Detailed magnetic dynamic behavior of nanocomposite iron oxide

M. B. FERNÁNDEZ VAN RAAP; CLAUDIA E. RODRÍGUEZ TORRES; F. H. SÁNCHEZ; LL. CASAS; A. ROIG; E. MOLINS

JOURNAL OF PHYSICS CONDENSED MATTER

Año: 2005 vol. 17 p. 6519 - 6531

0953-8984

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 sol?gel preparation method with supercritical drying. In the temperature range from 15 to 300 K the magnetic ac susceptibility  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 ô0 = 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 Keff ¡«= 1.02?1.4×105 J m-3 was determined for 5×20 nm and 4 × 20 nm average dimensions respectively. It is also shown that the temperature dependences of the relaxation time ô as obtained fromN´eel analysis of the magnetic absorption and Cole?Cole frequency dependence analysis are in good agreement with the value derived from Mossbauer effect spectroscopy at room temperature.