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[REPM2025_P2-69_Dirba.pdf](https://mdr.nims.go.jp/filesets/6fb5702a-12bb-4239-ab97-1650fd3803a0/download)

## Creator

Imants Dirba, Jürgen Gassmann, Oliver Diehl, Oliver Gutfleisch

## Rights

[Creative Commons BY Attribution 4.0 International](https://creativecommons.org/licenses/by/4.0/)

## Other metadata

[Iron nitride Fe16N2: intrinsic properties, synthesis, stability and bulk magnets](https://mdr.nims.go.jp/datasets/258f442c-4320-4a25-8db2-3c0739528d0f)

## Fulltext

Slide 1Temperature-dependent magnetic measurements confirm theXRD results. Decomposition of -Fe16N2 phase starts at 460 K transforming first into γ-Fe4N and finally into -Fe.   -Fe16N2 →-Fe + γ-Fe4N →-Fe + N2• Iron oxide Fe2O3 nanoparticles are used as a starting material • Reduction to pure -Fe by H2• Reduction conditions optimizedIron nitride Fe16N2: intrinsic properties, synthesis, stability and bulk magnetsSynthesis of α′′-Fe16N2 nanoparticlesReferences[1] I. Dirba et al., J. Magn. Magn. Mater. 379 (2015) 151–155. [2] I. Dirba et al., J. Appl. Phys. 117 (2015) 1–1. [3] H. Zhang et al., APL Mater. 4 (2016) 116104. [4] I. Dirba et al., Acta Mater. 123 (2017) 214–222. [5] I. Dirba et al., J. Magn. Magn. Mater. 518 (2021).[6] I. Dirba et al., J. Phys. D: Appl. Phys. 56 (2023) 025001.[7] I. Dirba et al., Nanoscale Adv. 2 (2020) 4777–4784.IntroductionTowards bulk α′′-Fe16N2 magnetsThe fascinating phase diagram of iron-nitrogen provides a variety of interesting materials with their magnetic properties tunable in a broad range depending on the nitrogencontent. Magnetism correlates with the iron amount, starting from nonmagnetic FeN to ferromagnetic γ′-Fe4N [1] with high magnetization and even further to α′-Fe8N [2].Perhaps the most attention is attracted by the ordered tetragonal superstructure α′′-Fe16N2 due to its unique combination of high saturation magnetization with enhancedmagnetocrystalline anisotropy [3]. Unfortunately, the α′′-Fe16N2 phase is metastable and therefore hard to produce in phase-pure form. Moreover, decomposition alreadybelow 200 °C [4] hinders large scale production of bulk fully dense magnets using conventional routes. Nevertheless, it has been studied for multiple potential applications,such as rare-earth-free permanent magnets, two-phase nanocomposite magnets [5] as well as biomedical applications [6]. In this contribution we provide a comprehensiveoverview and assessment of the potential magnet performance based on our own experimental results as well as study of the available scientific literature and patents. Wereport synthesis of α′′-Fe16N2 nanoparticles followed by production of bulk samples by low-temperature consolidation. The possibility of enhancing the anisotropy fieldusing shape anisotropy is explored in nanowires. Magnetic properties and stability are discussed in the context of possible application in permanent magnets.Imants Dirba1*, Jürgen Gassmann2, Oliver Diehl2, Oliver Gutfleisch1Functional Materials, Institute of Materials Science, Technical University of Darmstadt, 64287 Darmstadt, GermanyFraunhofer Research Institution for Materials Recycling and Resource Strategies IWKS, Aschaffenburger Str. 121, 63457 Hanau, GermanyFunded by the Deutsche Forschungsgemeinschaft (DFG, German Research Foundation), CRC 1487 (Project number 443703006), „Iron, upgraded!”, and project ID Nos. 471878653, 405553726-TRR 270.α′′-Fe16N2 thermal stabilityTheoretical potential and future directionsinitialAdvantages of Fe16N2 magnets• Cheap and abundant raw materials• Better temperature stability of magnetic properties thanNd2Fe14B due to the higher Curie temperature• High magnetizationChallenges to be addressed• Enhancing coercivity to avoid self-demagnetization• High production costs and complexity in nanoparticles• Particle alignment for production of anisotropic samples• Poor stability makes production of bulk magnets difficult*imants.dirba@tu-darmstadt.de-Feγ-Fe2O3H2High-pressure hydrogen reduction enables lower temperatures and particle growth can be successfully avoided [7]• Phase-pure α′′-Fe16N2 nanoparticles synthesized from the fine α-Fe precursors • No observable particle growth during the synthesis, crystallite size 50 nm-Fe -Fe16N2NH3• 663 K (ambient pressure): neck formation, significant coalescence into micrometer-range structures• 483 K: fine nanoparticles 50 nmNitrogenation in NH3 atmosphere𝐹𝑒2𝑂3 + 3𝐻2𝐻𝑒𝑎𝑡2𝐹𝑒 + 3𝐻2𝑂In inert atmosphere (Ar): -Fe16N2 is stable up to 463 K. Decomposition in -Fe + γ-Fe4N →-Fe + N2 In air:complete oxidation begins already at  433 K.Fe nanowires H2 treatment NH3 treatmentField alignmentAssessing the theoretically possible performance• Alloying to enhance the magnetocrystalline anisotropy field• Shape anisotropy using nanowiresSaturation magnetization 𝑀𝑠 ≈ 2.3 𝑇 Anisotropy constant 𝐾𝑢 ≈ 1.0𝑀𝐽𝑚3Hardness parameter: 𝑘 =𝐾1𝜇0𝑀𝑠2 ≈ 0.5 < 1Anisotropy field: 𝜇0𝐻𝑎 =2𝐾𝑢𝜇0𝑀𝑠=2∙1062.3≈ 1.1 𝑇Empirically (Brown’s paradox): 𝐻𝑐 ≤ 25% of 𝐻𝑎 ⟹ 𝐻𝑐 ≤ 0.27 𝑇 𝑯𝒄 ≪ 𝑴𝒔‘semi-hard‘ materialCoercivity sufficient for niche applications?Potential for coercivity improvements: (i) alloying and (ii) shape anisotropyFe nanowires as a precursor for synthesisFe16N2 to benefit fromshape anisotropyLab-scale production of bulk α′′-Fe16N2 magnetsα′′-Fe16N2 nanoparticles filled in a steel die Pressing below decomposition TLab-scale samples for characterization• Saturation Ms = 215 A m2 kg-1• Remanence Mr = 73 A m2 kg-1• Coercivity 0Hc ≈ 0.25 T• Curie Temperature Tc = 634 K• (BH)max = 28 kJ m-3 Nanocomposite magnets from Srhexaferrite as the ‘hard‘ and iron nitride as the ‘soft‘ phase.• Coercivity decreases from 0.78 T in the initial ferrite to 0.38 T in the case of 15 wt.% Fe16N2 addition. This is te highest coercivity reported  in  Fe16N2 samples.Coercivitiesreported in scientific literature for Fe16N2within the last two decades reach about 0.34 T.Reason for lower magnetization and lack of coupling issurface oxidation of the Fe16N2 nanoparticles.Fe₁₆N₂: Hype, Hope, or Heavy Hitter?Ha≈1.6 TOxidationDecompositionFe16N2 Fe4N + Fe Femailto:imants.dirba@tu-darmstadt.de