Discovery of process-induced tetragonality in equiatomic ferromagnetic FeNi

Abstract

Synthesis of a new tetragonal phase at the equiatomic composition in the archetypal binary Fe-Ni phase diagram is reported. This new phase is proposed as a transitional phase linking cubic FeNi with the chemically ordered tetragonal L10 FeNi compound, tetrataenite, of interest as a new advanced permanent magnetic material. This new tetragonal phase was created in a selection of nominally equiatomic FeNi alloys, made either from natural Fe and Ni or from natural Fe combined with the 62Ni isotope, via application of high-strain processing methods followed by an annealing protocol. High-resolution neutron diffraction affirms that all unprocessed samples adopt the A1-type cubic structure (space group Fm3¯m) while all fully processed samples adopt the chemically disordered A6-type tetragonal structure (space group I4/mmm). Magnetic characterization documents a decrease in the initial magnetic susceptibility of deformed samples after annealing, evidencing a processing-induced increase in magnetic anisotropy that may be entirely accounted for by the measured tetragonal distortion. It is proposed that this new phase is a precursor to the formation of tetrataenite (L10 FeNi, space group P4/mmm), a meteoritic mineral of high magnetization and appreciable magnetocrystalline anisotropy that requires extraordinarily long cooling periods to form in nature. These results furnish new fundamental information as well as engineering insight for terrestrial synthesis of tetrataenite on industrial timescales, with high relevance for the creation of next-generation permanent magnets comprised entirely of easily accessible, earth-abundant elements.