Magnetic Properties of L1₀ FeNi Phase Developed Through Annealing of an Amorphous Alloy

Chemically ordered L1₀ FeNi phase observed in Fe-based meteorite has the potential to replace high-cost rare-earth-based permanent magnets in the future. However, artificial production of this phase is extremely difficult due to negligible atomic diffusion around order-disorder transition temperature (~320 °C). Here, we report a method for producing high-quality L1₀ FeNi phase and its magnetic properties. We show that a highly disordered metastable state, that is, amorphous can be utilized to produce a highly ordered state, which is not possible with the conventional processing techniques. Amorphous Fe₄₂Ni_41.3Si_xB_12-xP₄Cu_0.7 ( x=0 to 8 at.%) alloy ribbons were studied. Crystallization of amorphous ribbons at 400 °C results in adequate atomic diffusion at low temperatures to precipitate L1₀ FeNi grains. Structural characterization revealed a high degree of chemical ordering (S ≥ 0.8 ), but the volume fraction of precipitated L1₀ grains is low. The crystallized ribbons of FeSiBPCu are composed of two magnetic phases (hard magnetic L1₀ FeNi grains embedded in a soft magnetic matrix). Alloys with higher concentration of Si are shown to produce high coercivity ( Hc ~ 700-750 Oe). The soft magnetic matrix strongly influences the H_c. The actual switching field (≥3.7 kOe) of L1₀ FeNi has been found to be much higher than that of H_c. In this paper, the L1₀ FeNi phase is shown to form at temperatures higher than the reported order-disorder temperature. Our results of temperature-dependent magnetization and thermal analyses suggest that the L1₀ FeNi phase can survive at temperatures ≤550 °C. The magnetization reversal mechanism was understood by angular dependence of Hc , and it is shown to be a domain-wall pinning type. Due to structural and magnetic similarities between L1₀ FeNi and L1₀ FePt, ribbon samples with low-volume fraction of L1₀ FePt grains in a soft magnetic matrix were prepared with a similar technique. Magnetization behavior of L1₀ FeNi is shown to be similar to that of L1₀ FePt.