XMCD and Magnetic Evidence for Cation Reordering in Synthetic Mg- and Al-substituted Titanomagnetites

Abstract:

The titanomagnetite (TM) solid solution (Fe_3-xTi_xO₄, 0 ≤ x ≤ 1) is one of the most important natural magnetic minerals used in paleomagnetic studies. Natural TMs with Mg- and Al-substitution have recently been shown to have Curie temperatures (T_C) that vary strongly with thermal history, and these variations have been indirectly linked to cation reordering in the crystal lattice (Bowles et al. 2013). Here we present the first direct evidence for cation reordering linked to these T_C variations. We have synthesized TMs with varying degrees of Mg²⁺ and Al³⁺ substitution to better understand the mechanism at work in the natural samples. T_C was determined by measuring magnetic susceptibility as function of temperature under Argon atmosphere. Isothermal annealing experiments between 10^-1 to 10³ h at temperatures between 300-500°C were conducted. Subsequent T_C measurements showed that T_C on warming increases for longer anneal times and higher anneal temperatures, whereas T_C on cooling decreases. These resulting T_C changes can be linked to cation ordering within the crystal structure. Splits of the same samples were studied via X-ray magnetic circular dichroism (XMCD), which is sensitive to both Fe valence state and site occupancy. Preliminary results suggest differences in Fe²⁺/Fe³⁺ site occupancy for samples of different compositions for different annealing treatments. Using the data collected for these synthetic samples we hope to gain further insight into the complex relationship between thermal history and cation distribution leading to changes in T_C. So far, our understanding of the acquisition of thermal remanent magnetization (TRM) in TMs is predicated on the assumption that T_C is a constant only depending on the mineral composition. However, the distribution of the magnetic Fe²⁺ and Fe³⁺ cations within the crystal lattice has a strong influence on the value of T_C and cation (dis-)/ordering processes can result in large changes in T_C. In natural samples, reordering results in changes in T_Cof up to >150°C on timescales and at temperatures relevant to both geological and laboratory processes. Thus, the cation reordering has major implications for the acquisition, retention, and demagnetization of partial TRM and thermoviscous remanence and may have an impact on many paleomagnetic studies using natural TMs.