XMCD and XAS examination of cation ordering in synthetic Mg- and Al-substituted titanomagnetites

Abstract:

Many paleomagnetic studies use natural magnetic minerals of the titanomagnetite (TM) solid solution series (Fe_3-xTi_xO4, 0 ≤ x ≤ 1). So far our understanding of the acquisition of thermal remanent magnetisation (TRM) in those minerals was based on the assumption that their Curie temperature (T_C) only depends on composition. However, it has been shown that T_C of some natural TMs with Al- and Mg-substitution also strongly depends on thermal history and T_C changes up to >150°C on timescales and at temperatures relevant to laboratory and geological processes were observed (Bowles et al. 2013). These variations in T_C are thought to result from cation reordering within the crystal lattice. Those processes may have major effects on the acquisition, retention and demagnetisation of partial TRM and thermoviscous remanence and may have significant consequences for many paleomagnetic studies. However, so far no direct evidence for this cation reordering has been established. To gain further insight into the mechanisms at work, we have synthesised TMs of varying Ti-content (0.25 ≤ x ≤ 60) and with different degrees of Al- and Mg- substitution. Measuring the magnetic susceptibility as function of temperature T_C was determined. Subsequently the samples were subjected to isothermal annealing at temperatures between 325-400°C for 10 to 10³ hours. We observe an increase of T_C on warming with anneal time up to 80°, whereas T_C on cooling decreases slightly. The magnitude of the change in T_C on warming varies with anneal temperature, and the temperature of maximum change depends on composition. Splits of the samples were studied using X-ray magnetic circular dichroism (XMCD) of the Fe L_2,3-edge, which is sensitive to valence state and site occupancy of the Fe cations. The results suggest different Fe²⁺/Fe³⁺ site occupancy for different sample compositions but there seem to be no consistent differences between the XMCD spectra of annealed and un-annealed samples of one composition. X-ray absorption spectra (XAS) of the Ti, Mg and Al show no significant differences for samples of different anneal stages, suggesting no reordering of the Ti⁴⁺, Al³⁺ and Mg²⁺. The lack of observed (re)ordering between A and B lattice sites suggests the process may be vacancy mediated or there might be short-range cation (re)ordering within the lattice sites.