SIZE DISTRIBUTION AND SURFACE EFFECTS IN ULTRAFINE PARTICLE NGR SPECTRA

Résumé
Les spectres Mössbauer de goethite en petites particules sont comparés aux prévisions de deux modèles : (1) le modèle de Mørup qui corrèle distribution de champ et distribution de taille des particules, (2) un modèle où la réduction du champ est attribuée à des effets de surface. Le second modèle semble mieux expliquer l'influence de la température.

Abstract
Presently two different approximations exist in order to explain the assymmetrically broadened lineshapes observed in ultrafine iron oxides and iron oxy-hydroxides. One, due to Mørup et al. /1/ uses a simple pre-relaxation model for the total sublattice spin of the particle, together with a particle size distribution. By adjusting suitable distribution parameters and the anisotropy constant, indeed a reasonable representation of the experimental results can be obtained. The second approach, as we proposed in previous papers /2, 3/ considers the observed reduced magnetic fields mainly to be due to the surface cations being gradually decoupled from the interior of the particle. Unfortunately neither a detailed shape of this field reduction near the surface can as yet been given, nor a clear description of the changes of the Mössbauer fraction f near the surface. However here too a field function can be proposed and suitable parameters be adjusted to match the experimental results ; these parameters are the maximum inside field H^o_i, a Gaussian dispersion σ_s of the surface field, a small dispersion σ_B on the interior field due to residual effects of particle size (which in this model is a second order effect), and the fraction p of surface cations relative to the total number of cations.