Investigation of the complex susceptibility of magnetic beads containing maghemite nanoparticles

We report on frequency and field-dependent complex susceptibility, χs(ω)=χs′(ω)-iχs″(ω), measurements of a magnetic colloidal system consisting of 200 nm spherical beads, containing maghemite (γFe2O3) nanoparticles. The relaxation properties of both the magnetic colloid and a free suspension of the γFe2O3 particles, are investigated over the frequency range 200 Hz–1 MHz.Under a polarizing field H  , an absorption peak is detected in the χs″ component at frequencies fmax between 1.1 and 1.7 kHz. We show that this absorption peak can be attributed to the Néel relaxation of the inner maghemite nanoparticles. It is also shown that the general trend for the value of fmax and the amplitude of both χs′ and χs″ is to increase with increasing H  . Furthermore, the relation between χs′(ω) and χs″(ω) and their dependence on frequency, ω/2π, is investigated by means of the magnetic analogue of the Cole–Cole plot and a measure of the Cole–Cole distribution parameter αs is determined.