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 resonance spectra arising from the ferromagnetic particles dispersed in a diamagnetic 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 distribution in particle shapes is shown to take an important part in determining the superparamagnetic resonance spectra. By computer simulating the resonance spectra of annealed iron-containing borate and sol-gel silica glasses, characteristics of size and shape distributions of magnetic nanoparticles are evaluated. As the anneal temperature is increased from 456 degrees C to 480 degrees C, the most probable diameter of the magnetic particles formed in the borate glass is increased from 3.4 to 4.7 nm, while their shapes become 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 borate glass. (C) 1999 Elsevier Science B.V. All rights reserved.