Size and shape distribution of magnetic nanoparticles in disordered systems: computer simulations of superparamagnetic resonance spectra

A new model is proposed in order to account for the superparamagnetic reson ance spectra arising from the ferromagnetic particles dispersed in a diamag netic matrix (e.g., ferrofluids, partially devitrified glasses). The model explicitly takes into account non-sphericity of magnetic nanoparticles in a joint distribution density of sizes and demagnetising factors. The distrib ution in particle shapes is shown to take an important part in determining the superparamagnetic resonance spectra. By computer simulating the resonan ce spectra of annealed iron-containing berate and sol-gel silica glasses, c haracteristics of size and shape distributions of magnetic nanoparticles ar e evaluated. As the anneal temperature is increased from 456 degrees C to 4 80 degrees C, the most probable diameter of the magnetic particles formed i n the berate glass is increased from 3.4 to 4.7 nm, while their shapes beco me better ordered. The mean size of particles formed in the sol-gel silica glass is small, ca. 3.0 nm, and their shapes are less ordered in comparison with the berate glass. (C) 1999 Elsevier Science B.V. All rights reserved.