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

Chemically ordered L1
₀
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 L1
₀
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 Fe
₄₂
Ni
_41.3
Si
_x
B
_12-x
P
₄
Cu
_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 L1
₀
FeNi grains. Structural characterization revealed a high degree of chemical ordering (S ≥ 0.8), but the volume fraction of precipitated L1
₀
grains is low. The crystallized ribbons of FeSiBPCu are composed of two magnetic phases (hard magnetic L1
₀
FeNi grains embedded in a soft magnetic matrix). Alloys with higher concentration of Si are shown to produce high coercivity (H
_c
~ 700-750 Oe). The soft magnetic matrix strongly influences the H
_c
. The actual switching field (≥3.7 kOe) of L1
₀
FeNi has been found to be much higher than that of Hc. In this paper, the L1
₀
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 L1
₀
FeNi phase can survive at temperatures ≤ 550°C. The magnetization reversal mechanism was understood by angular dependence of H
_c
, and it is shown to be a domain-wall pinning type. Due to structural and magnetic similarities between L1
₀
FeNi and L1
₀
FePt, ribbon samples with low-volume fraction of L1
₀
FePt grains in a soft magnetic matrix were prepared with a similar technique. Magnetization behavior of L1
₀
FeNi is shown to be similar to that of L1
₀
FePt.