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

This paper reviews a profound Mossbauer study on the magnetic properties of goethites with varying crystallinity and Al-content. It has been found tha t both the cluster-ordering and the superferromagnetic models reproduce the temperature dependencies of the average hyperfine fields quite well. Howev er, the magnetic transition temperatures predicted by the cluster-ordering model are in better agreement with those measured from thermoscan experimen ts. On the other hand, it has been found that the hyperfine field of maximu m probability at low temperatures follows the spin-wave law for antiferroma gnets. Linear relationships between the saturation hyperfine fields, the in ter-sublattice exchange interactions, and the Neel temperatures to the Al-c ontent and crystallinity could be derived. On the other hand, applied-field Mossbauer spectroscopy seems to suggest that the anisotropy field can diff erentiate between poor crystallinity and Al-content, and that a certain deg ree of ferrimagnetism occurs at higher Al-contents.