Factors influencing achievement of chemical order in tetragonal FeNi

Chemically ordered ferromagnetic compounds with the L1$_{\mathrm{0}}$ structure have attracted wide interest for rare-earth-free permanent magnet applications. In particular, L1$_{\mathrm{0}}$-structured FeNi is a promising candidate due to the abundance and low cost of the constituent elements and high theoretical maximum energy product (BH)max $=$ 42 MGOe [1]. Synthesis of L1$_{\mathrm{0}}$ FeNi has been hindered by extremely sluggish kinetics below the equilibrium order-disorder temperature T$_{\mathrm{O/D}}=$ 320 ${^\circ}$ and the phase is known to form in meteorites over millions of years. In this work, the thermodynamic stability of the L1$_{\mathrm{0}}$ phase and kinetics of the L1$_{\mathrm{0}}\to $fcc magnetostructural phase transformation are quantitatively determined via magnetic and thermal measurements of bulk L1$_{\mathrm{0}}$ FeNi extracted from meteorites. Influences on phase transformation kinetics, including effects of magnetism, will be discussed. \\[4pt] [1] L. H. Lewis, et al., IEEE Mag Lett 5 (2014) 5500104.