Cation Diffusion Processes and Self-reversal of Thermoremanent Magnetization in the Ilmenite-Haematite Solid Solution Series

Synthesized xFeTiO3 – (1 – x)Fe2O3 specimens having compositions near the centre of the solid solution series indicate that nucleation of cation-ordered clusters may occur several hundred degrees above the order–disorder transition temperature Tc(o−d), a finding in conflict with theory. In addition, the ‘x-phase’ is shown to have at most a narrow range of blocking temperatures about 100° above the Curie point of the ordered phase, a result incompatible with the model developed by Ishikawa & Syono. The following model, which well explains the experimental data, is proposed: At temperatures above Tc(o−d) but below the top of the solvus, short-range unmixing occurs with small ilmenite-rich and haematite-rich regions forming throughout the specimen. Upon cooling, the ilmeniterich regions order first and act as the ‘nuclei’ for further ordering. The haematite-rich regions, on the other hand, are the x-phase. It is argued that when these disordered haematite-rich regions are in contact with ordered material, self-reversal occurs.