A new class of natural magnetic materials: The ordering alloys

Abstract

Tetrataenite (∼FeNi), found in many meteorites, and Josephinite (∼FeNi3), found in many serpentinized peridotites and possibly in Allende, a carbonaceous chondrite, are ordering alloys. Knowledge of thermomagnetic contrasts between the ordered and disordered states is of some practical importance since misinterpretation would be the consequence of failing to recognize them. Magnetic characteristics of the ordered state are recorded during heating to the apparent Curie point, which coincides with the Curie point of the disordered state. Magnetic characteristics of the disordered state are recorded during cooling from the “Curie point” to room temperature. The ordered alloys can be annealed in the vicinity of 500°C in order to monitor the kinetics of disordering, and to obtain a magnetization record characteristic of the state of order. In the ideal scenario, the minerals acquire thermoremanence (TRM) on cooling below the Curie point (∼550°C for FeNi and ∼610°C for FeNi3), and order below the ordering temperature (∼320°C for FeNi and ∼500°C for FeNi3). In natural circumstances, more likely, the appropriate alloy may form below the Curie point and above the ordering temperature and also near or below the ordering temperature. Induced magnetic anisotropy is characteristic of the ordered state, reaching the extreme for the tetragonal, truly uniaxial anisotropy in FeNi. At the present state of understanding, we can only speculate on what effect atomic ordering might have on the original remanence.