A new interpretation of Mössbauer spectra of microcrystalline goethite: ``Super-ferromagnetism'' or ``super-spin-glass'' behaviour?

The temperature dependence of the magnetic hyperfine field of microcrystalline goethite has been studied in detail and the results have been compared to the behaviour of well-crystallized goethite. It is found that the magnetic properties of the microcrystalline goethite can not be described by existing theories for collective magnetic excitations and superparamagnetic relaxation. However, a model, in which the magnetic interaction among the particles is taken into account, may explain the results. Specifically, a modified Weiss mean field theory for interacting particles gives an excellent fit to the results. This model, which is denominated "super-ferromagnetism", gives a significantly better fit than a "super-spin-glass" model, in which a Gaussian distribution in the magnetic coupling constants is assumed. These models for interacting crystallites also suggest that an apparent reduction in the magnetic transition temperature may be observed in other fine-grained polycrystalline materials.