Perspective of Production of Nitride Iron Magnets

Introduction

Iron Nitride (Fe₁₆N₂) was first examined in the 1990s but faced skepticism due to inconsistent results. It was considered a mysterious material because of inconclusive debates during the 1990s, including several controversial reports at two specific symposia at the MMM conference. Consequently, the magnetic research community largely abandoned the topic.

Between 2002 and 2012, renewed research efforts clarified its properties, establishing it as a promising candidate for sustainable magnet applications. In 2020, Prof. Jian-Ping from the University of Minnesota presented a comprehensive analysis of the Iron Nitride compound, highlighting its potential as a rare-earth-free alternative for permanent magnet applications.

Since then, Niron Magnetics has been engaged in R&D activities to develop the best Iron Nitride magnet options to compete with current magnet materials used in the magnetic industry. Additionally, Niron Magnetics is working on the implementation of the manufacturing process for iron nitride magnets, which is a complex process requiring specialized knowledge in materials engineering, crystallography, mechanical engineering, and magnetic engineering, see Video 1.

Video 1. Niron Magnetics production perspective

Magnetic Properties of Nitride Iron Magnets

Iron Nitride (FeN) magnets have emerged as one of the most promising rare-earth-free alternatives for high-performance magnetic applications. With a theorical high saturation magnetization (Ms ~29→30 kG)* surpassing that of widely used in sintered NdFeB (Neodymium-Iron-Boron) magnets (Ms ~ 13 → 17 kG)*, Iron Nitride offers exceptional potential for several magnetic applications. The hypothetical high content of Fe (90%) in Iron Nitride magnets is the main factor to get a high saturation magnetization. Usually, in sintered NdFeB magnets the Fe content is approx. 75% which reflect a low saturation magnetization compared with the saturation of Iron Nitride magnets. Furthermore, a high Iron Nitride saturation directly leads to a hypothetical high residual flux density (Br) > 15 kG in the magnet material product of the reverse motion of the domains, and viceversa. Finally, a high saturation in Iron Nitride magnets is attributed to a unique combination of crystal structure, electronic configuration, and interstitial nitrogen doping.

However, the development of Iron Nitride magnets is still in the "experimental stage" due to significant challenges in phase stability, scalable processing, and the control of main magnetic properties like coercivities. It indicates that there is a lot of work to do to get Iron Nitride magnets with useful magnetic properties for actual magnetic applications.

Figure 1 shows a unit cell structure of an Iron Nitride (Fe₁₆N₂) crystal and Table I shows the magnetic properties of Iron Nitride magnets.

*Note: Saturation magnetization is an intrinsic property of the magnetic material, and it is the maximum magnetic moment per unit volume when all magnetic domains are aligned in the material. 

Manufacturing Process of Nitride Iron Magnets

Manufacturing typically begins with high-purity iron in wires, sheets, thin films, or nanopowder form, see Figure 2. Strain engineering is employed to tailor the iron lattice structure, creating favorable conditions for nitrogen interstitial incorporation called the “Nitriding Process”.  

Iron Nitride (Fe₁₆N₂) is typically synthesized through controlled nitridation of α''-Fe₁₆N or Fe powders using ammonnia gas (NH₃) or plasma-assisted methods at controlled intermediate temperatures (typically below 200 – 250 °C) to avoid decomposition. These conditions are carefully optimized to allow sufficient nitrogen diffusion without triggering decomposition into magnetically inferior phases like ε-Fe₃N or γ′-Fe₄N.

Following nitridation, the material undergoes a precisely controlled annealing process to promote atomic ordering and formation of the Fe₁₆N₂ phase. However, because this phase is not thermodynamically stable at ambient conditions, it tends to degrade unless carefully stabilized. To address this, researchers have explored the use of dopant elements such as Cobalt (Co), Titanium (Ti), and Aluminum (Al). These dopants serve multiple roles; for example, Co enhances magnetocrystalline anisotropy and thermal stability; Ti and Al aid in grain refinement, nitrogen retention, and stabilization of the desired phase by modifying the local bonding environment.

A particularly difficult step in the Iron Nitride magnet production route is compaction and densification. The compaction process of Iron Nitride magnets (Fe₁₆N₂) is a critical step in fabricating bulk or near-net-shaped permanent magnets from powders. Because Fe₁₆N₂ is metastable and decomposes at elevated temperatures, traditional sintering approaches like those used for NdFeB magnets are not viable.

Alternative compaction methods such as spark plasma sintering (SPS or SPS-DC), cold pressing, and low-temperature hot pressing are used instead, often under inert or reducing atmospheres to minimize oxidation and structural degradation.

Spark Plasma Sintering (SPS or SPS-DC): this method can be used for low temperatures (≤300 °C) and short dwell times. This method has been utilized for researchers to compact Iron Nitride powder applying pulses of current and die pressures > 100 MPa during the heating and sintering process, see Figure 3 and 4.

Even with successful phase formation and compaction, controlling coercivity, the resistance of the magnet to demagnetization—remains a major obstacle to practical deployment.  Iron Nitride magnets often exhibit relatively low coercivity, which limits its ability to maintain magnetization under operational stresses. Several strategies are being explored to improve this critical magnetic parameter:

Grain Size Control: Reducing grain size to the nanoscale (20–50 nm) impedes domain wall motion and increases coercivity. However, overly small grains may compromise anisotropy.

Grain Boundary Engineering: Introducing non-magnetic grain boundary phases or core-shell structures can isolate magnetic domains and reduce intergranular exchange coupling, thereby increasing coercivity.

Crystallographic Texturing: Aligning Fe₁₆N₂ grains along the easy axis of magnetization (typically [002] in the tetragonal structure) via magnetic-field-assisted alignment or hot deformation can enhance anisotropy and coercivity.

Dopant-Assisted Anisotropy Enhancement: Elements like cobalt not only help stabilize the Fe₁₆N₂ phase but also enhance magnetic anisotropy, directly contributing to higher coercivity.

Strain and Stress Modulation: In thin films and multilayers, residual stress from lattice mismatch or controlled strain can be engineered to improve magnetic performance.

Despite advances in these areas, achieving coercivities above 1 kOe (80 kA/m) is necessary for practical magnetic applications. Continued research into nanoscale structure control, dopant chemistry, and low-temperature processing is essential for unlocking the full potential of Iron Nitride magnets.

In summary, Iron Nitride magnets offer exceptional theoretical performance but face a complex set of challenges across synthesis, stabilization, compaction, densification, and magnetic property control. With increasing global and US interest in reducing dependence on rare-earth elements, sustained R&D efforts are crucial for overcoming the scientific and engineering barriers that currently limit Iron Nitride's commercial viability.

References

[1]   Jian-Ping Wang, “Environment-friendly bulk Fe16N2 permanent magnet: review and perspective,” Journal of Magnetism and Magnetic Materials, vol. 497, March 2020.

[2] Myung Hoon Han, Won June Kim, Eok Kyun Lee, Hyungjun Kim, Sébastien Lebègue, and John J Kozak, "Theoretical study of the microscopic origin of magnetocrystalline anisotropy in Fe16N2 and its alloys: comparison with the other L10 alloys," Journal of Physics: Condensed Matter, vol.32, no. 3, 2019.

[3]   Jian-Ping Wang, Shihai He, Yanfeng Jiang, "Iron nitride permanent magnet and technique for forming iron nitride permanent magnet," US 2018/0294078 A1, 2018.

[4]   Monson, T.C., Zheng, B., Delaney, R.E. et al. "Synthesis and behavior of bulk iron nitride soft magnets via high-pressure spark plasma sintering," Journal of Materials Research, vol. 37, pp. 380–389, 2022.

[5] T. Saito, H. Yamamoto, D. Nishio-Hamane, “Production of rare-earth-free iron nitride magnets (α″-Fe16N2),” Metals 2024, Vol. 14, no. 6:734, June 2024.