Curie Temperature Enhancement and Cation Ordering in Titanomagnetites: Evidence From Magnetic Properties, XMCD, and Mössbauer Spectroscopy

Previous work has documented time- and temperature-dependent variations in the Curie temperature (T_c) of natural titanomagnetites, independent of any changes in sample composition. To better understand the atomic-scale processes responsible for these variations, we have generated a set of synthetic titanomagnetites with a range of Ti, Mg, and Al substitution; a subset of samples was additionally oxidized at low temperature (150 °C). Samples were annealed at temperatures between 325 and 400 °C for up to 1,000 hr and characterized in terms of magnetic properties; Fe valence and site occupancy were constrained by X-ray magnetic circular dichroism (XMCD) and Mössbauer spectroscopy. Annealing results in large (up to ~100 °C) changes in T_c, but Mössbauer, XMCD, and saturation magnetization data all demonstrate that intersite reordering of Fe²⁺/Fe³⁺ does not play a role in the observed T_c changes. Rather, the data are consistent with vacancy-enhanced nanoscale chemical clustering within the octahedral sublattice. This clustering may be a precursor to chemical unmixing at temperatures below the titanomagnetite binary solvus. Additionally, the data strongly support a model where cation vacancies are predominantly situated on octahedral sites, Mg substitution is largely accommodated on octahedral sites, and Al substitution is split between the two sites.