The effect of Al-content and crystallinity on the magnetic properties of goethites

This paper reviews a profound Moessbauer study on the magnetic properties of goethites with varying crystallinity and Al-content. It has been found that both the cluster-ordering and the superferromagnetic models reproduce the temperature dependencies of the average hyperfine fields quite well. However, the magnetic transition temperatures predicted by the cluster-ordering model are in better agreement with those measured from thermoscan experiments. On the other hand, it has been found that the hyperfine field of maximum probability at low temperatures follows the spin-wave law for antiferromagnets. Linear relationships between the saturation hyperfine fields, the inter-sublattice exchange interactions, and the Neel temperatures to the Al-content and crystallinity could be derived. On the other hand, applied-field Moessbauer spectroscopy seems to suggest that the anisotropy field can differentiate between poor crystallinity and Al-content, and that a certain degree of ferrimagnetism occurs at higher Al-contents