Development of Magnetic Materials Based on the Ordered Fe$_{50}$Ni$_{50}$ Phase: Methodologies and Results

The L1$_{0}$ FeNi structure known as tetrataenite, usually found in meteorites, is reported to possess significant magnetocrystalline anisotropy suitable for hard magnetic properties. As part of the ongoing Advanced Research Project Agency-Energy project on FeNi-based magnets, melt-spinning was used to synthesize new FeNi precursors. The melt-spinning conditions were established in terms of wheel speed, ejection pressure, and atmosphere composition and pressure. The as-spun ribbons have a cubic crystal structure with a$=$3.584 $\pm$ 0.002 {\AA}, and (100) preferred grain orientation perpendicular to the ribbon. They also behave like soft magnetic materials, with coercitivities below 0.3 kOe. DSC response curves were essentially featureless, except for a thermal signature at about 515 $^{\circ}$C associated with the Curie temperature. In contrast, melt-spun FeNi ribbons that were subsequently ball-milled and annealed exhibited a more complex thermal behavior compared to the as-spun ribbons with a weak endotherm in the 300-350 $^{\circ}$C range followed by an exotherm at higher temperatures. These results are discussed in the context of a search for an order-disorder phase transition associated with the L1$_{0}$ phase, and preferred properties for permanent magnet applications. Although L1$_{0}$ phase formation was not observed at this point, the techniques established for processing FeNi will be further studied on modified FeNi alloys as a promising route to obtain the L1$_{0}$ phase.