${\ensuremath{\chi}}^{\ensuremath{'}}(\ensuremath{\omega})$ from a $+ve$ to a $\ensuremath{-}ve$ quantity. Here the complex susceptibility measurements, over the frequency range 0.1 to 6 GHz, of four colloidal samples, three of which display this resonant phenomenon, are presented. It is found that the experimentally determined resonant frequency is lower than that predicted by classical theory. This discrepancy is examined in the light of the work of Raikher and Shliomis. The presence of a pronounced loss peak in the ${\ensuremath{\chi}}^{\ensuremath{'}\ensuremath{'}}(\ensuremath{\omega})$ component of one of the samples, in the 320 MHz region, is interpreted as being a demonstration of the phenomenon of superparamagnetism.