Open AccessCurie Temperature Enhancement and Cation Ordering in Titanomagnetites: Evidence From Magnetic Properties, XMCD, and Mössbauer Spectroscopy
Author(s) -
Bowles J. A.,
Lappe S.C. L. L.,
Jackson M. J.,
Arenholz E.,
Laan G.
Publication year - 2019
Publication title -
geochemistry, geophysics, geosystems
Language(s) - English
Resource type - Journals
SCImago Journal Rank - 1.928
H-Index - 136
ISSN - 1525-2027
DOI - 10.1029/2019gc008217
Subject(s) - mössbauer spectroscopy , curie temperature , magnetic circular dichroism , materials science , octahedron , solvus , vacancy defect , crystallography , valence (chemistry) , magnetization , condensed matter physics , ferromagnetism , chemistry , crystal structure , microstructure , magnetic field , physics , spectral line , metallurgy , superalloy , organic chemistry , quantum mechanics , astronomy
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 Fe 2+ /Fe 3+ 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.