However, the results of the study offer a solution for the development of next-generation hard magnetic materials because the alloys are free from rare-earth elements, and the technique is well suited for mass production, the researchers concluded in their open-access paper published in Nature’s Scientific Reports.

Iron meteorites with Widmannstaetten type of structure are mainly composed of iron and nickel. This structure is formed through an extremely slow cooling rate of about one Kelvin per million years in the universe. The Widmannstaetten structure observed in Octahedrite-type meteorites is peculiar, and results from the apparent phase separation of the α-phase (bcc-FeNi with a mineral name: kamacite) and γ-phase (fcc-FeNi: taenite) at the FeNi interface. The taenite phase lamellae observed in the meteorite have varying Ni concentration zones (28–50%). Both the disordered fcc and ordered L1₀ phases of Fe-Ni were detected. Interestingly, the L1₀ FeNi phase, which is also known as tetrataenite is a hard magnetic with a high saturation magnetization (Ms ~ 1270 emu cm⁻³) and a large uniaxial magneto-crystalline anisotropy (Ku ~ 1.3 × 10⁷ erg cm⁻³). The theoretical maximum magnetic energy product of L1₀ FeNi (~42 MG Oe) is close to the best rare-earth-based hard magnets recently developed.

Due to shortage of rare-earth elements, which are currently used to produce high-grade permanent magnets, magnets free of rare-earth elements must be developed (i.e., hard magnets based on L1₀ FeNi). Practically, it is impossible to produce L1₀ FeNi magnet similar to meteorites industrially because the order-disorder transition temperature of L1₀ FeNi is too low 320 °C. The diffusion coefficients of Fe and Ni are extremely low around this temperature, and in reality, no diffusion takes place, which is why the ordered L1₀ FeNi phase requires billions of years to form in cosmic products (meteorites). Since the discovery of the L1₀ FeNi phase in the 1960s, several attempts (which might trigger atomic migration) such as irradiation with high-energy beams, a nanoparticle technique, mechanical alloying, thin films comprised of mono-layered atoms, and high-pressure torsion technique have been tried to artificially produce this phase. However, L1₀ FeNi-based hard magnets with high degree of chemical order have yet to be produced.

It seems that the production of L1₀ FeNi-based hard magnets via conventional material synthesis utilizing atomic diffusion in the crystalline state is extremely difficult, if not impossible.

—Makino et al.

Now, however, Makino’s research group has succeeded in producing the magnet by utilizing high atomic diffusivity at low temperatures, when crystallizing from amorphous state. Although the volume fraction of L1₀ FeNi phase is low (8 ~ 13%), the hard magnetic L1₀ phase is both “academically and industrially novel”.

• The Tohoku technique is able to produce the L1₀ FeNi phase at a much faster rate than the natural process (millions of years are required for meteorites).

• The artificial L1₀ phase has a much higher chemical order than natural meteorites.

• The non-equilibrium processing technique provides a new method to create a low temperature phase (such as L1₀ FeNi), which is difficult to obtain using conventional processing.

• The results also shed light on hard magnetic materials, which have been stagnating since the discovery of rare-earth-based magnets almost 30 years ago.

… the realization of hard magnets free of rare-earth metals may help in resolving the global issues of resource exhaustion, which should become a critical in the near future. Hence, the successful synthesis of the chemically ordered L1₀ FeNi phase is one-step closer to the field of materials science for realizing a safe and sustainable society in the 21^st century.

—Makino et al.

Resources

• Akihiro Makino, Parmanand Sharma, Kazuhisa Sato, Akira Takeuchi, Yan Zhang & Kana Takenaka (2015) “Artificially produced rare-earth free cosmic magnet” Scientific Reports doi: 10.1038/srep16627