Tetrataenite

Tetrataenite forms naturally in iron meteorites that contain taenite that are slow-cooled at a rate of a few degrees per million years, which allows for ordering of the Fe and Ni atoms.^[5][14] It is found most abundantly in slow-cooled chondrite meteorites,^[15] as well as in mesosiderites.^[5] At high (as much as 52%) Ni content and temperatures below 320 °C (the order-disorder transition temperature^[9]), tetrataenite is broken down from taenite and distorts its face centered cubic crystal structure to form the chemically ordered, tetragonal L1₀ structure.^[16][14]

In 2015, it was reported that tetrataenite was found in a terrestrial rock – a magnetite body from the Indo-Myanmar ranges of northeast India.^[14]

It is reported that the L1₀ phase can be synthetically produced by neutron- or electron-irradiation of chemically disordered (A1) FeNi below 593 K,^[9][10][11] by hydrogen-reduction of nanometric NiFe₂O₄,^[14] or by combined application of mechanical stress and magnetic field during annealing of the chemically disordered A1 phase.^[17]

It has been reported that the combined application of mechanical stress and a modest magnetic field during the annealing process can accelerate the formation of the atomically ordered L1₀ phase in bulk samples.^[17]

In 2022, it was reported that mixing iron and nickel together in specific quantities, with a phosphorus catalyst, and smelting the mixture, formed tetrataenite in bulk quantities, in seconds.^[18][19] However, in late 2024, the article originally reporting this result was retracted by the journal^[20] due to 'misinterpretation of the experimental data'. A subsequent Comment, published by a group containing many of the original article's authors, provided both reinterpretation of the original data as well as new measurements, and showed that the Bragg peaks originally attributed to presence of tetrataenite in the samples were, in fact, caused by the presence of phosphides.^[21]

Tetrataenite has a highly ordered crystal structure,^[16] appearing creamy in color and displaying optical anisotropy.^[5] Its appearance is distinguishable from taenite, which is dark gray with low reflectivity.^[14] FeNi easily forms into a cubic crystal structure, but does not have magnetic anisotropy in this form. Three variants of the L1₀ tetragonal crystal structure have been found, as chemical ordering can occur along any of the three axes.^[13]

Tetrataenite displays permanent magnetization, in particular, high coercivity.^[22] It has a large uniaxial magnetocrystalline anisotropy^[12] and theoretical magnetic energy product, the maximum amount of magnetic energy stored, over 335 kJ m⁻³.^[22] The L1₀ phase has a theoretical Curie temperature of over 1000 K,^[12] resulting in a magnetic anisotropy which is predicted to remain large up to and beyond room temperature.

Tetrataenite is a candidate for replacing rare-earth permanent magnets such as samarium and neodymium since both iron and nickel are earth-abundant and inexpensive.^[23]

• Glossary of meteoritics
• Superlattice