The material at the heart of the study is alpha double-prime iron nitride, Fe16N2, an ordered tetragonal phase that has fascinated magnet researchers for decades. It is made entirely of abundant, inexpensive elements, yet it boasts a theoretical saturation magnetization of roughly 290 emu per gram and a magnetocrystalline anisotropy near 1.0 times ten to the seventh erg per cubic centimeter, figures that rival or exceed those of many rare-earth compounds. The catch has always been coercivity, the field required to demagnetize the material, which determines whether a magnet can actually hold its grip in a motor. Powders of Fe16N2 made by scalable methods have historically shown coercivities too low for demanding applications, and closing that gap has become one of the central challenges in the search for supply-chain-proof permanent magnets.

The physics of the problem is well understood. Coercivity in ferromagnetic particles rises as particle size shrinks toward the single-domain limit, which for Fe16N2 sits at roughly 73 nanometers. That insight has driven a generation of bottom-up synthesis efforts, from coprecipitation to hydrothermal routes, all aimed at producing nanoscale iron oxide precursors that can later be reduced to iron and nitrided with ammonia. The trouble is that oxide nanoparticles love to agglomerate and coarsen during hydrogen reduction, destroying the fine features that made them attractive in the first place. The original size and shape of the nanoparticles are rarely preserved, and the resulting nitride inherits a compromised microstructure and a disappointingly low coercivity.

Aerosol-assisted synthesis offers an appealing way out because it produces micrometer-scale particles that resist coarsening while remaining compatible with continuous, high-throughput manufacturing. Previous studies explored salt-assisted porosity and hollow particle structures to aid nitridation, but the coercivities reported for aerosol-derived Fe16N2 powders remained modest, generally between 1.2 and 1.47 kilooersted. Most of that work focused on raising the fraction of the magnetic Fe16N2 phase rather than on engineering the particle microstructure itself. The new study, published in Advances in Industrial and Engineering Chemistry by researchers at the Korea Institute of Materials Science together with a collaborator from Hyundai Motor Company, takes the opposite approach: keep the chemistry of the final conversion steps identical, and instead vary the microstructure of the oxide precursor at the very start of the process.

The team prepared three distinct hematite, alpha-Fe2O3, precursors using two aerosol techniques and two different iron salts. The first route was ultrasonic spray pyrolysis, in which a 0.1 molar solution of iron nitrate was nebulized at 1.7 megahertz and carried by compressed air through a quartz tube reactor held at 700 degrees Celsius. Inside the hot zone, each droplet underwent solvent evaporation and thermal decomposition in a single step, yielding solid, spherical hematite particles with smooth, dense surfaces and an average diameter of just 0.5 micrometers. The second route was spray drying, a gentler two-step process in which droplets were atomized with a two-fluid nozzle and dried at only 220 degrees Celsius, leaving intermediate salt particles that were subsequently calcined in air at 450 degrees Celsius for four hours to form the oxide. Spray drying was performed with both iron nitrate and iron oxalate solutions at a much higher solid content of 10 weight percent, producing substantially larger particles, around 3.9 and 4.8 micrometers respectively.

The choice of salt turned out to matter enormously. The spray-dried nitrate powder retained its spherical shape but developed a rough surface of enlarged primary grains as hematite grew during calcination, with an average crystallite size of 31.6 nanometers measured by X-ray diffraction. The oxalate-derived powder, by contrast, emerged with a crumpled, fractured and highly porous texture. The reason lies in the thermal chemistry: iron oxalate decomposes at a higher temperature than iron nitrate, and as it breaks down it releases copious carbon monoxide and carbon dioxide gas that punches fine pores into the particle as it escapes. Because the decomposition happens later under identical calcination conditions, oxide formation is delayed, iron cation diffusion is limited, and grain growth is suppressed, leaving the oxalate-derived hematite with significantly finer crystallites of 20.7 nanometers on average.

All three precursors then passed through exactly the same conversion sequence. Hydrogen reduction at 330 degrees Celsius for four hours transformed the hematite into body-centered cubic alpha iron, and after a brief argon purge the gas flow was switched to ammonia for nitridation at 175 degrees Celsius for fifteen hours, yielding the body-centered tetragonal Fe16N2 phase. X-ray diffraction with whole-pattern profile fitting showed that every sample contained more than 90 weight percent of the desired phase, with the two nitrate-derived powders reaching purities of roughly 94 to 95 weight percent and the oxalate-derived powder coming in at about 90 percent. Remarkably, the final nitride crystallite size converged to a narrow window of 17 to 18 nanometers for all three samples, regardless of the crystallite size in the starting oxide. The researchers attribute this reset to the reduction step itself, in which oxygen removal and iron atom rearrangement govern the formation of new iron crystallites whose size is largely independent of the initial oxide state, while the subsequent low-temperature nitridation preserves that fine structure.

Electron microscopy revealed that the distinctive personalities of the three precursors survived the entire journey. The spray-pyrolyzed particles stayed as smooth spheres of about half a micrometer, merely roughened at the surface. The spray-dried nitrate particles shrank from 3.9 to 2.7 micrometers as oxygen left the lattice and kept their rough, porous surfaces. The oxalate-derived particles shrank only slightly, from 4.8 to 4.5 micrometers, but their porosity intensified during reduction and nitridation, producing a labyrinth of pore walls, edges and rough surfaces. Most of this architectural change occurred during hydrogen reduction, when vacancies and voids formed as oxygen was stripped away, while the low-temperature nitridation made only minimal further alterations. In effect, the microstructure engineered at the oxide stage was faithfully transmitted to the final magnetic powder.

The magnetic measurements delivered the study’s punchline. With crystallite size and phase purity held nearly constant across the three samples, coercivity varied systematically and dramatically: 1.14 kilooersted for the spray-pyrolyzed nitrate powder, 1.46 kilooersted for the spray-dried nitrate, and 1.86 kilooersted for the spray-dried oxalate. Because coercivity in iron nitride materials generally rises with Fe16N2 fraction, the fact that the least pure sample showed the highest coercivity rules out phase purity as the explanation, and the uniform crystallite size rules out that variable as well. The team points instead to domain-wall pinning, the mechanism by which defects, grain boundaries and surface irregularities impede the movement of magnetic domain walls and make the material harder to demagnetize. The oxalate-derived particles, with their abundant pore walls, edges and rough surfaces, simply offered far more pinning sites than their smooth-surfaced cousins. Notably, both spray-dried powders exceeded the 1.2 kilooersted recently achieved with salt-assisted spray pyrolysis, and they did so without any sacrificial templates or added process steps.

The trade-offs are honest and instructive. The high-coercivity oxalate powder showed a saturation magnetization at 2.5 tesla of about 153 emu per gram, lower than the roughly 190 and 185 emu per gram of the nitrate-derived samples, a consequence of its highly porous morphology reducing the density of magnetic material per unit volume alongside its slightly lower phase fraction. Even so, the study demonstrates something potentially transformative for magnet manufacturing: coercivity, the property that has most stubbornly resisted improvement in scalable iron nitride synthesis, can be tuned simply by choosing the aerosol route and the precursor salt. Spray pyrolysis and spray drying are both continuous, industrially mature processes that operate on gram-per-hour scales today, and the entire workflow, from dissolved iron salt to nitrided magnetic powder, involves nothing but water, air, hydrogen and ammonia. For a field racing to build high-performance magnets without rare earths, the message is that the path to a better magnet may begin not in the final nitridation furnace but in the microscopic architecture of a particle of rust.

Subject of Research: Aerosol synthesis and precursor microstructure control to enhance coercivity in rare-earth-free alpha double-prime Fe16N2 magnetic powders

Article Title: Scalable aerosol routes to enhance coercivity in α′′-Fe16N2 powders through α-Fe2O3 microstructure control

Article References: Jeong, J., Park, H. J., Koo, H. Y., Kim, D., Lee, J.-G., & Baek, Y.-K. (2025). Scalable aerosol routes to enhance coercivity in α′′-Fe16N2 powders through α-Fe2O3 microstructure control. Advances in Industrial and Engineering Chemistry, 1(1), Article 23. https://doi.org/10.1007/s44405-025-00026-9

Image Credits: AI Generated

DOI: 10.1007/s44405-025-00026-9

Keywords: Fe16N2, rare-earth-free magnets, coercivity, aerosol synthesis, spray pyrolysis, spray drying, hematite precursor, iron nitride, domain-wall pinning, permanent magnets, microstructure control, hydrogen reduction