Curie temperature variations in synthetic titanomagnetite single crystals

Paleomagnetic measurements provide very important methods to study the evolution of and variations in the Earth’s magnetic field throughout time. A vital tool used in paleomagnetism are natural magnetic minerals, such as the titanomagnetite (TM) solid solution series (Fe_3-xTi_xO₄, 0 ≤ x ≤ 1). The main source of magnetic information in TMs is the thermal remanent magnetisation (TRM) they retain whilst being cooled below their Curie temperature (T_C) during their formation.

The key factor determining the T_C  is the composition. However, recent studies on natural and synthetic TM powders [1,2,3] have shown that their T_C  is also heavily influenced by their thermal history. Annealing various natural and synthetic TM powders at temperatures between 300°C and 425°C for timescales of hours to months resulted in changes in their T_C  of up to 150°C.

The accuracy of many paleomagnetic measuring techniques, such as geomagnetic paleointensity estimates and paleomagnetic paleothermometry, depends on the exact knowledge of the Curie temperature. Changes in T_C  of such a considerable extend could deeply impact those techniques or even render them doubtable. So far, vacancy-mediated chemical clustering at the octahedral site of the TM structure has been postulated as the mechanism causing this phenomenon [2,3]. To further investigate the underlying processes, we synthesised a large (~6.5 mm diameter;  ~27 mm length) TM single crystal using an optical floating zone furnace. Via SEM-EDX techniques it was established that the crystal was homogenous over its whole length with a composition of  Fe_2.64Ti_0.36O₄. Using a Physical Properties Measurement System (PPMS) the Curie temperatures of several pieces of the crystal were determined after different annealing treatments. For the first time it has been possible to detect systematic changes in T_C  with annealing in a TM single crystal.

Additionally within the scope of this project it was possible to determine the relationship between the extend of change in T_C  and the microstructure for polycrystalline samples.

 

[1] Bowles, J. A., Jackson, M. J., Berquó, T. S., Solheid, P. A. and Gee, J. S. (2013), Nature Communications, 4, 1916. https://doi:10.1038/ncomms2938

[2] Jackson, M. J., and Bowles, J. A. (2018), J. Geophys. Res., 123, 1-20. https://doi:10.1002/2017JB015193

[3] Bowles, J. A., Lappe, S.‐C. L. L., Jackson, M. J., Arenholz, E., & van der Laan, G. (2019). Geochem. Geophy. Geosy. 20. https://doi.org/10.1029/2019GC008217