Curie temperature variations in synthetic titanomagnetite single crystals
Authors: Lappe Sophie-Charlotte , Winkens Georg , Persson Joerg , Nandi Shibabrata , Petracic Oleg 
Published: March 2020

&lt;p&gt;Paleomagnetic measurements provide very important methods to study the evolution of and variations in the Earth&amp;#8217;s magnetic field throughout time. A vital tool used in paleomagnetism are natural magnetic minerals, such as the titanomagnetite (&lt;em&gt;TM&lt;/em&gt;) solid solution series (Fe&lt;sub&gt;3-&lt;em&gt;x&lt;/em&gt;&lt;/sub&gt;Ti&lt;em&gt;&lt;sub&gt;x&lt;/sub&gt;&lt;/em&gt;O&lt;sub&gt;4&lt;/sub&gt;, 0 &amp;#8804; &lt;em&gt;x&lt;/em&gt; &amp;#8804; 1). The main source of magnetic information in &lt;em&gt;TM&lt;/em&gt;s is the thermal remanent magnetisation (&lt;em&gt;TRM&lt;/em&gt;) they retain whilst being cooled below their Curie temperature (&lt;em&gt;T&lt;sub&gt;C&lt;/sub&gt;&lt;/em&gt;) during their formation.&lt;/p&gt;&lt;p&gt;The key factor determining the &lt;em&gt;T&lt;sub&gt;C&lt;/sub&gt;&amp;#160; &lt;/em&gt;is the composition. However, recent studies on natural and synthetic TM powders [1,2,3] have shown that their &lt;em&gt;T&lt;sub&gt;C&lt;/sub&gt;&amp;#160; &lt;/em&gt;is also heavily influenced by their thermal history. Annealing various natural and synthetic &lt;em&gt;TM&lt;/em&gt; powders at temperatures between 300&amp;#176;C and 425&amp;#176;C for timescales of hours to months resulted in changes in their &lt;em&gt;T&lt;sub&gt;C&lt;/sub&gt;&amp;#160; &lt;/em&gt;of up to 150&amp;#176;C.&lt;/p&gt;&lt;p&gt;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 &lt;em&gt;T&lt;sub&gt;C&lt;/sub&gt;&amp;#160; &lt;/em&gt;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 &lt;em&gt;TM&lt;/em&gt; 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;&amp;#160; ~27 mm length) &lt;em&gt;TM&lt;/em&gt; 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&amp;#160; Fe&lt;sub&gt;2.64&lt;/sub&gt;Ti&lt;sub&gt;0.36&lt;/sub&gt;O&lt;sub&gt;4&lt;/sub&gt;. Using a Physical Properties Measurement System (&lt;em&gt;PPMS&lt;/em&gt;) 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 &lt;em&gt;T&lt;sub&gt;C&lt;/sub&gt;&amp;#160; &lt;/em&gt;with annealing in a &lt;em&gt;TM&lt;/em&gt; single crystal.&lt;/p&gt;&lt;p&gt;Additionally within the scope of this project it was possible to determine the relationship between the extend of change in &lt;em&gt;T&lt;sub&gt;C&lt;/sub&gt;&amp;#160; &lt;/em&gt;and the microstructure for polycrystalline samples.&lt;/p&gt;&lt;p&gt;&amp;#160;&lt;/p&gt;&lt;p&gt;[1] Bowles, J. A., Jackson, M. J., Berqu&amp;#243;, T. S., Solheid, P. A. and Gee, J. S. (2013), Nature Communications, 4, 1916. https://doi:10.1038/ncomms2938&lt;/p&gt;&lt;p&gt;[2] Jackson, M. J., and Bowles, J. A. (2018), J. Geophys. Res., 123, 1-20. https://doi:10.1002/2017JB015193&lt;/p&gt;&lt;p&gt;[3] Bowles, J. A., Lappe, S.&amp;#8208;C. L. L., Jackson, M. J., Arenholz, E., &amp; van der Laan, G. (2019). Geochem. Geophy. Geosy. 20. https://doi.org/10.1029/2019GC008217&lt;/p&gt;