Synthesis of Fe₁₆N₂ compound Free-Standing Foils with 20 MGOe Magnetic Energy Product by Nitrogen Ion-Implantation

Rare-earth-free magnets are highly demanded by clean and renewable energy industries because of the supply constraints and environmental issues. A promising permanent magnet should possess high remanent magnetic flux density (B_r), large coercivity (H_c) and hence large maximum magnetic energy product ((BH)_max). Fe₁₆N₂ has been emerging as one of promising candidates because of the redundancy of Fe and N on the earth, its large magnetocrystalline anisotropy (Ku > 1.0 × 10⁷ erg/cc), and large saturation magnetization (4πMs > 2.4 T). However, there is no report on the formation of Fe₁₆N₂ magnet with high B_r and large H_c in bulk format before. In this paper, we successfully synthesize free-standing Fe₁₆N₂ foils with a coercivity of up to 1910 Oe and a magnetic energy product of up to 20 MGOe at room temperature. Nitrogen ion implantation is used as an alternative nitriding approach with the benefit of tunable implantation energy and fluence. An integrated synthesis technique is developed, including a direct foil-substrate bonding step, an ion implantation step and a two-step post-annealing process. With the tunable capability of the ion implantation fluence and energy, a microstructure with grain size 25-30 nm is constructed on the FeN foil sample with the implantation fluence of 5 × 10¹⁷/cm².