Detailed magnetic dynamic behaviour of nanocomposite iron oxide aerogels

The magnetic properties of maghemite particles hosted in silica aerogel pores are studied in depth, with most of the focus on the magnetic relaxation mechanisms. It is shown that at room temperature, the system behaves as an ensemble of non-interacting superparamagnetic particles, indicating that particle aggregation can be avoided by using a sot-gel preparation method with supercritical drying. In the temperature range from 15 to 300 K the magnetic ac susceptibility chi(T) displays a broad peak that shifts to higher temperatures on increasing the ac applied field frequency. Transmission electron microscopy reveals acicular shaped maghemite particles exhibiting unimodal size distributions, from which energy barrier distributions can be inferred. The distribution of activation energies has been independently obtained from scaling plots of the frequency dependent out-of-phase ac susceptibility component using tau(0) = 10(-11) s. The value obtained is in agreement with the one derived from the particle size distribution. Combining the two distributions and assuming magnetic volume anisotropy, an effective anisotropy constant K-eff congruent to 1.02 - 1.4 x 10(5) J m(-3) was determined for 5 x 20 nm and 4 x 20 nm average dimensions respectively. It is also shown that the temperature dependences of the relaxation time tau as obtained from Met analysis of the magnetic absorption chi"(T, omega) and Cole-Cole frequency dependence analysis are in good agreement with the value derived from Mossbauer effect spectroscopy at room temperature.